Good sleep, good learning, good life
- 1 Introduction
- 2 Importance of sleep
- 3 Why do we need to sleep?
- 4 Sleep deprivation is lethal
- 5 Circadian and homeostatic sleepiness
- 6 Formula for good sleep
- 7 Sleep habits
- 7.1 Constellation of sleep habits
- 7.2 Body clock
- 7.3 Components of sleep in phase disorders
- 7.4 Lark-owl misconception
- 7.5 Charting sleep
- 7.6 24-hour sleep cycle
- 7.7 Preference for night sleep
- 8 Biphasic life
- 8.1 Biphasic human
- 8.2 Biphasic learning
- 8.3 Biphasic sleep periodogram
- 8.4 Biphasic learning and sleep
- 8.5 Biphasic sleep
- 8.6 Monophasic sleep with biphasic learning
- 8.7 Biphasic circadian graph
- 8.8 Two components of biphasic sleep propensity
- 8.9 Biphasic performance in sleep deprivation
- 8.10 Summary: Napping is good!
- 9 Optimum time for brainwork
- 10 Healthy siesta
- 11 Best time for siesta
- 11.1 Value of siesta
- 11.2 Circadian nap phases
- 11.3 Napping rules
- 11.4 One nap per day is enough
- 11.5 Conclusions: napping
- 12 Napping myths
- 13 Factors that affect sleep
- 13.1 Stress
- 13.2 Eustress
- 13.3 Alcohol
- 13.4 Caffeine
- 13.5 Sleeping pills
- 13.6 Melatonin
- 13.7 Nicotine
- 13.8 Exercise
- 13.9 TV
- 13.10 Cannabis
- 13.11 Sex
- 13.12 Diet
- 13.13 Learning
- 14 How long should we sleep?
- 14.1 Optimum length of sleep
- 14.2 People who sleep less live longer?
- 14.3 Jim Horne and Daniel Kripke
- 14.4 Effects of sleep duration and sleep phase on learning
- 14.5 Sleep block length distribution
- 15 Baby sleep
- 15.1 How to make babies sleep well?
- 15.2 Sleeping throughout the night
- 15.3 Development of a healthy circadian cycle
- 15.4 Co-sleeping as a circadian solution
- 15.5 Best timing for feeding
- 15.6 Child's own bed
- 15.7 What about the mom?
- 15.8 Why babies sleep so much?
- 15.9 Conclusion: Perfect formula for baby sleep
- 16 Insomnia
- 17 Sleep inertia
- 18 Shift work and jet lag
- 19 Sleep apnea
- 20 Hypersomnia
- 21 Delayed Sleep Phase Syndrome
- 21.1 DSPS Problem
- 21.2 DSPS in teenagers
- 21.3 Solution to the DSPS problem
- 21.4 Is DSPS a disease?
- 21.5 Asynchronous DSPS
- 21.6 Synchronous DSPS
- 21.7 28 hour day schedule
- 22 Advanced Sleep Phase Syndrome
- 23 Curing DSPS and insomnia
- 24 Charting phase shifts
- 25 Segmented sleep
- 25.1 Wehr experiment
- 25.2 Interpretation of segmented sleep
- 25.3 Segmented sleep and Borbely model
- 25.4 Segmented sleep and two-component model
- 25.5 Examples of segmented sleep
- 25.6 Application of segmented sleep
- 26 Function of sleep
- 27 Sleep and learning
- 27.1 How sleep affects learning?
- 27.2 Studying sleep and learning with SuperMemo
- 27.2.1 Long sleep results in poor learning?
- 27.2.2 Learning reduces the demand for sleep?
- 27.2.3 Approximating the sleep phase
- 27.2.4 Timing of repetitions
- 27.2.5 The impact of SleepChart
- 27.2.6 Recall vs. Consolidation
- 27.2.7 Alarm clock vs. learning
- 27.2.8 Learning in free running sleep
- 27.2.9 Alertness multiplier
- 27.2.10 Learning overload
- 27.2.11 Alertness vs. learning
- 27.3 How learning affects sleep?
- 27.4 Sleep and school
- 27.5 Learning in alpha state
- 27.6 Learning during sleep
- 28 How do we fall asleep?
- 28.1 Initiation of sleep
- 28.2 Circadian cycle
- 28.3 Borbély model
- 28.4 Phase response curve (PRC)
- 28.5 Recursive phase response curve (rPRC)
- 28.6 Two-component model of sleep in SleepChart
- 28.7 REM rebound hypothesis
- 28.8 Sleep-wake flip-flop
- 28.9 Suprachiasmatic nucleus (SCN)
- 28.10 Dorsomedial Hypothalamic Nucleus (DMH)
- 28.11 Ventrolateral Preoptic Nucleus (VLPO)
- 28.12 Nucleus of the Solitary Tract (NTS)
- 28.13 Adenosine
- 29 NREM sleep and REM sleep
- 30 Neural optimization in sleep
- 30.1 Sleep as a neural optimizer
- 30.2 Sleep and memory
- 30.2.1 NREM and memory
- 30.2.2 REM and memory
- 30.3 Synaptic changes in sleep
- 30.4 Optimization of memory
- 30.4.1 Hippocampal lesions
- 30.4.2 Temporally graded retrograde amnesia
- 30.4.3 Memory processing in sleep
- 30.4.4 Catastrophic forgetting
- 30.4.5 Two-stage memory processing in sleep
- 30.4.6 Optimizing memories
- 30.4.7 Garbage collection
- 30.4.8 Unihemispheric sleep
- 30.4.9 Problem solving in sleep
- 30.4.10 Conclusion
- 31 Scientists who contest memory optimization
- 31.1 Is sleep a neural optimizer?
- 31.2 Robert Vertes and Jerome Siegel
- 31.2.1 1. Sleep does not serve a role in declarative memory?
- 31.2.2 2. REM sleep deprivation does not lead to cognitive impairment?
- 31.2.3 3. Sleep-dependent enhancement of procedural learning has not been proven?
- 31.2.4 4. Learning in waking is far more significant than overnight enhancements?
- 31.2.5 5. Sleep models should be simple
- 31.2.6 How can random impulsations in REM make a sense in dreams?
- 31.2.7 Dr Siegel's theory of sleep
- 31.2.8 My personal bias
- 31.2.9 Olive branch
- 31.2.10 More reading for skeptics
- 32 Circadian oscillator genes
- 33 Myths and Facts
- 34 Summary
- 35 Glossary
- 36 Incremental writing
- 37 References
I have for years been interested in sleep research due to my professional involvement in memory and learning. This e-book attempts to produce a synthesis of what is known about sleep with a view to practical applications, esp. in people who need top-quality sleep for their learning or creative achievements. Neurophysiology of sleep is an explosively growing branch of science. Many theories that are currently contested will soon be forgotten as a result of new findings. Consequently, this text is likely to grow old very quickly (compare old versions from 2000 and 2012). Still, some basic truths about sleep are well-established, and practical conclusions can be drawn with the benefit to human creativity and intellectual accomplishment. In this text, I provide some links to research papers and popular-scientific articles that advocate disparate and contradictory theories. Please consult other sources to be certain you do not to get a one-sided view! This text includes some indications on how to use free running sleep in the treatment of insomnia, advanced and delayed phase shift syndromes, and some other sleep disorders. If your own experience can contribute to the ideas presented herein, I will gladly hear from you (esp. in the context of learning and creativity).
Importance of sleep
Dreams of good sleep
It is everyone's dream to wake up fresh, happy, and ready for action on a daily basis. Sadly, in the modern world, only a small minority lives that dream. Yet the dream is within reach for most healthy people given:
- a bit of knowledge, and
- a readiness to make some lifestyle sacrifice.
I hope that this article compiles all the basic ingredients of knowledge that are helpful in accomplishing refreshing sleep. As for the sacrifice, it is important to begin with the understanding that one cannot eat one's cake and have it too. Healthy sleep may be incompatible with some modern habits, some cravings, or some lifestyle choices. At worst, refreshing sleep may be incompatible with one's job or even long-term goals. Due to the latter fact, this article cannot provide a solution for everyone. Moreover, having a happy and fresh mind on a daily basis is a difficult thing to accomplish even with the whole arsenal of scientific knowledge and a full focus on good sleep. However, let me state it emphatically:
This article was originally written a decade ago. I have always been interested in memory, learning, and sleep. In addition, in my job, sleep is as important as oxygen. As we all move deeper into the Information Age and Knowledge Economy, the issues discussed herein will become more and more important for each of us. After writing the original article, I had the great pleasure of getting in touch with hundreds of people experiencing various sleep problems. I came to see first hand how knowledge of sleep helps solve their problems. I could also see how the industrialized age lays obstacles in one's quest for good sleep and high productivity. I have witnessed a true epidemic of sleep phase disorders, an explosion of interest in polyphasic sleep, and an exponential increase in interest in the matters of sleep in general. Despite my pleas, many people just cannot avoid using an alarm clock, running all-nighters before exams, waking their kids cranky for school, popping pills before sleep, leaving babies in their cots to cry it out for sleep, etc. The picture would be pretty sad and alarming were it not for the fact that there is hope in knowledge. With a degree of determination, everyone can improve his, her, or their kids' sleep.
This article is a compilation of the most important and the most interesting things about the biology of sleep. It is supposed to help you gain knowledge needed to achieve high quality refreshing sleep that will boost your mental powers. The article explains why sleep is vitally important for health and for the brain. It argues that sleep deserves highest respect, and that most people could get excellent sleep if they only followed the prescribed rules.
Since writing the original Good sleep, good learning, good life, tremendous progress has been made in the science of sleep. My own work with tools such as SleepChart and SuperMemo has shed some interesting light on the connection between sleep and learning. As I kept addressing the progress in sleep science in minor articles and FAQs, some visitors to super-memory.com complained that valuable nuggets of information are dispersed throughout the site instead of being organized in a more encyclopedic manner in a single article. Here then comes a comprehensive compilation, in which I would like to retain the focus on practical knowledge that is helpful in achieving good sleep. However, I would still like to smuggle in some lesser known research findings that might be inspiring for an average reader and/or a scientist working in the fields of sleep, memory, and learning. If you believe I left out anything important that others should know, please let me know.
As the article grew to be insanely long, you may wish to begin with the summary at the bottom of the article. And if even that is too long, here are the highlights:
- respect sleep as your tool for high IQ and good learning
- free running sleep can help you resolve many sleep problems
- biphasic sleep schedule is probably the healthiest schedule for creative people
- do not wake up kids for school; if they cannot wake up in time, let them skip a class or two, or consider homeschooling
- let babies and young children sleep on demand, co-sleeping is a great idea (even if many pediatricians will tell you otherwise)
- exercise, learning, and sleep are your best tools for brain growth!
- avoid regulating sleep and alertness with substances, esp. sleeping pills, alcohol, illegal drugs, nicotine, and caffeine
Too few people realize how important sleep is! The alarm clock is an often-used fixture in an overwhelming majority of households of the modern world. By using electric lighting, alarm clocks, sleeping pills, and shift-work, we have wreaked havoc on the process of sleep.
Figure: Four examples of sleep logs that illustrate that modern human sleep patterns are as varied as snowflakes
Over the last hundred years of the twentieth century, we have intruded upon a delicate and finely regulated process that was perfected by several hundred million years of evolution. Yet only recently have we truly become aware that this intrusion may belong to the most important preventable factors that are slowing societal growth in industrial nations! In a couple of years from now, we may look at alarm clocks and "sleep regulation" in the same way that we look today at other "great" human inventions in the league of cigarettes, asbestos materials, or radioactive cosmetics.
Check this list below and see which applies to you:
- I often have problems with falling asleep at the right time
- I often find it painful to get up in the morning due to sleepiness
- I am often awfully drowsy at school or at work
- I regularly cut my sleep by 2-3 hours as compared with what my body seems to need
- I use the alarm clock and truly hate it
- I drink buckets of coffee or coke
- I often take 2-4 hour naps in the evening
- for me, at least one of the above is a source of regular stress or reduced productivity
I bet that chances are around 90% you could subscribe to one of the above. Perhaps this is why you are reading this article. It is also highly likely you have already learned to accept the status quo, and you do not believe you can do much about it. This article may hint at some remedies. However, the bad news is that for a real solution you will probably need to change your family life, your work, your boss, or some social rules!
Sleep isn't just a form of rest! Sleep plays a critical physiological function, and is indispensable for your intellectual development! Those who do not respect their sleep are not likely to live to their full mental potential!
Modern society has developed a set of well-entrenched rules that keep sleep in utmost disregard. This has been driven to pathological levels in American society. Here are some bad rules that hurt sleep:
- it is ok to use an alarm clock to cut sleep short
- it is ok to work in shifts
- it is ok to travel people around the world without much attention to the jet lag problem
- it is ok to save time by sleeping less and working more
- it is ok to pull kids out of bed in time for school
- it is ok to skip nights before important exams, etc.
Cutting down on sleep does not make people die (at least not immediately). It does make them feel miserable, but the ease with which we recover by getting just one good night of sleep seems to make sleep look cheap. Even the reports from the Guinness World Record attempt at sleeplessness (Randy Gardner's awakathon in 1964 lasted 11 days) trivialized the effects of sleeplessness. Many books on psychiatry and psychology still state that there aren't any significant side effects to prolonged sleeplessness! This is false! The Guinness Book of Records has since withdrawn its sleep deprivation category due to the involved health risks.
In 1992, when Bill Clinton was running for president, he proudly admitted that he went 48 hours without sleep because he really wanted to become the next president. Former Senator Bob Dole "improved" the record in 1996 presidential campaign: We have been going 78 hours. We've got to go 96. We have been going around the clock for America. Dole's feat was matched by Vice President Albert Gore Jr., who kept campaigning for three days before the election day of November 7, 2000. After the election, Gore still kept on his feet by going into extra hours of the concede-retract cycle of his cliffhanger contest against Governor George W. Bush of Texas. When Barack Obama was asked about his most desired Christmas gift after over a year of campaigning for president, he answered without hesitation: 8 hours of sleep.
The bad example of disrespect for sleep comes from the most important people in the nation!
Yet some dramatic facts related to sleep deprivation have slowly come into light. Each year sleep disorders add $16 billion to national health-care costs (e.g. by contributing to high blood pressure and heart disease). That does not include accidents and lost productivity at work. For this, the National Commission on Sleep Disorders estimates that sleep deprivation costs $150 billion a year in higher stress and reduced workplace productivity. 40% of truck accidents are attributable to fatigue and drowsiness, and there is an 800% increase in single vehicle commercial truck accidents between midnight and 8 am. Major industrial disasters have been attributed to sleep deprivation (Mitler et al. 1988)(incl. Three Mile Island, Chernobyl, the gas leak at Bhopal, Zeebrugge disaster, and the Exxon Valdez oil spill).
It has been known since the 1920s that sleep improves recall in learning. However, only at the turn of the millennium, research by Dr Robert Stickgold, Associate Professor of Psychiatry at Harvard Medical School, has made international headlines. Dr Stickgold's research proves a fact that has long been known yet little appreciated: sleep is necessary for learning (Stickgold 2005)! With less sleep, we reduce the recall of facts we learned before or after a shortened night. Studying nights before an exam may be sufficient for passing the exam, yet it will leave few useful traces in long-term memory. The exam on its own replaces knowledge as the main purpose of studying!
A change in societal sleep habits can spell a social revolution in learning, health, and productivity on a scale that few imagine! "Judging from history, it would seem that fundamental changes in the way we think about sleep will be required for policy changes that would protect society from sleepy people who make catastrophic errors in industry and transportation" (Merrill Mitler, PhD)
I have studied student personalities among users of SuperMemo for over twenty years now. There are a couple of determinants that make a good, efficient and persistent student. Here are some characteristics of a person who is likely to be successful in learning:
- highly optimistic
- sleeps well
- knowledge hungry
- energetic, but able to slow down at the time of learning
Here are some unfortunate characteristics that do not correlate well with the ability to study effectively:
- prone to depression or mood swings
- problems with sleep (esp. insomnia)
- high levels of stress
- hyperactive and unfocused
- low stress tolerance (smokers, abusers of mood altering substances, drinkers, etc.)
Why do we need to sleep?
Role of sleep
For many years, the physiological function of sleep has not been clear. In most people's mind, sleep is associated with rest and time for mental regeneration. Restorative, protective and energy-conserving theories of sleep have been quite popular until quite recently, when it has become apparent that one long-lasting sleep episode with suppression of consciousness does not seem to be the right way for evolution to tackle depleted resources, toxic wastes, or energy conservation. For example, muscles do not need to shut off completely to get rest. The critical function of sleep is dramatically illustrated in experiments in which rats chronically deprived of sleep eventually die usually within 2.5 weeks (for more see: If you do not sleep, you die!).
In evolutionary terms, sleep is a very old phenomenon and it clearly must play a role that is critical to survival. Only quite recently, it has been proven beyond doubt that the function of sleep is related to learning!
Researchers have long known about the importance of the hippocampus, a small brain organ, for memory formation. Yet it has always been difficult to find out what is special about the hippocampus that distinguishes it from other areas of the cerebral cortex that also show synaptic plasticity, i.e. the ability to store memories.
A collective effort of a number of researchers resulted in the proposition of the concept of neural optimization in sleep (see the next section for a metaphorical explanation: Disk and RAM metaphor). Ground-breaking theories of Dr György Buzsáki and his two-stage model of memory trace formation have shed new light on what might actually be happening during sleep (Buzsáki 1989)(important: do not confuse this two-stage model with the two-component model of memory (Wozniak et al 1995) or with the two-component model of sleep regulation (Borbely 1982) below). Using his knowledge of neural networks, ingenious experiments on neuronal firing, and sophisticated mathematical analysis of spatiotemporal neuronal firing patterns, Buzsáki provided a good model explaining how the two components of sleep, REM and NREM sleep, work together to optimize memories. The hippocampus acts as the central switchboard for the brain that can easily store short-term memory patterns. However, these patterns have to be encoded in the neocortex to provide space for coding new short-term memories. This complex process of rebuilding the neural network of the brain takes place during sleep. Unlike rest or conservation of energy, this highest feat of evolutionary neural mathematics requires the brain to be shut off entirely from environmental input (in most animals)! This automatic rewiring is the main reason for which we sleep and why there is no conscious processing involved! During sleep, the brain works as hard as during SAT or GRE exams. It rewires its circuits to make sure that all newly gained knowledge is optimally stored for future use.
Sleep is controlled by a complex set of brain nuclei. Interference with this system disrupts the function of sleep. For more see: Sleep control system
For more see:
Disk and RAM metaphor
A metaphor can help understand the role of sleep and why alarm clocks are bad.
If you do not care about your intellectual performance, you may want to know that there are many other biological reasons for which using alarm clocks is unhealthy. Many people use alarm clocks and live. Yet this is not much different from smoking, abusing drugs, or indulging in fat-dripping pork. You may abuse your brain with alcohol for years, and still become president. Many of mankind's achievements required interrupted sleep. Many inventions were produced by sleepy brains. But nothing is able to change the future as much as a brain refreshed with a healthy dose of restful sleep.
Bad sleep kills and costs billions
Sleep deprivation is a killer! It kills precious life via airplane crashes, nuclear power station failures, car crashes, oil spills, etc. Sleep deprivation can change the course of history. Charles Lindbergh would have been just a footnote in history if he had failed to recover the Spirit of St. Louis from a dive caused by microsleep. Sleep deprivation has changed the future of nuclear fission and the future of oil exploration. Poor sleep kills as many people on the roads as alcohol. 1550 annual fatalities in the US can be attributed to drowsy driving. That's nearly an equivalent of six WTC collapse tragedies in a decade! Amazingly, as the pain and suffering is diluted in the population, drowsy driving does not nearly make as many headlines as a terrorist attack. At least a third of Americans have fallen asleep behind the wheel at least once! During the shift to DST in spring, car accidents increase by 9%. Sleep deprivation carries an astronomical cost to industrialized societies. There are zillions of hours wasted on unproductive learning in schools, and zillions of man-hours wasted on futile tossing and turning in bed. There is also a cost to grumpy behaviors and snappy outbursts. The quest for better sleep provokes desperate solutions such as the Uberman polyphasic sleep, "safe alarm" contraptions, hundreds of books and thousands of blogs with good advice on falling asleep fast, getting up early, or sleeping little. At the same time real solutions are simple and obvious! Read portions of this article and try free running sleep for at least a month to quadruple your knowledge about sleep and its potential to change your life for the better.
In a comment to the conclusion of a sleep deprivation debate organized by the Economist, Karen M. wrote: "We don't get enough sleep, and we are not going to "change our ways" because there are already too few hours in most people's days to do things they enjoy. Call it a sad fact of life because that's what it is". Even though Karen attempted to represent the entire population saying "we", many readers of this article will disagree and do their best to get as much sleep as physiologically necessary. Otherwise my writing effort would not be needed. Good sleep makes us nicer, smarter, and saves lives!
See: 10 Things to Hate About Sleep Loss from WebMD.
Sleep deprivation is lethal
If you do not sleep, you die!
Nearly everyone pulls an all nighter once upon a time. Even if this is often an unpleasant experience, it nearly always ends up with 100% recovery after a single night of solid sleep. It is therefore a bit surprising to know that a week or two of sleep deprivation can result in death! Sleep researchers constructed a cruel contraption that would wake up rats as soon as they fell asleep. This contraptions showed that it takes an average of 3 weeks to kill a rat by sleep deprivation (or some 5 months by REM sleep deprivation alone)(Rechtschaffen 1998). Dr Siegel demonstrated brain damage in sleep-deprived rats (Siegel 2003). Due to an increase in the level of glucocorticoids, neurogenesis in some portions of the brain is inhibited by lack of sleep. In short, sleep deprivation is very bad for the health of the brain.
Sleep deprivation is a well-known form of torture. Yet, for ethical reasons, the rat experiment could not be reproduced in humans (to its ultimate end). However, we have a rough idea as to the degree of human durability in sleep deprived state due to fact that we can study the effects of sleep disorders. One of them is fatal familial insomnia, in which a mutation causes the affected people to suffer from a progressively worsening insomnia that ends in death within a few months. Another example is the Morvan's syndrome in which an autoimmune disease destroys neuronal potassium channels that lead to severe insomnia and death (unless the disease progresses into remission).
You may have heard of reports of people who do not sleep at all. These reports are certainly inaccurate or false. Those who report never sleeping are either boasting or experiencing a sleep state misperception that leaves them with an illusion that they do not sleep while resting in bed.
Brain's garbage collection
Why is sleep deprivation ultimately fatal? Death of sleep deprivation is like death of an old age in general. Very often, multiple causes conspire to produce the final inevitable outcome. Probably nobody knows the exact answer to this mystery. However, research into the role of sleep gives us pretty strong hints.
During the day, we learn new things, memorize, acquire skills, figure things out, set new memories through creative associations, etc. After a long day of waking, the brain is full of disorganized pieces of information that need to be integrated with things we have learned earlier in life. Without this re-organization, the brain would harbor chaos, and would quickly run out of space to store new memories. This neural role of sleep is so fundamental that sleep deprivation affects nearly all functions of the body that are governed by the nervous system. Without a regular garbage collection, individual networks begin to malfunction. These initially minor malfunctions can add up to a serious problem for the entire organism. Most prominent effects of sleep deprivation are problems with thermoregulation, decline in immune function, hormonal changes (e.g. increase in glucocorticoids and catecholamines), metabolic changes, malnutrition, hallucinations, autonomic system malfunction, changes in cell adhesion, increase in inflammatory factors (e.g. IL-6, TNF, C-reactive protein, etc.), skin lesions, oxidative stress, DNA damage, etc. Those problems become serious enough to kill. Metaphorically speaking, if we compared a less developed organism to a WW1 bomber, we could imagine that the process of evolving into a human being is like acquiring the software needed to fly a B-2 bomber. Even though B-2 is ages ahead of a plane constructed during the life of Orville Wright, it is enough to plant a bug in its software to make it fall out of the sky. Human body in sleep deprivation is like a B-2 with a progressive software malfunction. It may be technologically advanced, it may be smart, and yet it is very vulnerable. The reliance on advanced software or neural function is always dangerous! Luckily, all we need to eliminate the danger is to just go to sleep every day. For more see: Neural optimization in sleep.
There is a second layer of trouble in sleep deprivation. Due to the importance of sleep, all advanced organisms implement a sleep protection program. This program ensures that sleep deprivation results in unpleasant symptoms. It also produces a remarkably powerful sleep drive that is very hard to overcome. Staying awake becomes unbearable. Closing one's eyes becomes one of the most soothing things in the universe. Are these symptoms a result of network malfunction? Definitely not. If they were, the drive to sleep might malfunction as well. Moreover, recovery from sleep deprivation would not be as fast, as easy, and as complete! Sleep protection program is there, and it can make the effects of sleep deprivation worse. Like a cytokine storm in an overzealous immune system, sleep protection program can potentially add to the damage caused by the network malfunction in sleep deprivation.
Last but not least, sleep has evolved to become a chief anabolic state of the organism. Without it, the body keeps using itself up, without much time to rebuild. Turning on anabolic state does not require turning off the consciousness, however, the time of night rest seems to be the best time for the body to do all the rebuilding. As we must sleep anyway, that anabolic functions became consolidated with other functions of sleep, and now may be indispensable. The anabolic state, and the nighttime increase in GH or testosterone, also affects the neural networks and the status of our "mind software". Hormonal changes stimulate and/or inhibit neural growth. Dr Michael Stryker, best known for demonstrating the role of sleep in brain development (Stryker et al. 2001), says that nighttime hormonal changes may "play a crucial role in consolidating and enhancing waking experience". One of the leading causes of death in sleep deprivation seems to have been opportunistic bacterial infections caused by a decline in the immune function (e.g. no febrile response). That decline could be caused equally well by (a) poor neural control of the immune function or (b) straight effect of hypercatabolism. Whatever the cause, scientists have quickly figured out that application of antibiotics did not help much in preventing death from those infections. Sleep deprived rats would die anyway. The infection might speed up death that was otherwise inevitable.
Why do we die without sleep?
It is impossible to quantify the contribution of those three factors to the fatal outcome of prolonged sleep deprivation:
- network malfunction, or
- secondary effects of sleep protection program, or
- continuous catabolic state.
Even though the latter two could possibly be remedied pharmacologically, there is no way around network remolding in sleep. Researchers who hope to find a remedy against sleep are plodding a blind path. Without some serious nanotechnology bordering on science fiction, sleep is here to stay with human race for many years to come. Even though, sleep deprivation could kill, sleep is good news. It makes us smarter! We should all embrace the blessings of healthy unrestrained sleep. After all, there are few better things in life than a good night sleep after a well-spent day. Sleep should be listed among basic human rights!
Circadian and homeostatic sleepiness
Two components of sleep
Electric lighting and stress are the two chief culprits that have converted the natural process of sleep into a daily struggle for millions. In the new millennium, we can rarely hope to get a good night sleep without understanding the science and the art of sleep. Currently, the societal understanding of sleep and its functions is as dismal as the understanding of the health risks of cigarettes in the 1920s. A majority of the population inflict pain, misery and mental torture on themselves and their children by trying to regulate their sleep with alarm clocks, irrational shift-work patterns, sleeping pills, alcohol, caffeine, etc.
For a chance to break out from unhealthy sleep habits, you need to understand the two-process model of sleep regulation.
There are two components of sleepiness that drive you to bed:
- circadian component - sleepiness comes back to us in cycles which are usually about one day long
- homeostatic component - sleepiness increases with the length of time we stay awake
Only a combination of these two components determines the optimum time for sleep. Most importantly, you should remember that even strong sleepiness resulting from the homeostatic component may not be sufficient to get good sleep if the timing goes against the greatest sleep propensity determined by the circadian component.
There are around hundred known body functions that oscillate between maximum and minimum values in a day-long cycle. Because these functions take about a day's time to complete, the term circadian rhythm was coined by Dr Franz Halberg of Germany in 1959 (in Latin circadian means about a day). The overall tendency to maintain sleep is also subject to such a circadian rhythm. In an average case, the maximum sleepiness comes in the middle of the night, reaches the minimum at awakening, and again increases slightly at siesta time in the afternoon. However, the circadian sleepiness is often shifted in phase as compared with your desired sleep time. Consequently, if your maximum sleepiness comes in the morning, you may find it difficult to fall asleep late in the evening, even if you missed a lot of sleep on the preceding day. In other words, the optimum timing of your sleep should take into consideration your circadian rhythm.
Homeostasis is the term that refers to maintaining equilibrium or balance in physiological and metabolic functions. If you drink liquids containing lots of calcium, homeostatic mechanisms will make sure that you excrete calcium with urine or deposit it in the bones. This is used to make sure your blood levels of calcium remain the same. Similar mechanisms are used to regulate overall sleepiness and its multiple subcomponents. The longer you stay awake, the more you learn, the more you think, the higher your tendency to fall asleep. On the other hand, caffeine, stress, exercise and other factors may temporarily reduce your homeostatic sleepiness. The homeostatic mechanism prepares you for sleep after a long day of intellectual work. At the same time it prevents you from falling asleep in emergencies.
Clock and Hourglass metaphor
A metaphor is useful in explaining the two components of sleep (for a more scientific explanation see: Borbely model).
The fundamental theorem of good sleep
Let us now formulate the fundamental theorem of good sleep:
- strong homeostatic sleepiness: this usually means going to sleep not earlier than 15-19 hours after awakening from the previous night sleep
- ascending circadian sleepiness: this means going to sleep at a time of day when you usually experience a rapid increase in drowsiness. Not earlier and not later! Knowing the timing of your circadian rhythm is critical for good night sleep
You should be aware that using the circadian component will only work when all its physiological subcomponents run in sync (as it is the case in free running sleep). People with irregular sleep hours and highly stressful lives may simply be unable to locate the point of ascending circadian sleepiness as this point may not exist! For a visual illustration of circadian and homeostatic components, see section Two-component sleep model in SuperMemo. For more on the two components of sleep see: Borbely model.
When good sleep might not come?
You may be surprised to find out that your internal circadian oscillation is based on a period that is closer to 25 hours than to 24 hours! To be exact, it varies between individuals, seasons, and other daily factors such as stress, timing of sleep, timing of the light period, intensity of light, exercise, and many more. Usually it falls into the range from 24.5 hours to 25.5 hours.
Most of us are able to entrain this 25 circadian rhythm into a 24-hour cycle by using factors that reset the oscillation. These factors include intense morning light, work, exercise, etc. German scientists have named these factors zeitgebers (i.e. factors that give time). As a result of the influence of zeitgebers, in a well-adjusted individual, the cycle can be set back by 30-60 minutes each day. However, the entrainment to the 24-hour cycle may come with difficulty to many individuals due to factors such as:
- blindness (i.e. the inability to use the main zeitgeber: light)
- short-sightedness (i.e. reduced sensitivity to light zeitgeber)
- increased demand for sleep (e.g. as a result of intense learning, highly creative job position, exercise, etc.)
- endocrine disorders
- sleep disorders
A great deal of sleep disorders can be explained by entrainment failure (i.e. the failure to reset the 25-hour circadian rhythm to the 24-hour daylight cycle). In other words, in the interdependence between sleep disorders and entrainment failure, the cause-effect relationship will often be reversed! Due to the physiological function of sleep, which is the rewiring of the neural networks of the brain, we can naturally expect that the demand for sleep be associated with the amount of learning on the preceding days. This link may also explain a decreased demand for sleep in retirement due to a decrease in intellectual activity. This age-related drop in the demand for sleep is less likely to be observed in highly active individuals. For similar reasons, the entrainment failure can often be found among students during exams. It is not clear how much of this failure can be attributed to stress, or to the desire to do more on a given day, or to the actual increase in the demand for sleep.
Formula for good sleep
Free sleep is good sleep
Free running sleep is defined by the abstinence from all forms of sleep control such as alarm clocks, sleeping pills, alcohol, caffeine, etc. Free running sleep is a sleep that comes naturally at the time when it is internally triggered by the combination of your homeostatic and circadian components. In other words, free running sleep occurs when you go to sleep only then when you are truly sleepy (independent of the relationship of this moment to the actual time of day). Night sleep on a free running schedule lasts as long as the body needs, and ends in natural awakening. No form of sleep disruption is allowed. In particular, any use of an alarm clock is the cardinal violation of the free running sleep principle.
The greatest shortcoming of free running sleep is that it will often result in cycles longer than 24 hours. This eliminates free running sleep from a wider use in society. However, if you would like to try free running sleep, you could hopefully do it on vacation. You may need a vacation that lasts longer than two weeks before you understand your circadian cycle. Even if you cannot afford free running sleep in non-vacation setting, trying it once will greatly increase your knowledge about natural sleep cycles and your own cycle in particular. You should also know that it is possible to entrain one's sleep to a desired sleep bracket (e.g. early rising). However, the entrainment requires iron self-discipline and the religious adherence to the entrainment rules.
Free running sleep
Free running sleep is sleep that is not artificially controlled to match our schedules and desires. It is a sleep without alarm clocks and sleeping pills. Mankind has practised free running sleep for as long as it existed. Our ancestors were gently encouraged to retire to bedtime at sunset, and would wake up naturally, probably after having spent no less than 8-10 hours in bed (see also Segmented sleep). All departures from that healthy practise were an imposition of culture, habit, religion, and/or tradition. Despite our ancestors' lives being fraught with danger, superstition, wars and disease, we should pause and ponder the marvellous impact of this naturally undisturbed sleep on their health. The arrival of fire and candlelight did not provide much incentive to stay up except for those few that have always had much to do in the evening: the first bookworms and artists. Only the genius of Edison and the like brought in the true sleep scourge: the electricity. With the wide dissemination of printed matter and electric lighting, millions would find their evening book far more interesting than sleep. Enter the web. In 2012 AD, we have an endless spectrum of entertainments and distractions that lure everyone away from bed and healthy slumber. More and more, we want to squeeze sleep into designer brackets. We wish to fall asleep at a specific time, and wake up at a specific time. Amazingly, a big chunk of the population does not realize that this is not possible without a detriment to health! Luckily, nearly everyone has the intuition that sleep is vital for healthy living. Those who would want to dispense with sleep altogether form a tiny minority. Nearly all creative people would wish to wake up fresh and ready for action. Preferably at a specified time. The same people wish to be less tired in the evening before sleep, and fall asleep instantly. Preferrably at a specified time. Let me then state it in bold print:
If we exclude unhealthy techniques:
- It is not possible to fall asleep whenever we wish.
- It is not possible to wake up whenever we wish.
- It is not possible to eliminate evening sleepiness.
However disappointing this might be, everyone would do better in life if those truths were assimilated. If we agree to wake up naturally at one's body's preferred time, it should be possible to be fresh and dandy from the waking moment. However, a decline in mental capacity over the waking day is inevitable. It is natural. Midday dip in alertness is also inevitable. And the optimum bedtime is hardly movable. If you try to advance it, you will likely experience insomnia. If you try to delay it, you will cut down on sleep and possibly wake up unrefreshed. If you try to wake up earlier than your natural hour, e.g. by employing an alarm clock, you will wake up with a degree of sleep deprivation that will affect the value of sleep for your learning and creativity. Don't be fooled by the illusive boost in alertness caused by the alarm clock. Yes. This happens to some people, some of the time. This perpetuates the myth that it is possible to wake up fresher with the ring of the alarm.
This simple formula is called free running sleep. For many people, after years of sleep abuse, even free running sleep can be tricky. It will take a while to discover one's own body's rules and to accept them. You will know that you execute your free running sleep correctly if it takes no more than 5 min. to fall asleep (without medication, alcohol or other intervention), and if you wake up pretty abruptly with the sense of refreshment. Being refreshed in the morning cannot be taken for granted. Even minor misalignment of sleep and the circadian phase will take the refreshed feeling away. After months or weeks of messy sleep, some circadian variables might be running in different cycles and free running sleep will not be an instant remedy. It may take some time to regulate it well enough to accomplish its goals. It cannot even be excluded that after years of shift-work or jetlag, some brain cells in the sleep control centers might have died out making it even harder to achieve well aligned refreshing sleep. In addition to all these caveats, stress is one of the major factors contributing to destroying the fabric of sleep. In free running sleep, stress will make you go to sleep later, take longer to fall asleep, and wake up faster, far less refreshed. Combating stress is one of the most important things in everyone's life for the sake of longevity and productivity.
Partners and spouses can free run their sleep in separate cycles, but they will often be surprised to find out that it is easier to synchronize with each other than with the rest of the world (esp. if they have similar interests and daily routines). If they are co-sleeping, one of the pair will usually get up slightly earlier and work as a strong zeitgeber for the other. The problem will appear only when the length of the naturally preferred sleep cycles differs substantially between the two. In such cases, instead of being a zeitgeber, the other person becomes a substitute for an alarm clock.
Even if you are not convinced, you should try free running sleep to better understand the concept of the sleep phase, and how the sleep phase is affected by various lifestyle factors. You will often notice that your supposed sleep disorder disappears! Note that the free running sleep period is not solely genetic. Various factors in the daily schedule are able to shorten or lengthen the period. Of the obvious ones, bright light in the morning or melatonin in the evening may shorten the cycle. Exciting activities in the evening will lengthen it. The period changes slightly with seasons. It will also change when you leave on vacation. It often gets shorter with age. Try free running sleep to understand your own sleep parameters. This will help you synchronize with the rest of the world, or at least get quality refreshing sleep. Please read more about free running sleep in this article. Throwing away the alarm clock is not a panacea. You may need to learn a bit about the hygiene of sleep.
Should we free run our sleep?
As it will be discussed later, free running sleep can be used to solve a number of sleep disorders except for those where there is an underlying organic disorder that disrupts natural sleep mechanisms. However, you will often hear two arguments against adopting the use of free running sleep:
- Argument 1 - free running sleep will often result in a day that is longer than 24 hours. This ultimately leads to sleeping in atypical hours. This seems to go against the natural 24-hour cycle of light and darkness. Less often, the cycle will be less than 24 hours
- Argument 2 - sleep can be compared to eating. Your body will always try to get more than it actually needs. This will result in spending more time in sleep than necessary. In other words, free running sleep is time-inefficient
Argument 1: Phase shifts
It is true that free running sleep will often run against the natural cycle of light and darkness. However, the departure from the natural rhythm is a direct consequence of using electric lighting and modern lifestyle. Our ancestors could expect little but darkness and boredom past sunset. Prolonged darkness and boredom are quite efficient in lulling humans to sleep. If we stubbornly refuse to use electric lighting beyond a certain hour, we will still find it difficult to run away from the excitements of modern lifestyle. To shut your brain to sleep efficiently in the early evening you would probably need to quit your current job and pick some uninspiring one, give up your intense family life, give up your hobbies and interests, give up the Internet, evening TV, etc. We live more stressful and more exciting lives than our grandparents. Turning the lights off in the early evening would probably only be wasteful. Additionally, shortsightedness, the ailment of the information age, makes us less sensitive to the light zeitgeber and artificially prolongs the circadian cycle. There are a number of downsides to free running sleep. The worst shortcoming is a difficulty in establishing an activity cycle that could be well synchronized with the rest of the world. Stabilization of the cycle is possible with self-discipline in adhering to cycle-reset rules such as morning exercise, bright light, sleep protective zone in the evening, etc.
Argument 2: Excessive sleeping
It is true that people who try to free run their sleep may find themselves sleeping outrageously long in the very beginning. This, however, will not last in a healthy individual as long sleep is a body's counter-reaction to various sleep deficits resulting from sleep deprivation. Unlike it is the case with foods, there does not seem to be any evolutionary advantage to getting extra sleep on days when we can afford to sleep longer. In the course of evolution, we have developed a tendency to overeat. This is a protection against periods when food is scarce. Adipose tissue works as a survival kit for bad times. However, considering the function of sleep, the demand for sleep should be somewhat proportional to the amount of new learning received on preceding days. In ancient times, we did not have exam days as opposed to lazy days. Consequently, the link between learning and demand for sleep is quite weak. The body clock will still make us sleep 7-8 hours on nights following the days of total inaction. Secondly, every extra minute of sleep might improve the quality of neural wiring in the brain. Sleep would better be compared to drinking rather than eating. We do not have much capacity to survive without drinking due to our poor water storage ability. Similarly, we cannot sleep in advance in preparation for a double all-nighter before an exam or important deadline. The claim that free running sleep increases the natural need for sleep is false! If you happen to sleep longer in free running sleep, it indicates that you were sleep deprived before running free. This longer sleep stage is transient. On occasion, if you go to sleep very early, you can also clock an excess number of sleeping hours. For more see Excessive sleeping.
In my view, everyone should always free run his or her sleep unless it makes it impossible to function in society along one's chosen profession, specialization, education, etc., or where it makes it impossible to take care of the young ones.
Free running sleep is stressless
Someone suggested that if any change is stressful, switching to free running sleep would be stressful too. The opposite is the case. Perhaps after an exclusion of the initial adjustment period in which people with lesser understanding of chronobiology make mistakes that may result in a decline in their sleep quality. Saying that any change is stressful is a generalization that goes too far. Changing your T-shirts daily does not imply stress. In addition, the degree of change is important. The same change can produce overstress or be a welcome factor in life depending on its degree. Letting your sleep free run does not imply any degree of stress, unless free running sleep itself produces changes in your schedule that might be stressful. If you eat your moderate meals frequently when you feel hungry, you are likely to experience less stressful change than when you eat them at pre-set lunch hours. Free running behaviors, by definition, free your organism to adapt behaviors to body's internal needs. As such, these can be considered anti-stress factors. It refers equally to sleep, eating habits, exercise, and other physiological needs
Free running sleep algorithm
- Start with a meticulous log in which you will record the hours in which you go to sleep and wake up in the morning. If you take a nap during the day, put it in the log as well (even if the nap takes as little as 1-3 minutes). The log will help you predict the optimum sleeping hours and improve the quality of sleep. Once your self-research phase is over, you will accumulate sufficient experience to need the log no longer; however, you will need it at the beginning to better understand your rhythms. You can use SleepChart to simplify the logging procedure and help you read your circadian preferences.
- Go to sleep only then when you are truly tired. You should be able to sense that your sleep latency is likely to be less than 5-10 minutes. If you do not feel confident you will fall asleep within 10-20 minutes, do not go to sleep! If this requires you to stay up until early in the morning, so be it!
- Be sure nothing disrupts your sleep! Do not use an alarm clock! If possible, sleep without a bed partner (at least in the self-research period). Keep yourself well isolated from sources of noise and from rapid changes in lighting.
- Avoid stress during the day, esp. in the evening hours. This is particularly important in the self-research period while you are still unsure how your optimum sleep patterns look. Stress hormones have a powerful impact on the timing of sleep. Stressful thoughts are also likely to keep you up at the time when you shall be falling asleep.
- After a couple of days, try to figure out the length of your circadian cycle. If you arrive at a number that is greater than 24 hours, your free running sleep will result in going to sleep later on each successive day. This will ultimately make you sleep during the day at times. This is why you may need a vacation to give free running sleep an honest test. Days longer than 24 hours are pretty normal, and you can stabilize your pattern with properly timed signals such as light and exercise. This can be very difficult if you are a DSPS type.
- Once you know how much time you spend awake on average, make a daily calculation of the expected hour at which you will go to sleep (I use the term expected bedtime and expected retirement hour to denote times of going to bed and times of falling asleep, which in free running sleep are almost the same). This calculation will help you predict the sleep onset. On some days you may feel sleepy before the expected bedtime. Do not fight sleepiness, go to sleep even if this falls 2-3 hours before your expected bedtime. Similarly, if you do not feel sleepy at the expected bedtime, stay up, keep busy and go to sleep later, even if this falls 2-4 hours after your expected bedtime.
Cardinal mistakes in free running sleep
- do not go to sleep before you are sleepy enough - this may result in falling asleep for 10-30 minutes, and then waking up for 2-4 hours. Ultimately you can experience an artificial shift forward in the entire cycle!
- unless for natural reasons (no sleepiness), do not go to sleep well after the expected bedtime. This will result in missing the period of maximum circadian sleepiness. Your sleep will be shorter and less refreshing. Your measurements will be less regular and you will find it harder to predict the optimum timing of sleep in following days
- do not take a nap later than 7-8 hours from waking. Late naps are likely to affect the expected bedtime and disrupt your cycle. If you feel sleepy in the evening, you will have to wait for the moment when you believe you will be able to sleep throughout the night
Sleep logging tips
In free running conditions, it should not be difficult to record the actual hours of sleep. In conditions of entrainment failure, you may find it hard to fall asleep, or wake up slowly "in stages". In free running sleep, you should be able to quickly arrive to the point when you fall asleep in less than 10 minutes and wake up immediately (i.e. without a period of sleep inertia). In other words, you can remember the hour you go to bed, add 5-10 minutes and record it as the hour you fell asleep. As soon as you open your eyes in the morning, you should record the waking hour. Usually you should not have any doubts if you have already awakened for good (as opposed to temporarily), and you will usually not fall asleep again (as it may be a frequent case in non-free running sleep). The graph below shows an exemplary free running sleep log in a graphic form:
An exemplary 5-month free running sleep cycle graph. In the picture, the average time of night sleep is 7 h 5 min, time before the midday nap is 7 h 48 min, the average nap takes 25 minutes and the time before the nap and the night sleep is 9 h 46 min. The whole cycle adds up to 25 hours and 4 minutes. Note that the distance between the nap and the night sleep in the graph is less than 9 h 46 minutes due to the fact that the blue retirement-line refers to the previous day sleep as compared with the red nap-line. Consequently, the nap-to-sleep band is horizontally shortened by 64 minutes, i.e. exactly as much as the daily phase shift in the cycle.
If you have collected your own free-running sleep data with SleepChart, I would be very grateful for your submissions that will be useful in further research (sending data from SleepChart takes just a single click).
Constellation of sleep habits
There are many different sleep habits in human population. Some of the differences have an important underlying biological cause. Scientists use the term chronotype to differentiate between sleep time and duration preferences that characterize different individuals. One person's chronotype might make him a short sleeper, another's chronotype will make him an owl. Yet another's chronotype will result in a diagnosis of a sleep phase disorder. Despite a seeming variety, a small set of underlying variables should make it rather easy for you to figure out your own basic chronotype. Your chronotype may determine your suitability for certain occupations. Luckily, you do not need to determine your chronotype before you choose your major or your job. Many people instinctively gravitate towards activities and professions that match their natural sleep habits. A physician or a fireman needs to tolerate shift work and interrupted sleep. Milkmen get up early, while gym or disco owners need to stay up late, while a writer may be of any chronotype as (s)he can adapt his or her writing hours to his/her sleep patterns. To illustrate individual sleep patterns I use a freeware application called SleepChart that you can download here to visually chart your own sleep (Wozniak et al. 2003). If you collect a few months of data, I would be very happy to receive your data file for analysis and future research. Sending SleepChart data requires a single click in the program.
Figure: Four examples of sleep logs that illustrate that modern human sleep patterns are as varied as snowflakes
The cycle of sleep and waking is regulated by the body clock. Body clock is located in the brain and is primarily based in the suprachiasmatic nucleus (see the chapter devoted to the SCN). The clock has a period of about 24 hours. During a single 24 hour day we have a period of 5-10 hours when we are very sleepy. This is the time when we normally sleep. During the remaining 14-19 hours we are usually awake or take a nap at siesta time. As mentioned earlier, only a small portion of the waking time is suitable for top-quality intellectual effort (see: Optimizing the timing of brainwork). The period of maximum alertness may last as little as 2-4 hours. We should plan our day in such a way so that sleep comes at the time of maximum sleepiness, while activities that demand maximum focus or creativity fall into the hours of maximum alertness. It is very difficult and usually very unhealthy to force the body and the body clock to change the timing of waking activities and sleep. It is far easier to do the opposite: adapt one's life to the natural cycle governed by the body clock. That adaptation will depend on the unique properties of one's own body clock. In the following sections I will try to show different types of sleep habits determined by the properties of the body clock that characterize a given individual.
Components of sleep in phase disorders
There are two main mechanisms that regulate sleepiness (see: Two components of sleep). One is the body clock, and the other is the "wake-meter". Body clock produces increased sleepiness every 24 hours. The wake-meter increases sleepiness with prolonged wakefulness (i.e. the longer we do not sleep, the sleepier we are). In sleep literature, these two mechanisms are called the circadian and homeostatic components of sleep propensity.
Sleep control components:
- circadian clock - circadian clock produces sleepiness in 24 hour cycles
- homeostatic control - wake-meter measures the period in which we stay awake and triggers sleepiness after we stay up for long enough
- circadian clock runs in periods far different from 24 hours. For example, in DSPS people, the circadian clock may be set to 25-26 hours.
- circadian clock is not sensitive to time resetting factors (termed zeitgebers). Normal people reset their clock in the morning by light and activity. In addition, darkness and inactivity in the evening provide further clues for the clock. Normal people with normal lifestyles can easily synchronize their sleep with the day-night cycle.
- Homeostatic wake-meter has an unusual time constant. Sensitive wake-meters will make people get tired very quickly after awakening. Insensitive wake-meter may make people tend to stay up for long. Caffeine abuse could contribute to a fast decline in alertness via adenosine receptor downregulation.
- Lifestyle has a dramatic effect on the behavior of the circadian and homeostatic sleep regulation mechanisms. The same individual will show a different sleep pattern depending on such factors as: using artificial lighting, exercise, level of stress, timing of exciting activities, napping, diet, climate, changes in ambient temperature, health status, etc.
Research shows that 15% of people would classify themselves as "morning type" or lark. Another 20% would call themselves "evening type" or owl. The remaining 65% are indifferent or "mid-range". What is your type? You can find many lark-or-owl tests on the net. However, I have not yet seen even one that would be well-designed to truly answer the question of your genetic predispositions. In particular, the same person on a work-week schedule may be classified as a different chronotype than when he or she is on a free running schedule.
Few people know that they can easily adapt to a completely different schedule by means of chronotherapy (e.g. by shifting their sleeping hours by 30-45 minutes per day). If you ask a typical owl to go to sleep 30-45 minutes later each day, the owl will keep shifting its bedtime to later hours. Initially, it will sleep during the day. That sleep will shift gradually to even later hours until the owl finds itself going to sleep in the very early evening just to get up before the larks! Surprisingly, even the most committed owl can then comfortably stick to the early waking hours for quite long! There is little natural preference as to the sleeping time of the day!
However, there is a factor that drives people into believing they are of a given sleep-time preference type. This is the length of the circadian cycle and their ability to entrain it to 24 hours. As mentioned earlier, typical circadian clock period lasts longer than 24 hours. Those people whose cycle is particularly long tend to go to sleep later each day. They push the limit of morning hours up to the point when their compulsory wake-up time results in unbearable sleepiness. In other words, people with long cycles will tend to work during the night and sleep in the morning as long as it is only possible.
A smaller proportion of people will experience short circadian periods and experience extreme sleepiness in the early evening. This is the lark type. Life forces larks to go to sleep slightly later than their natural preference (family, work, light, etc.). This keeps larks in line with time and they will often claim that the quiet of the morning, the singing of birds or the beauty of the sunrise keeps them getting up early. Yet it is still possible to forcibly push a lark to gradually shift sleeping hours and behave like an owl!
In a modern society, only a small fraction of people can boast a perfectly steady and regular natural sleep pattern. Not only are these the healthiest people around, they are also creatures of habit in reference to their sleep and waking rituals. They obey their rituals religiously, avoid alarm clocks, avoid evening entertainment, avoid medication that affects sleep, etc. Unlike those well-regulated individuals, owls shifted to a morning schedule will gradually tend to advance to their standard late-night rhythm. Similarly, larks will quickly shift back to getting up with the birds.
Some correlation studies showed that owls (as defined by the timing of melatonin release) exhibit slightly higher IQs than larks (Roberts and Kyllonen 1999).
Understanding the control mechanisms that produce sleep and wakefulness is extremely helpful in understanding sleep habits. It is particularly useful in individuals suffering from a number of sleep disorders, esp. insomnia and phase-shift disorders. Simple measurements of circadian variables and simple tools of chronotherapy may bring sound sleep to those who often struggled for years with insomnia, unsatisfying sleep, or sleep in wrong hours. Better understanding of chronobiology could also help extinguish dangerous practices such as poorly planned shift-work, disrespect for health consequences of the jet lag, cumulative sleep deprivation and the Internet fad of Uberman sleep.
To illustrate various sleep habits I use charts from a freeware program SleepChart. You can download SleepChart here and begin your own analyses today. All you need to do in the program is to click the beginning and the end of the sleep block in the graph. See the bottom of the SleepChart window for exact time corresponding with the position of the mouse pointer. If you set a wrong block, select it with a click and press Del.
Using SleepChart data, I will try to explain the main reason for which healthy people may not be getting refreshing sleep: sleep phase problems.
SleepChart attempts to approximate the circadian bathyphase that correlates with maximum sleepiness, low body temperature, low ACTH, high melatonin, etc. The underlying assumption is that when you log your sleep with SleepChart, you do not attempt to artificially play with the sleep hours. Each intervention in the sleep schedule makes the tools used in SleepChart work with lesser accuracy. Here are the most important interventions that should be avoided:
- waking up with an alarm clock,
- combating sleepiness in the evening (e.g. in order to delay sleep), and
- controlling sleep with substances (e.g. alcohol, sleeping pills, etc.).
On those rare occasions when you delay sleep or use an alarm clock, you can disqualify the sleep episode with the appropriate markings. However, all attempts to modify the sleep schedule will partly fool the algorithm and your reading will be inaccurate or plain wrong. It is also very important that you do not attempt to follow the circadian approximation when determining your optimum sleeping hours! You should always give priority to your natural body signals, i.e. sleepiness. Following SleepChart approximations can result in a positive feedback of error. In other words, errors in the graph may be amplified by your attempts to follow the graph. This can disrupt the sleep cycle. At worst, you could even self-diagnose yourself with DSPS without actually suffering from the disorder! Your only and sole "go to sleep" criterion should be rapidly increasing sleepiness. You may use the graph to approximate the moment in which the readiness for sleep will occur so that you could "cool down" in time. You can also find SleepChart helpful in chronotherapy for ASPS or DSPS to make it easier to schedule your appointments without conflicting with your natural sleep rhythm.
Courtesy of the numerous contributors who sent in their SleepChart data, we can draw a number of interesting conclusions. The most compelling one is probably the confirmation of the hypothesis that we might be facing an epidemic of Delayed Sleep Phase Syndrome (DSPS) in younger generations, esp. among students and people employed in high-tech jobs. The epidemic is a result of an ever-growing discrepancy between the environment in which humans and their primate ancestors evolved over the last several million years, and the environment in which we live today with electric lighting, Internet, computers, TV, rat race, and 24-hour society. The increasing gap between lifestyles and biology leads many to seek radical solutions and take on drastic measures. A quick survey of those who attempted to adapt to an Uberman sleep schedule reveals an interesting truth. Although the idea to squeeze in more waking hours into a day is very appealing, most of the "experimenters" began their interest in polyphasic sleep as a result of troubles with achieving refreshing sleep!
Some people reacted with skepticism to the concept of using SleepChart as a sleeping prop: "it is just far too complicated and Ockham's razor needs to do a bit of shaving! Sleep is as natural as breathing air or drinking water and if you have to set up complicated charts and experiments, and utterly eccentric sleep-activity patterns just so as to get some decent shut-eye, then you must have a problem - but one more of a psychological than a physiological nature". It is true that sleep will occur naturally in a natural setting. The trouble begins when we interfere with nature using caffeine, alcohol, nicotine, artificial lighting, 24/7 society, night-time entertainment, etc. SleepChart may seem complex, but it might still be the easiest way to predict the optimum timing of sleep in free-running conditions for people who may have problems with sleeping. SleepChart will only ask you when you go to sleep and when you wake up (naturally). All computational complexity is hidden in the background. The approximation procedure needs no further input from the user and it predicts the circadian bathyphase as well as the optimum bedtime. SleepChart can even disentangle homeostatic and circadian components of sleep. Understanding these can also be helpful in planning healthy sleep.
I agree that the need to resort to tools such as SleepChart is a sign of troubled times. However, SleepChart has a proven record of helping people understand their seemingly irregular sleep patterns and organizing their sleep. Falling asleep might be natural, but there are many factors that mask sleepiness or magnify it. For people on very irregular sleep schedules this can pose an insurmountable obstacle!
People with sleep problems are often little understood by the naturals: "If you work solidly 8 hours a day, have 3 decent meals, have a proper family life, and treat other people as human beings, then in the evening you go to bed happily knocked out and wake up next morning happily refreshed. Surely this is as it always has been for most people throughout history and surely this is how it will always remain". This attitude towards sleep problems is not much different from telling a clinically depressed person: "Pull yourself together", or expect a heroin addict to go cold turkey and instantly return to normal life. A tortured insomniac will only get more upset with himself or herself if (s)he is told that sleepless nights come from "unsolid work", "indecent meals", "improper family life" or treating others "inhumanely". The trouble stems from the clash of biology with modern lifestyle. With the arrival of artificial lighting sleep disorder statistics skyrocketed. These were only made worse by television, computer games and the Internet. With the advent of mobile telephony and instant messaging, insomnia and sleep phase disorders seem to reach epidemic proportions. Fewer people are able to leave work behind, cope with stress, or give up evening activities. Without a major change in lifestyle or a breakthrough in circadian control methods, people affected with lifestyle-related sleep disorders are faced with a choice between a daily sleep deprivation misery and radical solutions such as throwing away the alarm clock. Certainly, we can expect science to come up with answers to the problem. Until that happens though, waking up "happily refreshed" remains a privilege of a shrinking subset of the population in industrialized nations.
SleepChart in SuperMemo
To make it possible to analyze the connection between sleep and learning, SleepChart has been integrated with SuperMemo speed-learning software. Instead of explaining SleepChart itself, I will shortly describe its functionality in SuperMemo. Keep in mind that some of the functions related to memory are not included in the freeware version due to the fact that it does not have access to your learning data.
SleepChart was included in SuperMemo a few years ago upon the understanding that sleep is vital for learning. To sleep well and to learn well, one needs to understand his or her own circadian rhythm. SleepChart in SuperMemo was designed with the view to assisting in that task. It can help you optimize the timing of sleep as well as to optimize the timing of your learning. Moreover, you can submit your sleep and learning data for analysis and have your own contribution in our research over the impact of sleep on memory. You can access SleepChart in SuperMemo with: (1) Tools : Sleep Chart on the Main menu, (2) SuperMemo commander, or (3) by just pressing F12.
Sleep blocks are marked in blue. Learning blocks are marked in red. Total learning time on individual days is displayed on the right. Selected sleep block is displayed in yellow. The length of that block is displayed at the bottom.
In SuperMemo, the learning timeline is generated automatically. Each time your make repetitions with SuperMemo, the learning block is added to the timeline (displayed in red on the graph). On the other hand, your sleep data must be logged in manually (displayed in blue). At minimum skill level, you can use SleepChart for a basic visual inspection of your favorite learning and sleep hours. However, more advanced functions such as optimizing the time for learning or the time for sleep require advanced analysis and understanding of circadian rhythms. If you start logging your sleep data today, you will be able to use future, more advanced versions of SuperMemo to study and understand your sleep and learning.
The timeline of sleep in SleepChart must be logged manually. To log a block of sleep, click the beginning of the block (sleep start) and then click the end of the block (sleep end). You can also start from clicking the end of sleep first. Sleep blocks above 22 hours are disallowed. Sleep blocks cannot overlap with repetitions timeline (you cannot learn with SuperMemo and be asleep at the same time). If you have already collected your sleep data in SleepChart Freeware, you can import this data to SuperMemo with File : Import : SleepChart file (you can also import data from a spreadsheet). If you import files from SleepChart Freeware, you can test for sleep and learning overlaps with File : Verify : Block overlaps. Protection from block overlaps is an important advantage of using SleepChart in SuperMemo as opposed to a standalone SleepChart, in which it was very easy to fall out of phase in logging data (e.g. by failing to fill out a single day and noticing that only a month later). You can mark blocks of forcefully delayed sleep, as well as mark blocks cut short with an alarm clock or other factors. Please note that you can get best analytical results if you do not artificially regulate sleep (e.g. with an alarm clock, sleeping pills, etc.). Applied models will not fully account for artificial intervention. Last but not least, natural sleep is what you should aim for in learning as well as for the sake of maximum health and well-being.
Sleep and learning timeline
Combining sleep timeline with repetition data taken from SuperMemo opens an array of new research and optimization options.
Various sleep statistics pertaining to individual days can be displayed on the right. Sleep blocks can be consolidated with the Consolidate button on the toolbar. For example, if you woke up for 5-10 min. in the night, consolidation will make SuperMemo treat the entire night sleep episode as a single entity. Short nocturnal awakenings are a norm, even if we are not aware of them, and have little impact on learning. Sleep block consolidation often unmasks important properties of sleep (e.g. see Preference for night sleep). It helps treat successive sleep episodes as an expression of a single period of high sleep propensity.
In addition to sleep statistics, optimum bedtime can also be estimated in SleepChart. Two independent models are used to predict middle-of-the-night points as well as the expected optimum retirement and awakening times. Those approximations may be helpful in optimizing sleep in people who work shifts or sleep in irregular hours for various reasons. For example, after a week of irregular sleep, it may be difficult to determine the optimum retirement hour that is likely to produce best quality sleep. Going to sleep too early might result in premature awakening (which may often ruin the night sleep entirely). Going to sleep too late may result in short night sleep, sleep deprivation, and reduced alertness on the following day. Predicting optimum sleep time on the basis of sleep history is inexact science, and the two models used may produce different outcomes. Important! Your natural instinct should always take precedence over mathematical models. Moreover, best results in sleep optimization are accomplished in free-running sleep. If you use an alarm clock, or force yourself awake through the night, or take sleeping pills, the models may not adequately account for the chaotic change that is occurring in your sleep control systems.
Blue and red continuous lines are predictions of optimum sleep time using the SleepChart model (based on sleep statistics). Yellow continuous line shows the prediction of the maximum of circadian sleepiness (circadian middle-of-the-night peak) using a phase response curve model. Note that theoretically, yellow line should roughly fall into the middle between blue and red lines. However, when a disruption of the sleep pattern is severe, those lines might diverge testifying to the fact that it is very hard to build models that fully match the chaotic behavior of the sleep control system subjected to a major perturbation. point to the predicted daytime dip in alertness (i.e. the time when a nap might be most productive).
The circadian graph in SleepChart can help you better understand your sleep patterns, as well as to visualize the degree of cycle instability (i.e. how difficult it is for your sleep-wake cycle to fit into 24 hours). You will need a few months of data before the graph becomes meaningful. In addition, subjective night approximation lines in the sleep log are subject to substantial hysteresis. If your lifestyle changes dramatically (e.g. as a result of a therapy), you may need a few weeks for the approximation lines to align properly with data. The circadian graph may then be more difficult to interpret. In such cases, you can use From the first day and To the last day options to demarcate the period of interest. This will limit the analysis to a selected period characterized by a selected lifestyle.
Blue line shows the preferred time to fall asleep. It corresponds with sleep propensity derived from the number of sleep blocks falling into a given hour of the waking day, where zero on the horizontal axis refers to the hour of waking up. Percentage of sleep episodes initiated at any given time is displayed on the right vertical axis. The blue line roughly expresses your "tiredness of wakefulness". It also expresses your ability to fall asleep. Your own optimum bedtime hour is your personal characteristic as it differs between people. For most people the optimum bedtime falls into the range of 16-20 hours from waking. In the example, the most favored bed time occurs in the 18th hour of waking.
Red line shows the average length of sleep. This line is a rough reflection of the ability to maintain sleep, i.e. the longest sleep episodes occur during the subjective night. The average length of sleep is displayed on the left vertical axis. The graph will tell you that even if you are able to initiate sleep during the day, it will never last long. In most cases of regular sleepers, only after 11-14 hours of waking does the length of initiated sleep start increasing. Note that the sleep length graph is slightly phase shifted in reference to the preferred sleep initiation time due to the fact that long sleep is mostly achieved by initiating sleep early.
If you are trying to determine your optimum bedtime, find the evening peak in the blue curve and choose nearby points that produce sufficiently long sleep (red curve high enough). In addition, pay attention to the fact that your wake and sleep time should add up to 24 hours, otherwise you will experience phase shifts.
Some people take naps during the day. Others don't. In nappers, the blue curve should also point to the maximum mid-day alertness dip. Short nap time may actually be a sign of good nap timing as long as the nap is not taken too early in reference to the blue curve (see: Best nap timing). Non-nappers will also experience a peak of sleepiness around the 7th hour even though their blue curve will not show as a prominent bulge.
If the graph shows that your optimum nap time falls into the 8th hour, and you wake up at 6 am, you should take a break at around 14:00 (2 pm) and look for a secluded place for a few minutes of rest. You could also plan your lunch at around 13:00-13:30 to complete a perfect setting for a siesta.
Example 1: Unstable circadian cycle
In the exemplary circadian graph below, on average, the best night time sleep is obtained when it is initiated after 18 hours from the morning awakening (assuming the graph was created without any artificial form of sleep control such as an alarm clock, delaying sleep, etc.). The blue line shows that the 18th hour is the preferred time to initiate sleep, while the length of sleep (red line) is long enough to add up to a 24 hour sleep-wake cycle.
As blue peaks are of the same height, we can conclude that the graph represents a religious napper, whose optimum siesta time occurs 7 hours from awakening. In this case, for an awakening at 8 am, the siesta should begin at 3 pm, and the night sleep around 2 am. For both blue peaks, 7.4% of all sleep episodes being at the optimum hour, while the remaining 85% are suboptimum.
Maximum length of sleep can be achieved at the 16th hour, however, this does not indicate this is the optimum hour of going to sleep. If sleep is initiated too early, it may or may not catch on the full circadian low of the subjective night. In other words, there is a risk of a premature awakening after just a couple of minutes of sleep. Such an awakening makes it harder to fall asleep again. This is one of chief causes of insomnia. The difficulty in re-initiating sleep is due to a very rapid loss of homeostatic sleep propensity during sleep. In addition, sleep initiated before the full circadian low does not seem to be of more value than slightly shorter sleep initiated a bit later (e.g. as reflected by the subjective feeling of being refreshed in the morning, or as measured polysomnographically). The blue homeostatic line indicates that the sleep is more likely to be initiated effectively at the 18th hour, while its average length is then 6 hours. If your graph is generated without attempts to artificially regulate sleep, the second peak in the homeostatic curve will often indicate the optimum bedtime. The graph also indicates that if the sleep is delayed by an hour, it will be shortened by 10-30 minutes. It is possible, that even this little shortening will affect the performance during the day. If the sleep is advanced by an hour, it may be 10-30 minutes longer but its quality is not likely to increase proportionally.
The graph can also show how the length of the circadian period can be determined by the bedtime hour. The green line shows the set of breakeven points for a stable 24 hours sleep-wake cycle where the sleep and wake times add up to 24 hours. All the circadian graph points that lie to the right of the green line cause a phase delay, while points on the other side will cause a phase advance. Aqua blue line shows where the 24-hour-cycle green line crosses the red sleep length line. Due to the fact that the angle between green and red lines is large, this sleep pattern is pretty unstable. This means that going to sleep before the 18th hour will result in a cycle that is less than 24 hours long, while going to sleep after the 18th hour may lengthen the cycle and result in phase shift delays. For example, early bedtime (around the 15th hour) will result in a day that lasts 21 hours (15 hours on the horizontal axis corresponds with the average sleep length of 6 hours read from the red curve). Later bedtime (around the 18th hour) will result in a perfect 24 hours day, while a very long waking day (e.g. 20 hours) will produce a day lasting 25.5 hours. Naturally, all manipulations in the length of the day would better be avoided as early bedtime increases the chances of insomnia, while a very late bedtime increases the chances of sleep deprivation, and REM sleep deficit. Understanding one's sleep preferences can be very helpful for planning shift-work or combating jetlag in long-haul flights.
Example 2: Stable circadian cycle
The second graph shows a sleep pattern that is much more stable that the one from the first example above.
The graph shows a habitual napper who shows a preference for a waking day of 19 hours. As opposed to the graph shown earlier, the zone of stable sleep-wake cycle, demarcated by vertical aqua lines is much wider due to the fact that red and green lines are nearly parallel. This means that if the sleep is initiated after the 20th hour of waking, the night sleep will be shortened to fit the 24h cycle. Naturally, even if delayed sleep does not cause a phase shift, it will always result in lesser sleep quality due to stage compression. Such sleep will result in sleep deficits. Days lasting less than 20 hours may result in a phase advance. Despite running free, the longest average sleep period (initiated at around the 16th hour) isn't even 6 hours long. This illustrates that excessive sleeping is not a problem in free running sleep. In the graph, the optimum siesta time again falls in the 7th hour and is executed religiously (over 14% of sleep episodes executed "on the dot").
Phase shift disorders
As shown in both graphs above, with sufficient discipline, people with phase disorders should be able to accomplish 24 hour free running rhythm independent of the desired waking hour. In practice, due to various perturbations in lifestyle (exams, stress, socializing, etc.) as well as due to the stress of the need to wake up early, adherence to the optimum 24h sleep schedule may be very hard to achieve for people with severe phase shift problems. For those who need to wake up at a specific early hour, free running sleep may become unobtainable without the use of an alarm clock, melatonin, or other unwelcome measures.
24-hour sleep cycle
Perfect 24-hour cycle
Let us now consider an ideally synchronized 24-hour cycle. In the picture below, an octogenarian female wakes up naturally everyday around 3:00-3:30 am. She sleeps 5.4-5.5 hours per day, wakes up refreshed and is active throughout the day.
There is no synchronization with daylight as the waking hour falls into the period of darkness. The cycle is synchronized by evening activities, not daylight. The subject keeps in her mind a "must go to sleep" hour estimation that helps synchronize body clock with the time of day. This "psychological imprint" is illustrated by a smooth change in the sleeping rhythm after the end of the daylight saving time on Sunday October 27, 2002 (the graph disregards DST so that the waking hour before the change is set at 2:00-2:30 am).
Even though aging is said to increase nocturnal awakening, perhaps due to the cell loss in sleep control centers, this subject reported no awakening in the study period.
Circadian graph shows a single favored bedtime in the 19th waking hour. As the average nighttime sleep episode is 5 hours long, the sleep-wake cycle lasts exactly 24 hours, and daily fluctuations in bedtime are minimal. As the green breakeven line and the red circadian line are nearly parallel in the span of 3 hours, this sleep pattern is very stable, and all delays in bedtime occur at the cost of sleep time without causing a phase delay.
Sleep and stress
Stress can ruin the fabric of sleep. The following SleepChart graph demonstrates the impact of stress on a well-balanced 24 hour sleep pattern:
In the presented example, a middle-aged self-employed male wakes up naturally everyday around 6:00-6:20. However, on Jun 3, 2003, a severe family problem threw the rhythm into chaos as evidenced by frequent nocturnal awakenings. The rhythm returned to the norm one month later as soon as the family conflict was resolved.
Monophasic sleep graphs will often show a small siesta-time sleep propensity peak due to the fact that even the purest monophasic sleeper hits crisis days in which a postprandial nap brings a welcome relief. Due to their "crisis nature", such naps may last longer than in a habitual napper. The mid-day peak is particularly visible in irregular sleepers who show less discipline in sheltering their natural regular sleeping hours.
Preference for night sleep
Independent of the innate circadian cycle, light has a powerful impact on sleep. In particular, its phase-shifting capacity will always ensure that humans naturally gravitate towards sleeping at nighttime. Only the advent of lifestyle that involves electricity and 24h work cycles triggered the present epidemic of sleep disorders, which indirectly contributed to the appeal of concepts like "Uberman sleep".
The preference for sleeping in the night can best be seen in irregular sleepers, esp. those who suffer from phase shift disorders and run their sleep free, or those who are on a free running schedule and phase shift "by choice" (i.e. by not trying to fit any particular sleeping hours). In those cases, using the circadian graph in SleepChart, we can see the impact of nighttime on the ability to initiate and maintain sleep.
In the presented circadian graph, we see a clear preference for night sleep in free running sleep. The graph shows that sleep initiation (blue line) is easier at nighttime between 7 pm and 4 am, while the length of sleep (red line) is greatest if the sleep is initiated between 10 pm and 5 am.
The graph can also be interpreted as a phase space. It shows how difficult it is to achieve "wasteful" 8 hours of sleep in an efficient free running sleep pattern. It can also be used to demonstrate that no trajectory in the phase space will lead to an entrained polyphasic sleep. When alarm clock and/or sleep delay are introduced into the system, sleep control may become chaotic. However, in free running mode, it quickly stabilizes around a roughly biphasic rhythm, often with a degree of phase-shift dependent on the lifestyle. The timing of phase-shifting, excitatory and inhibitory stimuli, even if they are repetitive and regular, may still lead to a degree of chaos in the system. This occurs if the period of the stimulus cycle is different from the period of the entrained circadian rhythm.
In contrast to the first graph, the second example can be used to argue that artificial lighting can virtually eliminate the impact of natural light on the cycle in a well-disciplined sleeper with a more regular cycle and better adherence to free running sleep rules.
The question remains open to whether the nighttime sleep preference isn't to a large degree caused by social entrainment. Despite the fact that we live in a 24/7 society, there is still more fun to be had during the day or in the evening than during the night when still the larger portion of the population is asleep. A big clue comes from the fact that despite little difference in sleep initiation preference throughout the day, sleep initiated in the evening or in the night (8 pm - 6 am) is still likely to last up to twice as long as sleep initiated at 3 pm.
In this chapter, I will show why biphasic sleep is the best sleep pattern for high productivity and for the brain health.
Temperature changes in the course of the day in degrees centigrade (courtesy of: Dr Luiz Menna-Barreto, State University of Campinas, Brazil)
SuperMemo and SleepChart provide an excellent tool to verify the claim of the biphasic nature of human sleep-wake cycles. I have collected data from monophasic and biphasic sleepers that illustrate our biphasic nature.
SuperMemo alone makes it possible to see the biphasic character of the learning performance throughout the day by charting grades over time without the need to include sleep log data. In the presented example, a monophasic sleeper, a busy father of two, shows the best learning performance in the early morning around 6 am, i.e. shortly after his natural waking time. There is a big dip in average grade scores from 11 am to 1 pm. There is a second surge in the quality of learning at around 5-6 pm:
Biphasic sleep periodogram
SleepChart alone can also be used to demonstrate sleep biphasicity. Free running sleep logs can be subject to Fourier analysis to reveal the nature of sleep periodicity. An exemplary periodogram is shown in the graph:
Exemplary periodogram of human free running sleep reveals a biphasic nature of sleep periodicity. Two basic sleep frequencies dominate this particular sleep log. These roughly correspond to 12 and 24 hour cycles.
Biphasic learning and sleep
If we employ both SleepChart and SuperMemo, we can see how waking performance changes in reference to sleep phase. The biphasic grades graph from SuperMemo (as shown earlier) can be corrected for the circadian phase that can be pretty independent of the actual clock time, esp. in free running sleep. In the presented example, a biphasic sleeper, middle-aged male with irregular sleep patterns, shows the best learning performance in the early morning (roughly around the estimated end of the subjective night):
There is a big dip in the average grade scored some 7 hours from the morning peak. There is a second surge in the quality of learning in the evening. Finally, there is a steep decline in the quality of learning shortly before sleep.
SuperMemo for Windows makes it possible to correlate recall with the circadian phase as estimated by SleepChart, which has been integrated with the program. In the presented example, a biphasic 45-year-old male shows two major peaks in alertness and learning quality during the day:
The first peak occurs in the hours 3-4 from the estimated natural waking time, i.e. not the actual waking time, which may be different. The second, slightly longer peek spans hours 12-18. There is a pronounced depression in free recall at the 8th hour of the subjective day period (i.e. wake time estimated from the circadian data, not the actual waking period). The red line shows the estimated overall alertness derived from SleepChart's two component model. In this case, the estimated alertness nearly perfectly matches the recall measured during an actual learning process.
Monophasic sleep with biphasic learning
The height of the two alertness peaks may differ in a monophasic sleeper, who will also show the same depression in recall around the 8th hour of the subjective waking day. However, characteristically, a monophasic sleeper may not get the same performance boost in the evening as biphasic sleepers due to the effects of the homeostatic sleep drive component. Even a few minute nap can result in a major boost in alertness. This has already been noticed by a prominent napping expert Dr David Dinges in his comprehensive surveys comparing habitual nappers with non-nappers (Dinges 1989).
To illustrate the difference between biphasic and monophasic sleepers, see an analogous recall graph in which a monophasic 15-year-old non-napper shows the best performance in the morning hours with a sharp dip at the 8th hour of wakefulness coinciding with a subjective decline in cognitive function:
After a temporary dip, there is a sharp recovery, and a gradual decline in performance in the second half of the day. That decline is strongly accelerated by a homeostatic mechanism. The yellow line shows the estimated circadian component of alertness. In this case, the circadian benefits are muffled by the homeostatic decline in alertness, which is not shown in the graph. This is why the hypothetical circadian alertness and the actual alertness match only in the first half of the day.
Biphasic circadian graph
There is a biphasic twist to the two-process model of sleep regulation. In free running sleep, where sleep is a true expression of sleep propensity, it is possible to visualize both the homeostatic and the circadian components of sleep in a circadian graph:
In a habitual napper, the circadian biphasic nature is paradoxically expressed by the two-peak sleep propensity curve instead of the circadian curve. The reason for this role reversal is the physiological difference between the two circadian peaks in sleep propensity. In a habitual napper, sleep is initiated as easily at siesta time as it is initiated at night. However, the length of sleep at siesta time is very short (usually 15-80 min).
In the presented graph, the blue line corresponds with the ability to initiate sleep at any given circadian time. On the horizontal axis, it aligns well with the alertness graphs displayed in SuperMemo (as shown in earlier paragraphs). It aligns well with both the learning data, as well as with the two-process sleep model implemented in SleepChart.
The red line corresponds with the ability to maintain sleep. It reveals what is not visible in the alertness graph shown earlier: siesta naps cannot last long and will always be subject to an early natural termination (low red line under the first blue peak). In contrast, the period of subjective night is the only time of day when sleep can and should last longest (usually no less than 4-5 hours). The red peak is also the reason why polyphasic sleep adepts crave for "core sleep", wake up groggy, and need heavy alarm artillery to wake up in this critical subjective night period.
David Dinges, in his surveys noticed, that napping more than once within a day was extremely rare. Most nappers took naps lasting 15-120 min. Naps will be shorter if they are taken before the siesta peak. If they are taken after the peak, they will usually last longer, and may even integrate with the night sleep in cases of particularly large delay, or where there is a sleep deprivation, REM-sleep deficit, or any other form of "sleep debt".
Dinges noticed that both appetitive (habitual) and replacement (compensatory) nappers tended to time their naps 7-8 hours from waking (see: Best timing of naps). Even though napping habits may differ, the circadian timing of the siesta trough seems to be pretty similar across the population (Dinges 1992)
It is important to note again that the evening boost in alertness is magnified by a nap, but shows up also in non-nappers and can easily be deconvoluted in the two-processes model into its homeostatic and circadian components (as shown in the next two examples).
Two components of biphasic sleep propensity
The last two graphs show the impact of the circadian and homeostatic components on alertness.
In the first example, a free running female 29-year-old non-napper shows an alertness dip in 8-9 hours since waking. The red homeostatic estimate shows no dip and a steady decline over the waking day:
The yellow circadian estimate shows the expected position of the dip and the evening crest that explains a boost in the evening learning performance:
Both the evening recall boost and the evening circadian estimate align pretty well showing once again that the overall alertness depends on both homeostatic and circadian components of the sleep control system.
Biphasic performance in sleep deprivation
Mid-day slump is as prominent in conditions of severe as well as mild sleep deprivation. This graph shows a mid-day alertness slump in a 26 hour sleep deprivation study (Czeisler et al. 2006). The timing of the slump (hours 10-12 of waking period) indicates that the preceding sleep episode was positioned suboptimally (hence the need to interrupt sleep for the study). Natural awaking would probably take place 1-2 hours after a forced awakening in lab conditions. The graph also shows that sleep inertia caused by forced awakening from Stage 2 NREM or REM sleep causes a much greater cognitive decline than 26 hours of sleep deprivation.
Summary: Napping is good!
From the above charts we can conclude that human circadian pattern is definitely biphasic. Here are the implications:
- Humans are biphasic in nature and show a circadian boost in learning in subjective evening hours
- Non-nappers show a mid-day dip in performance and might also benefit from a siesta
- In free running sleep, healthy humans tend to fall into biphasic sleep pattern
- Siesta is a well-tried old practice in many cultures around the world. It is a healthy practice
Optimum time for brainwork
Optimizing the timing of brainwork
Circadian graph and brainwork
Figure: Optimizing the timing of brainwork with respect to the circadian cycle. This exemplary graph was generated with the help of SleepChart on the basis of 3-year-long daily measurements of a free-running sleep rhythm. The horizontal axis expresses the number of hours from awakening (note that the free running rhythm period is often longer than 24 hours). are actual sleep episode measurements with timing on the horizontal, and the length on the left vertical axis. Homeostatic sleepiness can roughly be expressed as the ability to initiate sleep. Percent of the initiated sleep episodes is painted as a thick blue line (right-side calibrations of the vertical axis). Adenosine-related homeostatic sleep propensity increases in proportion to mental effort and can be partially cleared by caffeine, stress, etc. Circadian sleepiness can roughly be expressed as the ability to maintain sleep. Average length of initiated sleep episodes is painted as a thick red line (left-side calibrations of the vertical axis). Mid-day slump in alertness is also circadian, but is biologically different and results in short sleep that does not register as red sleep maintenance peak. Sleep maintenance circadian component correlates (1) negatively with temperature, ACTH, cortisol, catecholamines, and (2) positively with: melatonin and REM sleep propensity. Optimum timing of brainwork requires both low homeostatic and circadian sleepiness. There are two quality alertness blocks during the day: first after the awakening and second after the siesta. Both are marked yellow in the graph. For best learning and best creative results use these yellow blocks. Caffeine can only be used to enhance alertness early in this optimum window (brown). Later use will affect sleep (caffeine half-life is about six hours). Optimum timing of exercise is not marked as it may vary depending on the optimum timing of zeitgebers (e.g. early morning for DSPS people and evening for ASPS people). For more details see: Biphasic nature of human sleep.
Best brainwork time
Optimum timing of brainwork requires both low homeostatic sleepiness and low circadian sleepiness. There are two quality alertness blocks during the day: first after the awakening and second after the siesta period. Both are marked as yellow blocks in the graph (above). For best learning and best creative results use these yellow blocks for brainwork. Caffeine can only be used to enhance alertness early in this optimum window. Later use will affect sleep (caffeine half-life is about six hours). Optimum timing of exercise may vary depending on your exercise goals and the optimum timing of zeitgebers (e.g. early morning for DSPS people and evening for ASPS people). In this example, the stress block is followed by the exercise block to counterbalance the hormonal and neural effects of stress before the siesta. Unmarked white areas can be used for the lunch (before siesta) and fun time unrelated to work in the evening at a time when the ascending circadian sleepiness makes creative work ineffective. That white evening protective zone should be free from stress, alcohol, caffeine, etc. Recommended activities might include fun, games, relaxation, TV, reading, family, DIY, housework, etc. For inveterate workaholics, less challenging and stress-free jobs might also work ok. The best litmus test for a well designed day is that all activities should be fun! Brainwork is fun only if your brain is ready. Sleep is fun if you are ready. Rest and entertainment feel in place only after a productive day. Even a bit of stress can be fun if it is properly dosed and timed. You do not need to be an adrenaline junkie to enjoy your stress and exercise slots. There is little exaggeration in saying that a good understanding of the circadian cycle is the key to a happy and productive day!
For more see: Natural creativity cycle
Balanced 24 hour cycle
The slanting green line separates the graph into the areas of phase advanced (right) and phase delays (left). The line is determined by points in the graph where the waking time (horizontal axis) added to the sleep time (left vertical axis) equals to 24.0 hours. The place where the green breakeven line crosses the red sleep length line determines the optimum balanced sleep cycle of 24 hours. In the presented example, 17.35 hours of waking, added to the expected 6.65 hours of sleep time complete a balanced full 24 hours sleep-wake cycle. The greater the angle between the green and red lines, the harder it is to balance sleep and fit it into the 24h cycle of the rotating earth. In the example, adding waking hours does not shorten sleep much enough to make the balance easy. This implies that a religious adherence to a 17.35 day may be necessary to balance the cycle. However, this shortened waking day may increase sleep latency and increase the probability of premature awakening, which can also tip the balance towards the phase delay. The vertical aqua line shows where the expected sleep time added to the waking time equals to 24 hours (crossover with the green line representing a perfect 24-hour day). In DSPS or ASPS that 24h balance may be hard to accomplish. For example, without medical intervention, only a large protective zone in the evening, early nap (or no nap), and intense morning exercise can help balance the day in DSPS.
Important! This graph is based on data that is true solely for a free running sleep condition. If you use an alarm clock to regulate the timing of your sleep, this measurements and recommendations may not apply! In addition, timing and the amplitude of changes differ vastly between individuals!
Sleeping against your natural rhythm
If you sleep against your natural rhythm you will often experience tiredness or drowsiness that can be resolved by adjusting the sleeping hours. In healthy individuals, the daytime alertness is primarily determined by:
- circadian phase and homeostatic sleepiness
- total sleep time the night before
- amount of slow-wave sleep the night before
- regular adherence to the sleep-wake schedule in preceding days
- sleep deficits accumulated in the preceding days (e.g. REM deficit, SWA deficit, etc.)
All those factors are closely associated with the sleep phase. Free running sleep provides the best way to maximize the alertness throughout a waking day. Free running sleep is likely to shift the minimum temperature point from the early morning closer towards the middle of the subjective night. You should notice increased sleepiness before going to sleep and no sleep inertia upon awakening! If you cannot free-run your sleep, it is very important to understand the relationship between your homeostatic and circadian sleep drives as compiled in the table below. In the course of the day, you should move in sync between the yellow areas of the table, i.e. from perfect alertness to maximum sleepiness, and then back to perfect alertness. The gray areas illustrate when your sleep falls out of sync:
|High circadian sleepiness||Low circadian sleepiness|
|High homeostatic sleepiness||Peak of the night: You are very drowsy and fall into refreshing sleep with latency of less than five minutes||Insomnia: You are tossing and turning in bed. You are very tired but you cannot fall asleep. Your temperature, blood pressure and pulse are raised. Your thoughts are racing
Solution: Wait for the arrival of the circadian phase. Delay going to sleep by 3-6 hours
|Low homeostatic sleepiness||Hypersomnia: You are drowsy throughout the day despite long sleep hours. Napping does not help. You show minimum energy levels. Your muscles are weak and atonic
Solution: Adjust your sleep phase to your circadian (e.g. try to go to sleep 3-6 hours later)
|Peak of the day: You are alert, energetic, and full of new ideas|
Kill the alarm clock!
Alarm clock epidemic
Few upwardly mobile people in the modern rat-race society can live without an alarm clock. With a shot of strong coffee and round-the-clock stress, most people learn to live and survive with an alarm clock. Half of the population wakes up with an alarm, 9% are woken by a partner, 4% by pets, 3% by children, etc. That leaves a minority that wake up naturally. Increasingly, time becomes the most precious commodity in society where achievement is often associated with speed and perfect time-management. However, alarm clocks introduce harmful side effects: stress, sleep debt, and worst of all, disruption of the natural physiological sleep function. At worst, those factors will result in physical damage to the brain (e.g. such sensitive structures as the hippocampus, your memory switchboard, may literally lose neurons as a result of disrupted sleep).
The art of time-management makes it possible to live at a high speed with an alarm clock at your side, and still be free from stress. However, the societal damage inflicted by alarm clocks and sleep deprivation is unforgivable. An alarm clock that interrupts your sleep damages your memories, your ability to learn, your creativity, your mood and temper, your relationships with other people, your ability to focus, and your overall intellectual performance!
Dr Robert Stickgold has shown that people who learn a skill during the day do not show significant improvement until they get 7-8 hours of good sleep. There was a noticeable correlation between the degree of improvement and the quality of sleep received. My own work with SleepChart also shows that the use of alarm clocks can dramatically reduce memory recall and consolidation. Forgetting is so painless that we rarely notice its effects. In a natural way, forgetting will proceed even if you get as much sleep as you need, and it is difficult to point to specific memories lost as a result of not sleeping enough. Moreover, sleep deprivation may leave your memories intact while their storage will be sub-optimum. The difference may be impossible to spot without measurement. We are more likely to notice sleepiness, reduced mental agility, or bad mood.
Disrespect for sleep has reached biblical proportions. This is most noticeable in the US and other highly industrialized nations. Men's Health's Dan Vergano writing for ABC News in "No More Rude Awakenings" suggests a seven-day system for fighting sleepiness: "The secret is to fuel that arousal system so it can beat the pants off the sleep system. By creating the kind of feel-good expectations that trigger hormones to wake the brain, you’ll override the need to sleep and be able to jump out of bed like a man on fire". The article suggests a "fresh" mind method that capitalizes on the fact that stress hormones help keep you alert. However, the only rational remedy for "rude awakenings" is simple: get enough sleep! Jumping like a man on fire is not likely to have a positive effect on your creative potential!
You may often notice that waking up with an alarm clock gives you a jumpstart for the day. You may then come to believe that using the alarm clock might help you stay alert later in the day. This is not the case. The alarm signal simply scares your brain into wakefulness, disrupting the carefully planned process of neural optimization that occurs in sleep. As a result, you get an immediate injection of adrenaline and your levels of ACTH and cortisol also increase. This is cortisol that peaks at awakening in natural sleeping rhythm that provides you with the fresh-mind impression. With passing time, this cheaply gained alertness will wear thin unless you continue abusing your physiology with more "remedies". You may use more scare tactics for keeping yourself alert, abuse caffeine, or even get a more profound effect with modafinil, cocaine, or amphetamines. Alertness should be achieved with the help of sufficient sleep, not despite the lack of sleep! Apart from your reduced ability to learn new things, all unnatural anti-drowsiness methods will produce a great deal of side effects that can be pretty damaging to your health in the long run.
All efforts to overcome sleepiness by means other than sleep itself can be likened to a chase of the first high in the use of psychoactive substances. If you drink buckets of coffee, do pushups, pour cold water over your head, or slap your face, you only dip into the last reserves of your alertness hormones that only worsen the effects of deprivation after the effects of the stimulation wear off, which is usually a matter of minutes. Rarely can you get a boost lasting more than an hour, and the more you perk up, the lower you fall in the aftermath.
If your life without an alarm clock may seem like an impossibility, you will probably need to use all methods in the book to be sure you get enough sleep and minimize the damage. If you need to wake up early at the cost of your brain, avoid the insomnia trap! Insomnia trap is a vicious circle of:
- going to sleep too early to get more sleep,
- failing to fall asleep in time (or worse, waking up prematurely),
- feeling even more tired on the next day, and
- going to sleep even earlier on the next day to catch up with the lost sleep.
It is better to go to sleep at a natural hour (i.e. a bit later), wake up early, suffer a degree of sleep deprivation, and hope for a phase reset that will make it possible to continue on the designer schedule. For a solution to the insomnia trap see Curing DSPS and insomnia.
If you cannot reset your phase and still feel tired when getting up early on a regular basis, consider choosing a job that is acceptable for your body, not the other way around. Your long-term health and well-being is at stake. If you absolutely cannot live without an alarm clock, you can at least start from changing your mindset about the importance of sleep and ensure you do not impose wrong habits on your children. Perhaps the young ones will be lucky enough to work in a flex-time system that will make it possible to get sufficient amount of undisturbed sleep. At least, do not set a bad example!
Wake up the President
President Bill Clinton was woken up twice by telephone during the night of April 22, 2000 before the infamous I.N.S. raid on the home of Miami relatives of the young Cuban exile Elian Gonzales. He was probably the most often disrupted and sleep deprived president in history. Only after a heart surgery did Clinton take diet, sleep and (real) exercise seriously. Those interrupted nights would definitely influence his performance and the quality of his decisions! Has anybody thought of a rule: Do not wake up the president? A rule that could only be revoked in a true national emergency? President G. W. Bush (b. 1946) was woken up when an American spy plane landed in China in 2001. He was also woken up after a suicide bombing in Jerusalem in 2002. George H. W. Bush (b. 1924) and Hilary Clinton made "waking up in the middle of the night" part of their presidential campaign and prowess. It seems that only Ronald Reagan had pretty strong rules for protecting his own sleep. He also famously napped during some cabinet meetings. He slept through a couple of international events without an apparent negative impact on his somewhat delayed decision-making. Critics would say he slept through the entire Iran-Contra affair. Was Reagan so protective of sleep because he understood the role of sleep better, or perhaps he was just a bit lazier than other presidents? I don't know. However, he sure set a good example.
Alarm clock monsters
Andrea K. wrote to me with skepticism: "Take the alarm clock away from a typical person and they won't just wake up on their own at their desired time and they will miss work, school, or whatever. An alarm clock can't be that bad for you because of the simple fact that most people use it and I never noticed any problem with them :) Everyone in my family has been using one since they were children, and no one suddenly went crazy or began to mutate into a monster (yet)!" As I wrote earlier, when you use an alarm early in the morning in order to get to work or to school, you cut off the later stages of sleep. If the intrusion into natural sleep is not large (e.g. from minutes to an hour), the damage may be limited. Alarm clock will do far more damage if it cuts deep into the middle of the night sleep. You can compare the use of alarm clocks to smoking or eating hot dogs. The harm is not great enough to be instantly noticeable. It took the public many years to largely accept that "smoking is bad" or "fast food is bad". It is hard to quantify the degree of damage. However, as we move to knowledge society where our intellectual performance becomes increasingly important, the effects of sleep deprivation will come under closer scrutiny and alarm clocks are bound to gradually fall out of favor. Unlike hot dogs, they are already universally hated by their users. Most people are able to somewhat adapt their sleep to their schedules if their routines are regular enough. When those people need to resort to the use of the alarm clock, they cut less of their sleep and the damage is proportionally smaller. Nevertheless, we should always strive at eliminating alarm clocks altogether. Most of all, we should protect our kids from suffering interrupted sleep!
Napping is good
The natural sleep-wake cycle makes you feel less alert at mid-day. This period can easily be visualized using EEG measurements. In many tropical, subtropical, and Mediterranean countries this is the time for siesta. The drop in alertness can be magnified by a rich meal and a short nap is likely to quickly restore full alertness. However, the industrial nations do not seem ready to adopt the healthy habit of a postprandial nap. Just the opposite, when the Mexican parliament debated the law on statutory napping, politicians and comedians north of the border had a good laugh about "lazy Latin Americans". Siesta Awareness in the UK abruptly cancelled their National Siesta Day 2009 upon a publication from China that showed that diabetics nap more. Myths galore. Napping is smart, and yet nappers are often considered lazy, or weak. The self-improvement guru, Tony Robbins, provides a typical misguided get-up-and-go advice on napping: replace a nap urge with press-ups. Press ups will improve circulation and raise the level of catecholamines. This will make you feel more alert for a moment. However, only a nap can provide a true neural boost to your cognitive powers. Nap is better than exercise. Nap is better than caffeine. Nap is irreplaceable.
Napping and evolution
There are few theories on the evolutionary purpose of the mid-day dip in alertness. Most people believe that humans, as all other highly developed tropical animals, have developed a siesta habit as a way of getting around the midday heat. This explanation has also some cultural background as napping is by far less popular in moderate and cold climates. However, the alertness dip can be resolved by a short nap in minutes. This can make us active again long before the mid-day heat is over.
Another explanation is that the alertness dip is an atavistic remainder of the polyphasic sleeping mode that might have characterized human ancestors. Many animals and newborn babies sleep many times during the day. This might seem advantageous for optimizing memory circuits. However, consolidating sleep into a single night rest period might have offered some evolutionary advantage too. Early humans might have been less efficient in hunting and gathering activities at nighttime. This is why it might be advantageous to spend nights on memory optimization. Possibly, the consolidation of sleep went gradually from polyphasic sleep, through biphasic sleep to semi-monophasic sleep in modern humans. Actually, similar consolidation can be observed as we get older. By the time of adulthood we are more or less monophasic with a clear dip in alertness that may be resolved with a short nap. As we near retirement, we again seem to tend to become biphasic. This may be a result of the fact that working people are forced to suppress their biphasic tendency. We remain strongly biphasic throughout the lifetime, and the monophasic model has largely been imposed by industrialization.
When I look at learning performance data collected with SuperMemo, I see that the homeostatic decline in cognitive powers throughout the day is steep enough to provide an alternative explanation: nap is cognitively beneficial, but not essential enough to boost it with a full-swing circadian support. As a result, we have developed a half-way sleep system that ensures the essential fully blown nighttime sleep, and a window for an optional mid-day alertness booster. As the circadian component of sleep drive is associated with some physiological functions of sleep, a system with homeostatic napping might not have been equally beneficial. As for the speed of homeostatic decline in alertness, it could be inherent to the networks involved and might depend on energy reserves, supply of neurotransmitters, size of the networks involved, etc. It should also depend on the degree of use. The heavier the mental effort, the faster the decline in cognitive performance. In other words, for the brain as it is, and for heavy mental loads, slower homeostatic decline may simply not be physically possible. The timing of the mid-day nap comes from the fact that splitting the day into two exact halves maximizes overall alertness. Here again, mid-day tropical heat might actually provide an additional evolutionary incentive.
Naps and brain power
The father of the napping science, Dr. D. F. Dinges has spent many years investigating the problem of alertness at workplace and has shown substantial benefits of napping in professions where the alertness may be the difference between life and death. His research showed a substantial alertness boost coming from a nap (Dinges 1989). He has also noticed relatively little impact of napping on the night-time sleep in regular nappers:
However, when Dr Matthew Walker published his research proving the value of napping for cognition (Walker and Stickgold 2005; Walker and Nishida 2007), Dr. Derk-Jan Dijk commented surprisingly: "there was no clear evidence that daytime napping offered a distinct advantage over sleeping just once over 24 hours (...) while the brain effect reported in the study might be spotted in a laboratory setting, the picture became more clouded in the "real world"". Today, you can measure the benefits on napping on your own using SuperMemo. Comparing recall graphs of nappers and non-nappers, we can clearly see how non-nappers power at half-steam through the second half of their waking day (see: Biphasic nature of human sleep). Dr Walker, who confirmed his point with later research, says convincingly: "It's as though the e-mail inbox in your hippocampus is full, and, until you sleep and clear out all those fact e-mails, you're not going to receive any more mail". Take it from a religious napper Mr Winston Churchill: "you get two days in one"! The value of the nap increases in proportion to the degree in which your work depends on your brain and the quality of your thinking.
To nap or not to nap? Nap!
Here is a short summary of pros and cons of afternoon napping:
- Siesta naps, rich in NREM sleep, result in a significant increase in alertness that will be highly appreciated by people in creative professions. By various measures that boost may be as high as 50%!
- As shown with SleepChart, napping improves recall and memory consolidation in the second half of the day
- Well-timed napping may help combat sleep deprivation. Some people even prefer to sleep in two 3-4 hour portions throughout their lives!
- Naps reduce blood pressure, improve cardiovascular health, and might, as a result, prolong your life!
- Badly timed (late) naps may worsen insomnia. This is why so many sleep experts counter-recommend naps. However, proper timing mitigates this problem
- Badly timed (late) naps may result in sleep inertia. This can be resolved easily with proper nap timing.
- Napping requires good rest conditions, conducive napping culture, and solid napping skills and habits. All these may be hard to come by in industrialized nations.
If you ever hesitate, to nap or not to nap, take a well-timed nap and see how it impacts your life. If you wake up groggy, remember that napping is also an art. Read about best timing of naps. Chances are, napping might become a beloved habit that will increase your productivity. Many great minds napped habitually. In addition to Churchill, notable nappers included Napoleon, Bill Clinton, and J. F. Kennedy. Interestingly, this group also includes a famous long-sleeper, Albert Einstein and a famous short-sleeper Thomas Edison. Even Bill Gates enjoyed taking naps under his desk in his creative programming years.
Napping in the corporate world
More and more companies in the US have already decided to make a switch from a coffee break to a napping break with special cubicles designed for nappers. In the future, this trend is likely to become more prominent as caffeine is not a fraction as effective as a nap in combating fatigue. For neural reasons, coffee, doughnuts, press-ups, and other methods taken together will never prove as efficient in mental restoration as a nap. At the same time, our society drifts strongly towards information processing where alertness is central to productivity. And when the productivity comes into the equation, corporations will definitely avail of the up-to-date research on napping.
- Do not use the alarm clock! Contrary to popular belief, well-scheduled nap will not result in sleep inertia (unless you are seriously sleep deprived). Alarm clock can seriously undermine the value of the nap!
- Measure exactly the optimum length of the period between the natural awakening and the nap to maximize the effectiveness of a nap. The nap should come at the nadir of alertness, which usually comes 7-8 hours from natural waking. Napping beginners often miss the right timing! Choosing 7 hours as your starting point will allow of a 60 min. margin in case you were late for one reason or another. See: Best time for napping
- Drink coffee or other caffeine drinks only after the nap. Even the tiniest amounts of caffeine in the system might interfere with the quality of sleep.
- If you drink alcohol before the nap, it should be largely metabolized (i.e. out of your system) by the time you fall asleep
- If you nap for more than 100 minutes, you probably need more sleep in the night (or you nap too late)
- Avoid stress 1-2 hours before the nap. Even things you love can make you excited and make it harder to avail of the benefits of napping
- Exercise is good. Try to finish your exercise at least 45 minutes before the nap. If you fail to cool down, your nap may end up prematurely
- Meal before the nap is recommended. Unless your doctor says otherwise, your main meal of the day should actually come before the nap (around the 6th hour of your waking day)!
- Sex before the nap is recommended
- Stick to your ritual (e.g. stick to your best sequence: exercise, beer, bath, meal, quiet place, nap, music, or similar)
- If the above advice does not work, you may need a month or so of trying. Mental slow-down is critical here! Many people do not discover the benefit of napping until some circumstances put them into the routine (e.g. heart condition diagnosis). Even if you cannot fall asleep, you may still need a nap! Sleeplessness may only be a result of a habit or your inability to forget the worries of the day. You may simply not fully understand your actual sleep needs.
- If it all won't make you fall asleep in 10 minutes even after a month of trying, you can probably safely give up napping for good
- When sleep deprived you may need 20 minutes of naptime per one hour of sleep lost (see: Power nap)
Best time for siesta
Value of siesta
Naps are a blessing for a tired brain. However, if taken at a wrong time, they can also contribute to messing up your sleep cycle. Many people believe that a nap is a nap is a nap. Whatever its timing, the nap will refresh your mind. This is false. Understanding the optimum circadian timing of naps is essential for naps to be your friend, not your enemy! The belief in the universality of naps sparked a dangerous ideavirus: lifestyle based on polyphasic sleep.
Napping is a skill. Many people cannot nap even if they are sleepy. Measuring the time between your natural waking and the nap should help you optimize the quality of a nap. Optimally, your tiredness might not even be perceptible enough to easily guess the optimum timing. If you measure the time between night sleep and the nap, you will notice that the length is always the same (minor variations depend on the quality of sleep in the night). In other words, the measurement helps you figure out the timing of your circadian dip even on days when you do not feel tired at midday. You may wonder, why nap in the first place then? The boost in cognitive powers is worth the investment (which may be as little as 10-20 min. on a good day).
In a healthy biphasic sleep, a nap taken at siesta time is an excellent boost to your mental energy and creative powers. It is important to know that the timing of the nap should not be determined by the clock that hangs on your wall. Your nap should come at around 7-8 hour of your natural waking time. To be precise, only you can determine that value precisely by comparing what happens if you try to take naps a bit earlier or a bit later. The optimum value may not hold if you cut your sleep short with an alarm clock, or fall asleep earlier than usual (e.g. because of an exhausting day), or delay going to sleep beyond your natural sleep hour.
Circadian nap phases
To optimize your nap taking, you need to understand the impact of the circadian phase on sleepiness. Below, I explain what happens if you take naps at different phases. In the text below, Phase 7 Nap denotes a nap that is taken 7 hours after natural awakening from sleep taken at natural hours. Refer to the following graph which illustrates the biphasic nature of human sleep propensity. This graph shows that there is only one optimum alertness valley (in red) conductive to sleep, usually in hours 7-8:
Figure: Recall at different times of day in a biphasic sleeper in relation to the circadian phase showing two major peaks in alertness and learning quality during the day. In monophasic sleepers, the same peaks can be observed, however, evening recall may be lower. This means that biphasic sleeper can expect higher overall learning or creativity in the evening. 58,339 SuperMemo repetitions were used to produce this graph. The alertness in purple color is determined by a sleep model based on the two-process model of sleep regulation. Alertness estimate combines the inverse of homeostatic and circadian sleep propensity
Phase 0: Waking time
Napping in Phase 0 is napping that takes place immediately after waking, i.e. circadian phase 0 in the graph above. Napping in Phase 0 is possible, and largely depends on the history of prior sleep. Phase 0 naps after a normal night sleep can be considered as a complement to the night sleep, e.g. if it was not effective enough. Such naps consolidate with the night sleep in sleep models and are an efficient way of extending the night sleep in cases when it was interrupted (e.g. by noises, bursting bladder, health issues, etc.). Phase 0 naps after a sleepless night can serve as an inefficient substitute for the night sleep. Such sleep will be short, unrefreshing and leave a sleep debt. It will also introduce unwelcome oscillations in the circadian system that may take a few days to clear up. Such sleep is often used by night-shift workers to get some mental boost for the day. Sleep in Phase 0 is still far better than no sleep at all. The rule is simple:
Phase 3: Creativity time
Napping in Phase 3 should never be possible in a healthy well-regulated system (see the red peak in hour 3 on the horizontal axis in the graph above). Successful sleep at this time is an indication of sleep deprivation, poor quality sleep (e.g. due to sleep apnea), sleep in a wrong phase (e.g. taken too early), sleep disorder (e.g. narcolepsy), etc. This is probably the hardest time to nap of all. However, I am not aware of any bad effects of such naps for health or for the sleep control system.
Phase 5: Pre-siesta
Napping in pre-siesta slot is possible. However, such naps are likely to be short and not as refreshing as Phase 7 naps. They are also more likely to be REM-rich for circadian reasons. Those early naps can probably be recommended to people who suffer from sleep-onset insomnia, and who still want to boost the second half of their day in terms of alertness and creativity. Those naps can also be executed "in a hurry" due to their short duration in cases where longer napping is undesirable, or later timing does not fit the day's schedule.
Phase 7: Siesta
Perfect time for napping. As it can be seen in the graph, Phase 7 is the period when the mental performance is at its mid-day nadir (aim at Phase 7 to make sure being late will still place the nap within the nadir). It is not true that the nadir is caused by a hefty lunch (even though meals have a big impact on sleep control). The nadir is a natural expression of the circadian wave. This circadian phase low time comes at the roughly same clock time as the subjective night nadir at a roughly 12 hour shift (e.g. if the middle of your night falls at 3 am, naps at 3 pm could be most effective). This is well explained in "How to nap". The benefits of a siesta have been confirmed by numerous studies. It has been practised for ages in many regions of the world. It will definitely trickle into the corporate world as human productivity becomes increasingly dependent on our creative powers.
Phase 11: Evening
This is not a good time for napping. In a healthy cycle, napping might be hard to achieve or impossible. However, even a minor degree of sleep deprivation will produce a nap that might trigger the control mechanisms responsible for the full-night sleep. Late naps are likely to be rich in NREM sleep and rob your night sleep of the vital SWS component (Werth et al. 1996). Those naps can last far longer than siesta naps. They can make you groggy. Worst of all, they can compound insomnia. Unfortunately, this is a type of a nap that a huge proportion of students take! Forced to wake up at indecently early times for school, kids and students struggle semi-conscious through school hours with negligible progress in learning. Learning in such a state only magnifies the pretty universal hatred of school. Phase 11 nap is then the only way to survive the day and get some actual learning done in the evening. The body clock shifts the subjective night to morning hours. The positive side effect is that evenings can be filled with effective studying. The negative side effect is that the student finds it impossible to fall asleep before 3-4 am, and welcomes the new bright school day with an alarm clock that rings in the middle of the subjective night. This perpetuates the cycle of suffering and school hate. Nobody has ever estimated the global consequences of this phenomenon that includes an impact on adolescent attitudes that are notoriously fraught with problems. Neither has anyone come up with a practical solution (shifting school hours usually results in kids "adapting" to the new cycle by shifting their bed time as well). I am not able to recommend a solution here either. Skipping evening naps might be better for the quality of night sleep and for the stabilization of the circadian cycle in the earlier phase, however, that would effectively rob those students of their only time in which they can learn. Those evening naps are also the only meager substitute for free running sleep that those young brains crave. The only time when the brain gets what it wants. If I was to answer: to nap or not to nap, I would probably have to admit that evening napping is the lesser evil in a majority of cases.
Phase 13: Pre-sleep
This is a particularly bad time for napping. Initiating naps at this time should be relatively easy. However, pre-sleep naps are likely to produce one of the following unwelcome outcomes: long-nap-short-night or long-night-early-waking (depending on the current status of the sleep control system). A pre-sleep nap is likely to result in triggering the night sleep sequence. However, this sequence is not unbreakable, and can result in early awakening combined with the difficulty in launching back to sleep (insomnia). This is particularly likely if the homeostatic sleep process generates substantial sleepiness while the circadian sleep process is not yet mature for the night sleep. As a result, such a pre-sleep nap can yield less total sleep than a normal night sleep. This long-nap-short-night will not entirely fulfill the physiological function of sleep. Consequently, your alertness levels for the next day are likely to dip substantially. The less unfortunate outcome of a pre-sleep nap is if you successfully trigger the uninterrupted night sleep sequence. However, you will likely prematurely run out of the homeostatic process before the circadian function of sleep is completed. You will probably wake up earlier than usual. This is the long-night-early-waking outcome that produces nights that are amazingly unrefreshing considering the fact that premature sleep is often much longer than an ordinary night sleep. The reason for this low sleep efficiency is probably the scarcity of REM sleep, which is strongly circadian. Moreover, for circadian reasons, your morning is likely to be unusually sleepy!
Phase 15: Segmented sleep
Phase 15 napping should be considered "early night sleep". If you go to sleep at this time you can expect any of the following (depending on the degree of sleep debt):
- if you carry no sleep debt: some unproductive time in bed, as you might not be able to fall asleep.
- pre-sleep outcome as in Phase 13 sleep: long nap with a short night, or long night with early waking. This outcome is likely to leave you less refreshed.
- segmented sleep: you will fall asleep, but will wake up for 1-2 hours in the night only to fall back asleep. This lucky outcome will leave you pretty refreshed.
- long night: on rare occasions, esp. if you are sleep deprived, early bedtime will result in a nice long refreshing night sleep.
Due to the precarious nature of Phase 15 sleep, it should rather be employed only in conditions of sleep deprivation, which provides good chances for a positive outcome. Otherwise, early bedtime may be unproductive at best, and bad for the quality of sleep, at worst.
Phase 18-24: Night sleep
If you try to nap in Phase 18-24, you are bound to trigger a normal healthy night sleep. This is ok as long as you do not get down to "napping" with the evil intent of stopping the process in 20-40 min. Here is where the pain of polyphasic sleeping becomes hardest to bear. As Dr Stampi noticed decades ago, it is not the problem with staying awake or with falling asleep that is most exasperating. The most painful part of a polyphasic life is when your brain wants to trigger the night sleep sequence and a polyphasic adept stubbornly disallows it (Stampi 1992)! This is as bad an interruption as any other abrupt stop to an all-or-nothing physiological process (urination, defecation, orgasm, swallowing, heartbeat, sneezing, coughing, childbirth, and the like). Many polyphasic bloggers note: "I noticed that when my naps get longer, I get groggy. So I try to keep them under 20 min". Duh! If you do not launch the night sleep sequence, you will not suffer the pain of interruption. Why nap in the first place then? It's easier to delay defecation than to stop it in the middle. The most unusual night-time nap control method I have encountered was... "I keep lots of junk in my bed. That keeps my naps short"!
- The napping phase is relative to the optimum natural waking time, not the actual waking time! For example, in a premature sleep, or in an interrupted sleep, an efficient nap can take place 10 hours from waking. If your sleep is not entirely natural, you need to take a correction for your circadian cycle that might be running in the background independently of your actual sleeping hours. Unfortunately, a vast majority of people with sleep phase problems have no idea how to efficiently measure and plot their natural cycle
- The two alertness valleys are biologically dissimilar! Only the night-time valley can produce a typical long-drawn periodic NREM-REM interplay with a gradual increase in the proportion of REM. The subjective night period is marked by a characteristic increase in the release of melatonin. The length of siesta sleep, in biphasic sleep, is 4-20 times shorter than the natural night sleep. Phase response is elicited by stimuli that precede or follow the night sleep. However, the same stimuli may affect the timing of the siesta nap, which in turn may have an indirect impact on the sleep phase
- Timing of naps will determine their structure. Some researchers believe that this can also affect the efficiency of naps for a particular type of mental task due to the fact that nap timing determines the proportion of NREM sleep to REM sleep. This might be true if you want to maximize value/time for specific tasks. However, with sleep optimizations, you should always go for the maximum total value. This is why the best nap is a nap well aligned with your midday circadian dip in alertness.
Good conditions for a nap are important. A nap in a semi-reclining position, or in a noisy room, or in bright lights, will also bring benefits to your alertness (on condition you actually manage to fall asleep, and perhaps pass Stage 1 NREM). However, a nap in a sleep-conducive environment will often last longer and be far more refreshing.
One nap per day is enough
Many people believe that every extra successful nap can be preciously helpful in restoring their mental energy. In a normal sleeper, who is not sleep deprived, an additional nap is indeed likely to bring increased alertness and improve mental performance. However, on a healthy schedule, all naps outside the siesta period should be very hard to accomplish. If the goal of sleep is defined as achieving maximum creative productivity, and if the night sleep can run its healthy course (i.e. there is no sleep deprivation), then any nap attempt at times other than the siesta time will be wasteful. This is because falling asleep should be difficult, and simply resting with the eyes closed does not yield a fraction of the neural benefit of an actual successful nap. Moreover, even if successful, an extra nap forced in in the morning is likely to interfere with the afternoon nap. Similarly, an evening nap may result in shortening of the night sleep. Those extra naps may bring incremental improvement in performance, but will reduce the overall efficiency of sleep and may cause ripples in the circadian system. Our biphasic nature makes it quite clear, we should strive at a single nap in the afternoon (in the 7th hour of waking). For some people, even this will be too much, and monophasic pattern is their optimum.
Many young creative individuals come up with their own designer sleep schedules. I often get mail with submissions of new sleep pattern propositions. For example, triphasic sleep: one main sleep episode of 6 hours (00:00-06:00) with two 30 minute naps after meals (12:00-12:30, and 18:00-18:30). Like most of artificial ways of making the sleep system work to design, this schedule is not likely to be efficient. Most people are strongly biphasic, and only biphasic or monophasic sleep works well for adults. However, if one throws away the second nap, the proposition will be pretty close to a natural biphasic rhythm: 0:00-6:00 and 13:00-14:00. Even though, designer schedules should always be avoided. The only exception is for designs that are an approximation of what SleepChart shows in free running sleep. As people differ in various parameters of their sleep control system, those who are very regular sleepers might indeed consider wiring a specific timing to their schedule as long as the timing is derived from their actual sleep pattern measurement. If sleep episodes in a designer schedule are not aligned with the circadian needs then they will often lead to a circadian chaos.
Once you run free, you will quickly determine if your prefer to sleep biphasically or monophasically. In creative lifestyle, biphasic sleep leads to higher productivity. See: Natural creativity cycle.
Myth #1: Only lazy people take naps
In some cultures, this harmful myth makes people feel ashamed that they are weak enough to need a nap. This myth must be abolished promptly. Naps have a great effect on cognitive function and productivity. If you want to take a nap, take it and announce it proudly. You are doing a smart thing. Naps make you smarter!
When Newt Gingrich was caught asleep on camera, commentators pointed fingers in all wrong directions: he is getting old, he is tired of the campaign, he might be suffering from Alzheimer's, etc. Gingrich was set to video-stream live via satellite hook-up to the American Israel Public Affairs Committee. During a preceding Panetta speech, he opted for a quick nap in a sitting position. His age or health did not need to have to play any role here. In fact, Newt was doing a smart thing: he was clearing up his brain before appearing before a demanding audience. However, despite a short span of time available, he managed to launch deep sleep and woke up with clear signs of sleep inertia. He did not know where he was and what he was about to do! He recovered pretty fast with stump lines attacking radical Islam. Those sleep inertia symptoms did not need to indicate he was tired of the campaign or that his sleep deprivation carried over from many days of sleep debt. Even a single night of lost sleep would be enough to put anyone in his position. His being a political old-timer worked against him. No novice would be able to overcome the stress of public sleeping to get a few zzzs. The only obvious mistake Gingrich made was to fail to get his full load of sleep on the preceding night. For more see: Why naps cause sleep inertia?
Myth #2: A nap is a nap is a nap
This myth says that every nap is a good nap. It does not matter when it is taken and how long it lasts. This myth lives deep in the psyche of inexperienced nappers who often do not realize the myriad of genetic, metabolic, neural, and hormonal processes that cycle through the human body throughout the roughly 24 hour period. In the section titled Best nap timing, I include a general partitioning of the circadian cycle with a short analysis of what processes occur when a nap is taken at each selected point of the cycle. Naps taken at different points of the circadian cycles are as different as chalk from cheese. Some are refreshing. Some are a waste of time. Some may be unhealthy (or at least inefficient). Some will last several hours!
Myth #3: Make sure you wake up from Stage 2 NREM
Some napping "experts" will tell you to use an alarm clock to make sure you wake up after 15-20 min from Stage 2 NREM. Supposedly, longer naps send you into deep sleep (Stage 3/4 NREM), and you wake up groggy. In reality, it is the timing of naps in reference to the circadian cycle, as well as the prior sleep deprivation and REM-sleep deficit that will determine the nap duration and the effects of the nap. On some occasions, it may happen that a nap cut short with an alarm clock will be somewhat refreshing and will prevent the ripples of a wrongly timed prolonged nap. However, it is always better to choose the appropriate time for a nap. It will usually be around 7-8th hour of the subjective day. This translates to 7-8 hours from waking in free running sleep. However, in conditions of sleep deprivation, or misaligned sleep cycle, it is safer to take an earlier nap or even skip the nap entirely to help cycle re-synchronization.
Did you hear a story in which Einstein supposedly napped with a pencil to wake up as soon as the pencil dropped? I doubt a great genius would make this mistake on a regular basis. I am sure he had a chance to compare the values of a well-timed natural nap and an interrupted nap. Perhaps a pencil dropped indeed. Once? Perhaps the genius brain was thus deprived of some new creative insight? Or conversely he was inspired by an interrupted mentation? Perhaps it was just a nice story to tell over a cup of tea? Whatever the truth, do not follow this example! Let your brain decide how long the nap should last!
Myth #4: The circadian cycle can be ignored or abolished
A harmful myth says that we could ignore the circadian cycle, so that the sleep can be reduced to one-dimensional homeostatic process. This myth comes from the lack of understanding of the two-process nature of sleep. It made many to believe that polyphasic sleep is a good long-term lifestyle choice. The myth comes from the lack of appreciation of the overwhelming power of the primary circadian sleep component. Consequently, the myth bears a belief that naps can be induced at will at any time that is sufficiently far away from the prior nap.
Have a peek at the following amazing picture obtained with the help of SuperMemo.
The graph shows the powerfully biphasic nature of the human circadian cycle. The horizontal axis shows the circadian time, i.e. the time that elapses from phase 0, i.e. the predicted "end of the night" time. The prediction comes from the circadian model employed in SleepChart, and is derived from the sleep log collected in SuperMemo. The red line is the predicted alertness derived from the same sleep log data using the two-process model of sleep developed for the purpose of sleep optimization in SuperMemo (the model is inspired by similar work of Alexander A. Borbely and Peter Achermann). The alertness is a resultant of the status of the two sleep-drive processes:
- the homeostatic process and
- the circadian process.
The blue dots are recall data taken from an actual learning process in SuperMemo. In other words: red is the model, blue is the data. Both tell the same story! For skeptics who do not believe in scientific models, blue-dot unprejudiced data should be the ultimate clinching argument. The graph says unequivocally that we got two major peaks of alertness during the day. It also states clearly that there are only two valleys conducive for sleep and napping.
Factors that affect sleep
Stress is a sleep killer. Hormones associated with stress, such as adrenaline, ACTH, cortisol, etc. increase alertness and reduce the sleep propensity. It is more difficult to initiate sleep in conditions of stress. Nighttime awakenings are more likely. The sleep structure may also change. The sleep may become shorter and less refreshing. Moreover, sleep deprivation will magnify the effects of stressful situations. Stress and bad sleep conspire hand in hand to make life miserable for quite a number of people in industrialized nations. Those who are sleepy and in stress are less likely to achieve their goals. That only adds to the misery.
In addition to its effects on sleep, stress may have a negative influence on creative work, learning, problem solving, etc. At its worse, stress can virtually shrivel your brain! Persistent stress and raised corticosterone levels have been shown to decrease BDNF in rat brains. This leads to the atrophy of the hippocampus - the chief memory switchboard! Stress can cut down your IQ. Not only for a day, but also, to a lesser degree, for a lifetime!
Stress is rooted in our emotional brain. Emotions are very difficult to control and will often determine a person's chances for success or failure. Negative emotions, such as anger, are counterproductive and contribute little to a person's growth. Positive emotions, such as well-dosed passion, help one overcome obstacles that are bound to be found on any race-course. Emotions are also transitive and tend to amplify in social groups. Anger begets anger. Cordiality begets cordiality. Your effort to beget positive emotions, in suitable circumstances, will send positive ripples through the social circles you interact with. Learn to capitalize on positive emotions and circumvent negative emotions. Invite all positive emotions that help you execute your grand plans. Condition yourself to love your work, people, and the world. Run away from sources of negative emotion.
The power of emotion comes from the fact that they are wired into the low-level brain structures that cannot easily be controlled by rational thinking governed by the prefrontal cortex. An angry individual can command its brain to cool down, however, he cannot instantly reduce the level of adrenaline that has already been released into the bloodstream. A drug addict can rationally decide to give up drugs, but when the physical effects of the craving hit his system his rational brain is often powerless.
As it is hard to combat one's reactions to stress, one of the best ways to deal with stress is to run away from it. Stress is so important to your well-being that, if possible, a change of a job, a change of friends, a change of residence, or a change of lifestyle must be considered! Without these, even the best counter-stress advice may not work. Some people are inherently prone to stress and may find it impossible to live a life without it. Others may thrive on stress (to a degree). This article cannot possibly even touch the tip of this troublesome iceberg. If you suffer from bad sleep and stress, tackling stress may be your top priority thing to do. There are tons of books and blogs that deal with the issues of stress. It may seem redundant to produce yet another list. However, I thought I would make a selection sorted by how much I believe it could be helpful, esp. with the view to sleep and creative work. I believe that prioritization by informative power as opposed to the curative power is important. For example, good health might be the most important factor in combating stress, however, you are probably already working hard on it. On the other hand, fewer people realize the effectiveness of pain in curing one's troubles! I am no expert in stress management, and I have been blessed with pretty strong stress tolerance that can make it hard for me to come up with a comprehensive list. If you think I could add something worth recommending to others, please let me know. Here is my list:
- exercise: if you impose a pain of exercise, your other pains may be seen from a new perspective. Try marathon or winter swimming. Few things can bring as much relief and pleasure as pain! Exercise helps you grow the brain - your best anti-stress ally. On the other hand, sleep deprivation and stress shrivel the brain making it harder to struggle through life.
- eliminate interruption: wherever possible, create protected zones in your day when you can focus on creative work. If possible, your entire morning should be protected to keep your mind clear. In that period, e-mail, phone calls, and meetings are all forbidden. If the protected zone follows a period of stress, you may never get back to your best mental shape. Take occasional creative vacations when nobody can reach you. If this is too much, try it once. Perhaps you will see your entire life in a new context.
- measure the day: instead of measuring your distance to the goal, measure how much you have accomplished on a given day. If your goals are ambitious enough, most of the time they will be pretty remote far on the horizon. With your eyes on the horizon, you do not have a good sense of progress. If you focus on the jobs for the day, you will get a better sense of the positive motion. Do not wait for the great trophy. Let the little good deed be its own reward. Once you reach a higher level of satisfaction with little things, your brain may work on a higher gear and perhaps carry you beyond your original target.
- simplify: simple living is more fun. Simplicity is a great escape from the rat race. Some ideas: ride a bike instead of driving a car, give up some of electronic must-have gizmos, try to live without deadlines, make fewer promises (including yourself), turn off your cell phone, take a creative vacation (one job, one goal, no distractions), etc. If you work on too many projects, try to finish one before you begin another. Learn to say "no" or avoid situations in which you have to say "no". Learn to delegate. If you are overloaded with information or e-mail, dump it all to SuperMemo and prioritize.
- creative work: I believe that few things are as fulfilling as creative work for a good cause. If you can sacrifice lesser things for a major creative goal, do it each time you have a chance. In the long run, productivity of the mind is one of the best formulas for a happy and healthy living.
- slow down: if you always keep running out of time for doing things, you might be loading too much on your plate. This magnifies your stress levels, and gives you an excuse to delay sleep. It is good to be fast and efficient, however, this cannot stretch beyond a certain biological limit where you shortchange your life for today's minor accomplishments.
- prioritize: one of great ways to pile up stress in the long-run is to procrastinate or work on fun things while neglecting the rest. You need to always prioritize honestly and religiously execute by priority. There will always be a long bottom of the list that will need to be ignored. Ignoring the overload and focusing on top-priority jobs is a thing everyone needs to learn. One of the ways to prioritize effectively is to use tasklist sorted by priority = value/time. Dumping tasks on a tasklist is both stressless (you always hope to execute at some point) and efficient (you always work from the top of the list).
- sleep: even though we speak of sleep as a goal, sleep is also great for clearing up stress. However, this weapon is effective only if you can run your sleep free. This is due to the fact that stress itself may cause phase delays. This means that you may need to delay sleep slightly and get up a bit later. If you need to get up early, sleep is less likely to be your ally. If you get into an unsettling situation that spoils the day, you can always hope for some solid exercise followed by some solid sleep. Hang on to the comfortable thought that human biology will work for you. Stressful situations often seem much more stressful when stress adds up. Stress multiplies stress. Good sleep gives you a new, more honest perspective on today's worries.
- be nice: if you have something positive to say to someone, say it! Help others go through the day with a smile. If you have something negative on your mind, ponder if expressing it is necessary. Small things are not worth fighting for. When you are in a bad mood, remember than even if you force onto yourself a bit of niceness for others, you will see that niceness is infectious, you will get some back and a day will feel sunnier. If this advice does not work, perhaps you are in a wrong environment?
- family: family is a chief source of stress for many. However, it can also be the best form of stress relief. If you are just about to build a family unit, keep in mind that stress-free life in a long-term should be one of your chief goals. If your relationship is a string of worries with no light in the tunnel, perhaps it is not your best long-term investment?
- nature: sadly, many of us lost the ability to feed on the beauty of the natural world. For a stressed workaholic, a trip to the woods or the mountains feels like a waste of time that only magnifies the stress. However, the love of nature is atavistically dormant in everyone. It can be reawakened. It can be re-learned. Once it is restored, contact with the nature is one of the best stress relievers. Needless to say, nature preserves should be mobile phone free zones!
- contentment: one of the best ways to boost one's resistance to stress is work on one's overall contentment. Research shows that contentment is primarily based on self-esteem, relatedness (relationships with other people), autonomy (being in control) and competence (being productive). To a degree, all these factors can be influenced. Contentment makes it easier to ignore minor annoyances and see the big picture and big goals.
- avoid conflict: go to battle only for major life-changing causes. Too much time on this planet is spent on battling for or against little things and/or minor principles. A great deal of conflict can be resolved by just ignoring it and focusing on more important things.
- avoid bad people: one of the chief sources of stress is other people. Some of them are good people that bring stress by virtue of their job, duty, good intent, ignorance, etc. Others are just plain bad people that would best be avoided. In your choices of a spouse, job, gym, friends, etc. make sure you stay away from people with a talent for spoiling your day. Those choices will determine your stress levels, your ability to sleep and your ability to focus on things that are important.
- plan ahead: when you wake up with a set goal for the day, your mind is taken away from stressful distractions and your productivity soars. Review your goal lists regularly to refocus the mind. You need separate goals for work, family, exercise, etc. They are all helpful in a smooth and productive sail through each and every day.
- time-management: solid time-management is essential for productivity. Low productivity begets low productivity. Once your efficiency drops, it will have a negative impact on your motivation undermining your progress even further. In terms of a nagging stress, few things are as bad as piling lists of things undone. Those nagging lists are best tackled with time-management (getting things done), and prioritization (getting rid of things than cannot be done). Consider Plan in SuperMemo for time-management, and Tasklists in SuperMemo to stresslessly cut the bottom of the to-do list.
- health: healthy lifestyle is vital. Healthy body makes stress management easier. Here sleep can be a part of a negative feedback loop (less sleep, more stress, less sleep), or a positive feedback loop (good sleep helps combat stress, stress management helps you sleep better, etc.)
A degree of stress can also be a positive force. Some forms of stress are great energizers. I believe that when optimizing one's day for good sleep, good learning, and good creativity, it makes good sense to take into account the timing of stress. Stress before sleep will have a negative impact on sleep. Stress before creative work or learning will have a negative impact on the results of brainwork. A vast majority of people do not have much influence on the timing of stress. Stress seems to pervade all our lives. However, if you belong to the lucky few who can decide when to open a letter, read mail, make a difficult call, schedule a tough meeting, tackle a stressful task, you should try to employ stress to work to your advantage. I believe that the best and the only right time for tackling stressful situations is before or at siesta time. This timing spares the most creative morning period, and provides a sufficient time buffer before the night sleep. Moreover, it helps you sail through the less productive section of the day, including the mid-day dip in alertness (if you cannot afford napping). If you are a napper, adding exercise after the stress slot could pretty efficiently flush away the effects of stress. A dose of stress can actually improve the efficiency of exercise, and if exercise is not efficient enough to erase the effects of stress, you will be sacrificing only the lesser component of your daily sleep cycle: the midday nap.
Alcohol and creativity
Alcohol is a major enemy of a creative individual! In excess it is highly toxic to the brain! Even small doses can reduce the quality and the density of REM sleep. Alcohol also interferes with the function of deep sleep, produces sleep fragmentation, and relaxes the upper airway muscles, which worsens snoring, and severity of obstructive sleep apnea. Apart from its negative impact on sleep, alcohol reduces cognitive powers, inhibits memory encoding, and should be particularly avoided at times of creative effort!
Alcohol and health
To add confusion to the picture, lots of research indicates that small doses of alcohol may benefit health. Actually, a drink a day may be the simplest known method of preventing arteriosclerosis, heart attack, and cerebrovascular disease. There are reports that moderate beer drinking, or perhaps even alcohol in general, may reduce the incidence of Alzheimer's (Breteler et al. 2002). A beer belly or aluminum in beer cans may have the opposite effect. In smaller quantities alcohol can improve the blood lipid profile, while, in contrast, excess drinking is associated with hypertension. Some physicians recommend daily alcohol in very small quantities (not more than a drink per day).
To a highly creative individual, alcohol poses then a health-vs-brain dilemma. Certainly it should be avoided 3-5 hours before sleep. It should also be avoided altogether before intellectual work if there is no intervening sleep period in between. This would leave place only for very moderate drinking in the early evening (assuming you do not do any brainwork later on) or before a siesta (i.e. assuming that this is the time you take a break from intellectual effort to take a nap).
Alcohol and exercise
Exercise is known to reduce drinking, possibly through its impact on the reward centers. However, it should also provide some protection against the toxic effects of alcohol on the brain. Exercise accelerates circulation and speeds up the conversion of alcohol into acetyl-CoA that can then be quickly used as a source of energy. This prevents a buildup of acetaldehyde that is the most toxic metabolite of ethanol. Acetaldehyde is partly responsible for the hangover and may have carcinogenic properties. Exercise also helps you reduce the level of blood triglycerides that might increase as a result of chronic drinking. Regular moderate drinking improves the metabolic machinery used to neutralize alcohol. On the other hand, binge drinking is equivalent to destroying one's own brain. If you ever get to the point of slurred speech, or experience a hangover, you know that bad things happen to your brain! The younger you are, the more damage you can expect!
Alcohol and sleep
Alcohol may accelerate sleep and bring a faster descent into deep sleep EEG, however, little is know about the structure of sleep induced with alcohol as EEG is only an approximate indication of the sleep stage. There can also be an increase in alpha waves which indicates that the function of NREM sleep might be disrupted This reference is used to annotate "I would never send my kids to school" (2017) by Piotr Wozniak
(Chan et al. 2015).
Alcohol is quickly metabolized, and produces an acetaldehyde rebound effect that will greatly increase chances of waking up during the night. This effect keeps alcoholics up at nights, deprives them from REM sleep, and may actually be responsible for delirium tremens or perhaps even Korsakoff psychosis. Even moderate amounts of alcohol will have a noticeable effect on the quality of sleep! Make sure that alcohol is out of your system before your night sleep! Reading a book before sleep may be as effective as a glass of wine!
A great deal of confusion comes from the fact that very small doses of alcohol may have a very different impact on the brain. The impact will also be radically different in alcoholics in withdrawal. Nevertheless, you can safely employ the following rule of the thumb:
Alcohol and learning
You should never drink before creative work or learning. Even a gulp of beer can affect your performance. Some users of SuperMemo claim they enjoy moderate drinking while learning in the evening. This is understandable if the function of evening learning is fun and relaxation without great expectations as to the learning effect itself. I am not sure if this worsens or alleviates the impact of memory overload on the hippocampus, and the adjoining networks. However, I know for a fact that the memory effects will be greatly reduced due to the descending evening circadian slope and due to the effect of alcohol itself. The only imaginable benefit might come from suppressing the remnants of stress from a hard day of work.
Some drinking rules you might consider:
- alcohol in the blood before the night sleep will seriously reduce the quality and function of sleep
- alcohol suppresses creativity
- alcohol suppresses learning
- hangover is bad for the brain and indicates a toxic impact of alcohol and its metabolites
- you cannot expect health benefit it you consume alcohol to the point of slurred speech or balance problems
- if you violate the excess rules (above), take a month-long break from alcohol to prove to yourself that you are not on the way to addiction
- try to balance each drink with at least one hour of exercise to protect the brain. A beer after marathon might actually be healthy
Read more: Alcohol: Health Benefit or Hazard?
Caffeine is the number one drug used against sleepiness! 90% of Americans use it in some form. It can be found in coffee and coke, as well as in smaller quantities in tea and chocolate. It is addictive and acts via similar channels as amphetamines and cocaine. It also affects the reward centers (including the nucleus accumbens (Goldberg et al. 2002)). As caffeine has a profound effect on the central nervous system by blocking adenosine receptors, it is widely used to tackle drowsiness. However, majority of people little realize that it works well in your struggle with the adenosine-related homeostatic component of sleepiness, while it is quite inefficient in overcoming circadian sleepiness! Moreover, used against the latter, it can actually be quite unhealthy! If you abuse caffeine or use it at the time when your body clock tells you bedtime, you will only experience the symptoms that gave caffeine all the bad rap. These include: heart arrhythmia, irritability, overwhelming tiredness, depression, increased risk of miscarriage, and a typical coffee abuser's "sickness in the stomach". No wonder the popular myth says that coffee is bad for health and can contribute to a heart disease.
Surprisingly, the research on the health effects of caffeine does not seem to confirm its harmfulness. Considering the way coffee is manufactured, it may seem surprising that its carcinogenic effect is insignificant. Some publications even indicate a positive impact of coffee on health and longevity. If the research seems contradictory, it probably comes from the fact that some people drink coffee at the "right time", while others try to compensate for sleep deprivation or to mask circadian sleepiness. The link between coffee and heart disorders is weak, may depend on individual genetic ability to metabolize caffeine, and may be attributed to caffeine abuse in the form of excess doses or wrongly timed doses. Some research has even found that 3-5 cups of coffee are optimum for lifespan. The same research was criticized for failing to notice that coffee is more popular in well-to-do households that favor longevity. There have also been reports of positive impact of caffeine on memory. Caffeine increases the levels of BDNF in the hippocampus, and might perhaps boost neurogenesis. It was found to be modestly preventive against the Alzheimer's disease. You can then assume that caffeine is rather harmless in smaller 200-400 mg/day quantities (equivalent of 2-4 cups of coffee). Note that 50% of Americans take more than that. For caffeine to be harmless, it must be taken at the right time!
Caffeine tends to drive many people into a vicious circle: you drink it, you get a boost in epinephrine, you feel more energetic, you get a boost in dopamine, you feel better, you feel you can stay up late, you sleep less, you are more sleepy on the next day, so you need more caffeine, due to downregulation you get less boost per cup, you wonder why it does not work this time, you increase the dosage, and the vicious addiction circle continues. Coffee drinkers may occasionally experience migraine-like headaches. These are caused by an increased activity of adenosine receptors on days when the supply of caffeine is less. This results in the dilation of blood vessels in the brain. Vasodilation or activation of purine receptors on sensory neurons produces the headaches. Half a normal dose of caffeine should help. Conclusion: if you want to go straight on coffee, do not go cold turkey. Allow of a couple of days for your body to gradually fight off the addiction. A rational approach to caffeine is: use it as a circadian enhancer! Even though I always advise to avoid using substances in regulating sleep, you can use caffeine to accelerate your transition from sleep to full mental alertness. Small dose in the morning will shoot your alertness slope up and the regular intake will produce mild addiction that should help you fall asleep in the evening. This approach should be largely neutral to your health, to your sleep architecture, and positive to your alertness (at least very early in the day). Never use caffeine to cover up for insufficient sleep! Current knowledge about caffeine supports the recommendation for a cup of coffee in the morning in otherwise healthy individuals. As black coffee can be irritant to the stomach lining, coffee should rather be drunk with milk or cream. In regular nappers who do not battle insomnia, the circadian rhythm should yet permit drinking coffee immediately upon waking up from an afternoon nap.
As an arousal drug, caffeine may induce insomnia. This is why it should never be taken later than 6-7 hours before sleep. Caffeine half-life is about 6 hours for a healthy individual, but can vary substantially from person to person! Taken too late, caffeine will suppress REM sleep with detriment to the quality of sleep and its effect on memory. At the same time, when taken regularly early in the day, it may actually produce mild withdrawal effects and promote sleep. Its impact on sleep structure should also be minor if administered early enough, however, even a morning intake will reduce deep sleep in the night (Dijk et al. 1995). Caffeine cannot serve as a weapon against sleep deprivation. Only a sufficient amount of night sleep can play that role. Caffeine should also not be used against the circadian sleep component. As argued throughout this article, circadian rhythm should best be left alone to run its course!
Some sleep experts recommend coffee before a nap to shorten its duration and/or to ensure waking during Stage 2 NREM. This may be ok in case you need a quick restorative nap in a hurry, e.g. in case of a drowsy driver. However, an optimum nap in a free running cycle will naturally last no more than 30 minutes, esp. in conditions of stress. The effect of napping may be short-lived if the nap is artificially cut short with a cup of coffee.
The only good time for drinking coffee is in the morning! (or after a nap in habitual nappers)
Never drink coffee to overcome circadian sleepiness!
Dr Stickgold says:
In all likelihood, the vast majority of people drinking coffee in the morning are doing so, consciously or unconsciously, to correct from sleepiness due to inadequate sleep quantity or quality"
It does not need to be the case. Even after a good night sleep, without sleep deprivation, coffee can crank up the creative powers of the brain. However, it most likely does so at the cost of attention. It may then help in creative problem solving, but it might also reduce one's attention during a morning lecture or magnify the effects of a stressful situation. Ultimately, you need to be your own judge and see if this is really your best morning drink.
Michael Jackson was a genius. He obsessively cared about his health. He wanted to be immortal or at least live past 150. He even contributed to a rumour that he slept in an oxygen tent to combat aging. And yet he committed a cardinal mistake of sleep hygiene: he used medication to control his sleep. This is why he died at 50. He could afford the best medical advice, and yet the genius of pop died of ignorance.
Millions of people on this planet take benzodiazepines to get themselves to sleep. Others drink themselves to sleep. Yet others take a puff of marijuana. Inevitably, the outcome is the same: unrefreshing sleep and daytime tiredness. Dr Kripke showed in his research that people taking sleeping pills die younger. Why then do so many people make the mistake of medicating sleep? For many, unrefreshing sleep is better than no sleep at all. More importantly though, as society, we have lost the true sense of what a refreshing night of sleep can do to our brains and bodies! All this damage is done at a time were very little is needed to get great sleep in a majority of healthy people:
- sleep at the right time, and
- sleep until you wake up naturally (for more see: Formula for good sleep).
Melatonin is the only proven natural sleeping pill with few documented side effects. No wonder it is getting more and more popular. In addition, its antioxidant properties have sparked interest in melatonin as an anti-cancer agent. In the wake of such interest, there is always a wave of cheap counterfeit drugs hitting the market, esp. via the Internet sales. Those may contain no melatonin whatsoever.
Melatonin is a natural sleep hormone synthesized in the pineal gland from serotonin by acetylation catalyzed by serotonin N-acetyl transferase to form acetylserotonin that is later methylated with participation of SAM to melatonin. Melatonin is released during that part of the circadian cycle that corresponds with the period of darkness in both nocturnal and in diurnal animals. Diurnal animals, like us, are those that are active during the daylight period. However, melatonin is only an intermediary in the complex process of sleep onset. It can accelerate the onset, and can slightly advance the sleep phase, however, it cannot produce sleep on demand, and the sleep it can trigger will often differ in structure from a normal healthy sleep. Melatonin's impact on sleep structure is probably the reason why many people who use melatonin as a sleeping aid report feeling less refreshed in the morning. The explanation of its limitations may be in the fact that melatonin is produced downstream from the SCN, and as such cannot be considered a universal sleep hormone that affects the root of the sleep onset mechanism. Melatonin produces phase shifts along its unique PRC:
However, it is not clear to what degree the phase shifts induced by melatonin are a result of the direct impact on the SCN where most of the receptors for melatonin reside, and to what degree it is a result of the phase shifting impact of the arousal in earlier waking (in evening administration). Whatever the answer, sleep induced with melatonin is not likely to be physiologically equivalent to natural sleep due to the bypassing of some of the stages of the circadian control. I guess it might be compared to sleeping in a slightly earlier phase with corresponding changes in the sleep structure.
Doses of up to 0.3 mg raise the serum melatonin to its natural nocturnal level. The half life of melatonin is around 40 minutes, which is important to know when timing melatonin administration to induce a circadian phase shifts. Side effects of high dosage of melatonin (above 1 mg) include cognitive impairment, drowsiness, nausea, headaches as well as troubling dream imagery. It is not clear if the negative impact of melatonin on cognition is caused by its effects on sleep structure or a direct effect of melatonin on the brain and/or other tissues. High doses might be counterproductive as they could produce phase delays caused by prolonged action on the delay side of the PRC that begins pretty early in the subjective night. Needless to say, for the same reason, additional administration on a sleepless night would act in opposition to the desired effect as compared with a timely evening use.
Melatonin can be used to remedy phase shifts in DSPS, however, it cannot be considered a cure. For its effects to continue, it requires continuous administration. The withdrawal might actually worsen the symptoms due to various downregulation issues and suppression of the endogenous release. Total sleep time does not increase while the subjective alertness may actually drop (Sack et al. 2007). Melatonin has also been considered for morning administration in ASPS, however, considering its impact on cognition, this application would almost certainly prove highly controversial.
For a creative individual, melatonin should only be used when absolutely necessary (e.g. in order to generate phase shifts needed to maintain a schedule needed to function in society). The timing and dosage are essential for the therapeutical effect. Those must be consulted with a qualified professional!
Smoking destroys sleep, destroys health and kills many good people all too early. Do you think Obama is a cool and rational customer? Note that he is in a powerful grip of nicotine addiction. Even though he claims to have quit just in time to be president, rumour says he still takes a secretive puff from time to time.
Did you hear of a great method for quitting smoking called the "incremental withdrawal"? Probably not. I coined the term on the spot. However, I saw many people succeed using this method. This is how it goes:
- Count exactly how much you smoke per day. Let's call that number Baseline Count
- Take 95% of the Baseline Count and call it your Current Count
- Every morning, make sure you put your Current Count into a cigarette box. Plan it as you wish, that box must be all you smoke on that day!
- Do you feel any better on your 95% level? Do you feel you could sustain this indefinitely? If you feel strongly about those questions, reduce your Current Count by one.
- Stick with your Current Count until you are really in a mood to make another step forward. Remember that reducing Current Count by more than one is risky to your resolutions. Remember that it makes sense to go through a whole constellation of days in your schedule (e.g. workdays vs. weekends) before you reduce your Current Count. Don't leap ahead before you are sure.
- Go back to Step 3 and stay in this cycle until your Current Count drops to zero. Remember that the key to success is always feeling great about the procedure. You need to sense progress, and enjoy it without suffering too much of withdrawal consequences.
I like the incremental withdrawal method because it increases the chances of success. In addition, cold turkey is not without risks. Quitting is always great for your cardiovascular system. However, withdrawal can put a tremendous stress on that system. It can actually kill you! I favor methods that fiddle less with dangerous aspects of human physiology. If you really want to prove your strength, go fast through the incremental procedure instead of quitting instantly. Quitting cold turkey is not only risky, it also increases the chances of a relapse. This is due to the immortal maxim: Easy come, easy go. Some more tips: How to quit smoking?
If you still cannot live without nicotine, Nicorette chewing gum may be the simplest over-the-counter way to tackle the addiction without the carcinogenic effect of cigarettes. Obama swears by Nicorette. Still Nicorette may even be more addictive than cigarettes on their own, and the short half-time of nicotine may result in overnight craving that disrupts sleep!
Remember that smokers usually experience a shallow and unrefreshing sleep. Smokers get less REM than non-smokers! Even though nicotine make you feel more creative, without REM sleep, your creativity and problem solving capacity will inevitably drop. Don't get fooled by the transitory effect of nicotine injections! You will be less smart in the long run! Nicotine will improve your alertness by acting on cholinergic receptors in arousal areas including an important sleep center, basal forebrain (see: Why do we fall asleep?). However, it will also cause night time withdrawal effect that often results in premature awakening. Do you often wake up after just 2-4 hours of sleep? If you quit, you might leave those premature awakening behind.
Interestingly, only 4% of users of SuperMemo are smokers (source). In addition, users who smoke spend much less time on learning with SuperMemo (an average of about 10 minutes per day as compared with the usual average of around 30 minutes). This is more related to the hormonal balance in the brain of a smoker than to smoking itself. Smokers simply do not have the patience for SuperMemo and are less likely to be in-depth learners. Yet there are strong indications that those who quit smoking show improvement in their perseverance in learning! That is one more reason to quit!
Exercise is good for sleep
If employed skillfully, exercise is a blessing for sleep. Exercise is good for health, and whatever is good for health is also good for sleep. Exercise is known to enhance deep sleep and promote the nocturnal release of growth hormone, which has been found to stimulate memory consolidation via its impact on protein synthesis. Exercise, deep sleep and GH have all been linked with neurogenesis (i.e. brain growth). A good night sleep following exercise causes an increase in release of BDNF and an increase in nerve cell proliferation (for example see: Running increases cell proliferation and neurogenesis in the adult mouse dentate gyrus (Van Praag et al. 1999), Sleep deprivation reduces proliferation of cells in the dentate gyrus of the hippocampus in rats (McGinty et al. 2003)). In other words, exercise builds both muscles and the brain! (Gambelunghe et al. 2001).
What is the best time to exercise?
There are different forms of exercise: the type, the intensity, and the timing will affect sleep. The number of possible permutations is huge. It would take another article to list them all. Read about your favorite form of exercise, and experiment on your own! I tend to favor exercise at the descending slope of the circadian curve, either before lunch or dinner, or in the early evening. This choice is dictated by the wish to preserve the period of highest alertness for creative work. In addition, peak alertness times are not necessarily peak physical performance times that come at a slightly later phase. Metaphorically, the brain wakes up faster than the cardiovascular and, most of all, musculoskeletal system. Exercise before a siesta-time meal can deepen the mid-day nap as long as there is sufficient "cooling" period. Late afternoon exercise might be best for exercise's sake. Endurance and strength tend to peak at that time while the chances of injury are lowest (one theory says that it is due to the least catabolic testosterone/cortisol ratio). Early evening exercise, if not too intense or injurious, will do wonders to the quality of night sleep. Moreover, evening exercise seems like a good filler for the time when the brain is already winding up its capacity for mental effort. Exercise may increase the demand for sleep even more than learning. However, high adrenaline, competitive, emotionally charged, or injurious exercise that comes too close to the subjective night sleep could be disruptive and reduce the quality of sleep. Also, with some exceptions, it might not be as healthy as exercise at a more suitable circadian phase. Late exercise may increase the risk of injury. It can also result in contradictory signals for the cardiovascular system that is also supposed to wind down for the night. Exercising before sleep is little less contradictory than exercise after a meal. This way a late evening marathon should be discouraged, while some calisthenics or moderate body building might be encouraged. Again, your own experimentation is essential. If your way of exercise feels great, and your creative work does not suffer, and you sleep great, chances are you are doing all things right, and you are more likely to persist in your exercise regimen for psychological reasons. Early morning exercise is great for people who battle with sleep phase problems. If you find it difficult to fall asleep early enough and need to resort to an alarm clock due to oversleeping, early jogging in bright sunshine can help you shift the sleep phase. Exercise and light are powerful zeitgebers. Naturally, this practise will also contribute to your running out of mental energy earlier in the day, which, at times, is exactly what you need to remedy sleep phase problems. Republican presidential candidate Ron Paul confessed that the best time for exercise for him is the very early morning. He regularly takes long walks at the start of the day. This may work for him as walking is not injurious and can indeed be taken on very early in the morning. Moreover, for a politician, this exercise does not need to quarrel with his creative regimen. Walking is great, for example, to reiterate main talking points for an evening debate. Paul's example shows that there is no one-for-all cookie cutter rule for everyone. Late evening exercise, for example, tends to delay the sleep phase and might be helpful in people suffering from ASPS.
Formula for best exercise
If you anticipate your exercise with enthusiasm or even euphoria, you know that you chose the right type, timing and intensity. If you feel pleasantly exhausted, if you fall asleep fast, and if your sleep is deep, you know that you are doing things right. If you cannot drag yourself for a jogging, try walking, or swimming. If water is too cold, perhaps try swimming in an indoor pool. If swimming is not challenging enough, try the adrenaline of team sports. Or perhaps a social setting of a gym will suit you better. If exercise worsens your insomnia, try it earlier, change the intensity, or the type of exercise (e.g. to one that is less injurious). Read relevant exercise tips (rehydration, preventing injuries, etc.). Exercise can be and should be addictive. Many people hate exercise because they never tried it properly. If you are one of those, try again, perhaps with a personal trainer (for a while). People hate exercise only if they do not know how to exercise!
Sleep is good for exercise
It is well-known, at least amongst body builders, that sleep is necessary for muscle growth and repair. If you do not get enough sleep, your body building effort will be ruined. The muscles need sleep, but we do not sleep because of the muscle needs. For the organism to cope with muscle regeneration, there is no need to shut off the central nervous system, and make one unconscious for a third of one's life. If REM paralysis was to play this role, it could be enforced at the level of the medulla oblongata without making us unconscious. If growth hormone secretion was to play a role, it can also be upregulated in abstraction from the state of the central nervous system. There are many other benefits of sleep for muscular regeneration but none will require the state of unconsciousness on its own. For evolution, using sleep for muscle regeneration would be as sensible as shutting down the government in order to fix a highway. The universal belief that sleep evolved to promote rest and regeneration comes from the feeling of being "broken down" and "unrefreshed" once you do not get enough sleep. However, you do not feel crushed after an all-nighter because of the damage inflicted by the lack of sleep. Your state is simply your body's own defense against not getting enough sleep. You can cheat those defenses to a limited extent. One night of good sleep, and your body seems to be back to shape. The only true damage inflicted by sleep deprivation is to the fabric of memory. Unfortunately, this damage is imperceptible, and the universal perception is that sleep is cheap and can easily be dispensed with. Whether you sleep for the sake of your memory or for the sake of your muscles, sleep is good.
Sex and exercise differ
Someone noticed that I contradict myself recommending sex before sleep and saying that exercise directly before sleep is not recommended. One only needs to observe that hormonally sex and exercise differ like chalk from cheese, the degree of stress in sex should be negligible (at least in a stable and harmonious relationship), chances of injury are not too high, etc. This kind of exercise before sleep I wholeheartedly recommend.
Many sleep experts say that a bedroom should only be used for sleep and sex. They also imply that there should be no TV in the place where you sleep. However, this advice seems to stem from a futile battle against sleep-onset insomnia that is so often caused by sleep phase problems. If you go to sleep very early, and you are not sleepy enough, it is quite natural that a TV or radio could provide a distraction or even wake you up prematurely (e.g. with an annoyingly loud advertising). The effect of a TV sound may be quite different when you sleep in the right phase (e.g. in free running sleep). In those circumstances, you go to sleep when you are really ready. If you run a pre-recorded material, and set the timer to turn the TV off in 5-15 min, you might be actually doing your sleep a good service. An important thing for a good night sleep is to leave all issues of the day behind. Even pleasurable thoughts related to your life can keep you up and excited. At the same time, repetitive news from foreign lands or a moderately interesting science program can effectively lull you to sleep in 2-4 minutes, which should be your healthy target. Some TV or radio news for an adult can be compared to soothing music for a newborn or a fable that mom reads to a child before sleep. This has even become a part of bibliotherapy. Many people read themselves to sleep, which is a good idea (as long as passionate reading does not go on till morning hours). An audio-book might also be effective in a different way. It is a very personal issue. You need to test it for yourself and avoid the dogma. If you need to wake up early and you have problems with falling asleep, you may follow the conventional advice. However, if you can afford to run your sleep free, you should go to sleep only then when you are truly sleepy. In those cases, to TV or not TV, is really not a question. You can get those 2-3 min. of news, or just fall asleep in absolute silence. It is up to you.
Marijuana is a well known sleep "remedy". It is particularly popular among DSPS sufferers who claim it helps them go to sleep earlier. Unfortunately, research seems to indicate that cannabis changes the structure of sleep (e.g. reducing the proportion of REM sleep (Feinberg et al. 1975; Fujimori and Himwich 1973), which can also be expressed by particularly lucid dreams in withdrawal). This results in lesser sleep efficiency and possible premature awakening after the administration of cannabis. Due to the suggested impact on the release of melatonin, soporific effect and possible premature awakening, cannabis might seem like a remedy that might stabilize the circadian cycle in DSPS. However, this stabilization would be achieved at the cost of quality of sleep and productivity. Subjective sleepiness reports seem to indicate that indeed marijuana smokers do wake up much less refreshed. Moreover, they experience unusually high energy levels and rich dreams in withdrawal, which is an indication of the negative impact of the drug on the quality of sleep.
In abstraction from other potential negative health effects of smoking marijuana, it should definitely be avoided in the "protected zone", i.e. in the hours preceding sleep. In that respect, it is not much different from alcohol or benzodiazepines, which are also well known to affect the sleep structure and the efficiency of sleep. The same rule applies in all these cases: treat your brain before sleep no worse than you treat your brain before creative work. Whatever is bad for creativity is likely to be bad for the quality of sleep.
Dr Buzsaki spoke in an interview: "Timing and network synchronization are the essence of all cortical computation, and the timing ability of cortical networks is reflected in the rhythms they produce. We have shown that deterioration of synchrony of hippocampal assemblies, e.g., induced by the active ingredient of marijuana, is reflected quantitatively by the field rhythms. In turn, the degree of impaired hippocampal oscillations is correlated with the deterioration of memory performance. [...] Oscillations constitute a robust phenotype that reliably 'fingerprint' an individual and expected to alter in most psychiatric disorders. Often such changes are most pronounced in sleep".
Sex is good for sleep, however, using sex as a "sleeping pill" may not be too good for sex itself. For circadian reasons, morning sex should be best (in free running condition). Testosterone peaks in the morning. However, sex is a powerful hypnotic, and morning sex may undermine morning alertness. On the other hand, sex before sleep is likely to help you fall asleep faster. Evening sex may be less "efficient". You are more tired and perhaps not in a mood. But sex is a great soporific! Sex is a very personal thing, but I believe that creative people perform better if they sexercise before sleep or at siesta time.
If you practice sex without procreative intentions, positive influence of sex on sleep may be your number one excuse for sticking faithfully to your conjugal duties. Here is also a recommendation to stick with a single partner. Longevity studies show that healthy stable monogamous sex life is one of powerful life expectancy determinants (even though, in this case, monogamous should stand in opposition to promiscuous rather than to polygamous). While monogamous sex is generally good for sleep, sex with your new great love may actually disrupt sleep. Apart from a healthy dose of endorphins, it will also raise your catecholamines that may fragment sleep cycles. For the same reasons, promiscuous sex may also fail to play the expected hypnotic role.
Many highly creative people opt to sleep in beds that are separate from their partners. This approach may undermine family cohesion and sex life, however, it is pretty understandable, esp. in people who love to burn the candle at both ends. Co-sleeping is probably a better choice healthwise as long as it does not affect the quality of sleep. In addition, both partners should sleep in similar hours and forgo the alarm clock. This is a hard personal choice that needs to balance a healthy tradition with the quest for productivity. I do not think I can recommend one choice over the other. It is too complex and too personal.
Foods that we eat affect our alertness, and our propensity to sleep. However, the role of foods is largely overappreciated. For example, it is very difficult to significantly affect the circadian cycle with basic foodstuffs. It is the timing of meals that may matter more. Homeostatic sleepiness can be enhanced with some foods, esp. when consumed in larger amounts. However, this is highly individual. For example, your glucose tolerance will determine the effect of glucose-rich foods. Your ability to metabolize alcohol or caffeine will determine the degree of the effect of these two frequently consumed mind-altering substances. Your food intolerance and food allergies can have a big impact as well. Your current satiety status, rehydration, caloric needs, etc. also play a role. Ages old recommendations, such as "a glass of warm milk before sleep" will only play a marginal role in helping you sleep well. It would take a separate article to describe all nuances and possible interactions and synergies. I will therefore limit this section to the following basic rule-of-the-thumb mnemonics:
- whatever is good for health is also likely to be good for your sleep (at least in long-term). Healthy body will ensure healthy sleep. Consult your favorite healthy diet book or website. Keep learning! You need to virtually memorize an encyclopedia of foods to truly understand all nuances of different diet choices.
- if you are on an obese side, avoid meals in the last third of your waking day. For hormonal reasons, this will help you keep your weight in check, and help you sleep better.
- even if you are on a weight loss program, before creative work you need foods that will gradually release glucose into the bloodstream while you work in high gear
- avoid alcohol and caffeine in the last third of your waking day
Variations in the healthy diet are unlikely to have a major impact on your sleep. You may only want to watch caffeine, alcohol, exotic herbal products, toxins, and all substances with a substantial effect on the nervous system. Otherwise, the diet should have only a minor impact on the demand for sleep, circadian patterns, homeostatic sleepiness (with a major exception of caffeine), progression of sleep stages (with a major exception of alcohol), or neural efficiency of sleeping. The reason for this is the same as in many other cases of homeostasis: the organism is striving at retaining the homeostatic balance throughout all systems. Rare foods, herbal preparations, pharmacological intervention, etc. can always change or unbalance internal equilibria, but a standard healthy diet is far less likely to do so. It takes an extraordinary nutritional error to stop the human heart. It is even harder to stop the gene-based body clock.
Lots is being said about vegan or vegetarian diets in reference to sleep. It is not true that herbivores sleep less, as there are many exceptions to the rule (there are herbivores that sleep three times as much as short-sleep carnivores). There is, however, a correlation, which says that the decrease in sleep time is faster with the increase in weight in herbivores than it is in carnivores. In other words, heavy herbivores, like giraffe, indeed sleep very little. This correlation may be explained by changes in metabolism, but it could also reflect a different lifestyle. A predator may eat once and then spend many hours on digestion, an elephant keeps munching all day long to sustain its energetic needs, while a gazelle needs to maximize vigilance to ward off an attack from a long sleeping lion. The correlation between the diet type (herbivore vs. carnivore) and the length of sleep links sleeping habits with eating habits of a species, not with eating habits of an individual. While humans are omnivorous, you won't become an herbivore, and allegedly a short sleeper, by enforcing new eating habits.
Sleep and glucose metabolism
While changes to a healthy diet do not have much impact on the quality of sleep, sleep has a powerful impact on the metabolism. sleep deprivation research tells us that adequate sleep is particularly important for healthy glucose metabolism. Sleep deprivation, shiftwork and jetlag all facilitate obesity and the development of type II diabetes. The possible reason is that sleep deprivation decreases leptin and increases ghrelin for the same caloric intake (Knutson et al. 2007). Those two hormones control the appetite and affect the homeostatic set point for the body fat level. In sleep deprivation we tend to eat more and achieve satiety at a point which will increase the body fat percentage. In caloric terms, those changes can be pretty dramatic. Halving one's sleep might increase the demand for food by 1000 kcal per day. There are also indications that the appetite switches to substantially favor high carbohydrate foods. This only magnifies the problem. Last but not least, sleep deprivation simply makes you lazy. You are less likely to expend those extra calories.
For more see: The Dream Diet: Losing Weight While You Sleep.
Most nutritionists will tell you that weight loss is easier if you avoid larger meals in the last third of your day. Others claim this is a myth. Epidemiological studies, as always, do not provide a clear cut confirmation. Ramadan fasting seems to favor weight loss despite nighttime eating. Controlled studies also provide seemingly contradictory outcomes depending on the design. The conviction that evening fasting might be beneficial probably originated in the 1970s when weight loss programs were shown to prove more effective when meals are eaten in the first half of the day, as opposed to the second half (for a discussion see: Weight Loss is Greater with Consumption of Large Morning Meals and Fat-Free Mass Is Preserved with Large Evening Meals in Women on a Controlled Weight Reduction Regimen (Keim et al. 1997)).
I believe that if you try evening fasting for yourself, you will quickly discover that it can do wonders to your sleep, its restorative powers, your weight loss targets, your morning energy, etc. Unless you are in this ravenous group that cannot sleep without a nighttime trip to the fridge or at least an evening snack, you will also notice that for circadian and psychological reasons, evening fasting is pretty easy to sustain once you get the hang of it. Fasting promotes the release of ghrelin (Bloom et al 2000), which contributes to the overall nighttime increase in the release of growth hormone (Norrelund 2005). Most of growth hormone release occurs in deeper stages of NREM sleep early in the night. This nighttime release is partly responsible for the anabolic mode of early sleep that helps you avoid abdominal obesity, strengthen your bones, rebuild your muscles, tendons, ligaments and other tissues subject to daytime wear and tear. Hormonally, evening fasting produces effects similar to those of overall calorie restriction, which has been shown to prolong life in mice. Older people seeking their youthful past may resort to growth hormone injections. Evening fast combined with a healthy free running sleep is definitely a healthier and simpler option. Try it for yourself, and if you have any doubts, please write to me.
If you are an insomniac or suffer from DSPS, you should also consider evening fasting as a factor that might help you maintain a healthy sleep schedule. See: Curing DSPS and insomnia. On the other hand, if you are troubled by early awakenings and short nights, you might defy a conventional nutritionist advice and listen to Seth Roberts who says the reverse. Roberts found that skipping breakfast helps him maintain a healthy sleep phase (Roberts 2004). Thousands of people follow Roberts' advice without realizing that a majority of them are likely to be at the DSPS end of the phase disorder spectrum, and his advice, while well researched and ideally suited for him, may have the opposite effect in their own case. Remember therefore that your fasting choices as well as other lifestyle changes that affect your sleep must be chosen to fit your chronotype.
Combating jetlag with diet
There is some evidence that rats can entrain their cycles to food with the help of the DMH, however, using starvation to combat jetlag is only a theoretical concept. The SCN rhythm is not maleable beyond minor phase shifts, and losing synchrony between the SCN and the DMH, if at all possible in humans, is not likely to be a good thing for health, esp. that humans do not seem to have evolved a mechanism to subject sleep to the timing of the availability of food. If you happen to have any success in combating jetlag with the timing of meals, please let me know.
Few things can be as tiring before sleep as a dose of heavy learning. However, a leading sleep expert, Dr Dement, in his guide to better sleep suggests: "Avoid heavy studying or computer games before bed, they can be arousing". This advice needs a slight amendment. There is no doubt that computer games are arousing and should be avoided. However, "heavy studying" may have many forms. If you study for an exam, and this brings stressful images of the exam itself, it can indeed be arousing. If you study a fascinating subject that monopolizes your thoughts, it can be arousing as well. Similarly, learning in a brightly lit room may slow down the descent to sleep. However, if you extract the pure learning process devoid of stressful associations, light, social aspects, etc., you will come to a different prescription.
Learning should help you sleep
The more you learn on a given day, the lesser your capacity to learn more (see: Learning overload). For that reason, the more you learn, the faster you will get seriously sleepy. However, you will not be able to sleep well until your circadian subjective night arrives. This means that you can advance your bedtime only slightly, e.g. by 20-60 min. You cannot generate multi-hour phase shift with learning!
If you want to use learning as a form of getting tired for sleep, and you do not mind the learning process to be less efficient, here are the suggestions:
- select some unexciting learning material (e.g. your French vocabulary could pass the test, unless you plan an exciting trip to Paris)
- subtract the high priority material (learning in a drowsy state can negatively affect the learning process in that subset)
- make sure that your monitor and your room are not too bright. Otherwise you may impact your sleep phase that will make sleep harder on the next day
Remember that learning in a sleepy state is actually a violation of the learning hygiene. Science has not yet conclusively answered the question if this is good or bad for your memory in the long run (Wozniak 2002).
Does learning increase total sleep?
Even though you may hear from me often that learning increases the demand for sleep, I have not been able to demonstrate the fact with SuperMemo data! I simply repeat what other scientists keep saying. Learning should indeed increase the demand for neural optimization in sleep, however, this may as well be done by increasing the intensity of processing (e.g. by increasing the density of REM sleep). Heavy learning may not necessarily increase the length of sleep. Learning may also be like exercise, it does not contribute much to the baseline demand. If you do not learn with a textbook, you still keep learning by noticing things, by thinking, by talking to people. If you do not exercise, you still burn lots of calories. It seems easier to prove that heavy exercise results in longer sleep than to prove that heavy learning increases total sleep. I have been able to show that learning contributes to homeostatic sleepiness. As such, it should contribute to earlier bedtimes and longer sleep. However, I still have no data to show it. For more see: How learning affects sleep?
How long should we sleep?
Optimum length of sleep
In this busy modern world, every minute of time seems precious. For some people, the bottleneck resource is time (not money, material resources, people, etc.). Time becomes a limiting factor, and everyone looks for ways to decongest one's life. As sleep takes a third of our lives, a widespread ignorant solution is to cut down on sleep to economize more time for work. This might work in a short run for someone who needs his legs or arms more than his brains. It will definitely backfire for those who use their brains as the primary tool. Even for someone who believes he can accomplish more on limited sleep, life must feel like a race without the rays of happy sunshine.
Millions of young lives wasted on the fronts of World War I must have equalled to less of a meaningful contribution than a few hours of programming on the part of Tim Berners-Lee. Imagine all those lives spent on more productive pursuits! By cutting down on sleep, we undermine our chances for a meaningful creative contribution.
Recommended 8 hours of sleep
Sleep researchers often look for a recommended amount of sleep. Using surveys or lab sleep data, they often come with a recommendation of 8 hours of sleep per night. However, this recommendation opens a minefield of problems. To get their 8 hours, some people may wish to go to sleep too early and thus exacerbate their insomnia and related stress. The 8-hours-per-night recommendation is also scorned by some researchers who promulgate the false claim that sleep is like food and we will always want more even if we do not need it.
Energy conservation theory of sleep is patently wrong. Benefits of sleep, unlike the benefits of food, cannot be accumulated in advance and there is no evolutionary advantage in getting more sleep than necessary. An all-nighter will be as painful after a month of oversleeping as it is after just 2-3 nights of good sleep. Conservation of energy is minimal, and the brain may actually use more oxygen during some sleep stages than when working on a complex task. Even though lions might sleep 20 hours per day when there is shortage of food and water, humans, in normal circumstances, can only binge on sleep after periods of sleep deprivation, or when sleeping in a wrong circadian phase, or when they experience health problems. In theory, neural network optimization could benefit from some additional sleep, however, the brain does not seem to crave that extra optimization. Just the opposite, it clearly looks for the optimum balance, and attempts to fill all the remaining time of the day with pursuits based on the learn drive.
In the natural creativity cycle, the brain clearly goes for the optimum amount of learning combined with the optimum amount of consolidation and restructuring. Sleep control mechanisms are set to make sleep last for a limited period of time each day, even if we tried hard to get more sleep. Not only there is no advantage, there are huge costs to sleeping too much: we are most vulnerable and defenseless in sleep. Even though sleep can be compressed, proving it is not perfectly efficient, there are no natural and healthy methods of sleep compression. The best sleep is accomplished when all circadian, homeostatic, genetic, and neural mechanisms run in synchrony at the right time on a prescribed course. This can only be accomplished with free running sleep. With dozens of SleepChart submissions, I can demonstrate easily that once a regular sleep schedule is adhered to, the total amount of free sleep drops and becomes pretty steady. In the exemplary graph, a DSPS subject runs her sleep free and gets on average the same regular recommended 7.9 hours of refreshing sleep per night even though, before running free, she was convinced that she needed 9 hours, and that, even on 9 hours, she would still be tired throughout the day:
Saving time with polyphasic sleep
Uberman sleep schedule was proposed with a view to gaining more hours in a day. Polyphasic schedules are very appealing in theory, and many people tried them out just to give up within a week or a month (depending on the ability to suffer through the mental misery). Those who try to adjust to any unnatural schedule will suffer an unspeakable torment of the mind. Polyphasic sleepers regulate their sleep with an alarm clock until they reach the breaking point. Human self-experimenting guinea pigs collapse into a sound life-saving 5-8 hour sleep towards the breaking point and then resume the polyphasic schedule with a sense of guilt. That sense of guilt is calmed with exculpatory terminology such as "weekend break", "re-energizer", "bonus sleep", etc. Polyphasic experimenters may happen to sleep less but their intellectual performance will be dramatically undercut. Some polyphasic sleep theories are based on the false premise that the body can adapt to any sleeping rhythm. Researchers tried to find a natural polyphasic rhythm that would minimize the pain of sleeping little. For that purpose they have studied the phase response curve of the circadian rhythm, where the impact of various sleep affecting factors is shown to move the sleeping schedule forward or backward. The obvious conclusion is that we can rather painlessly move the major circadian sleepy time little by little in a desired direction. However, a healthy normal individual will not be able to chop the rhythm into a desired number of pieces. Monophasic sleep or biphasic sleep are the norm in healthy individuals. Biphasic sleep is rarely composed of two major sleep episodes. Usually it has a form of a major episode (nocturnal sleep) and a minor episode (siesta). Great catnappers nap when they feel they need to. Often, they can accurately predict when and how much they will need to nap. If you want to minimize time spent sleeping and maximize your learning results: free run your sleep. Polyphasic sleep is not the answer. Get rid of the alarm clock!
Minimizing sleep time
All forms of sleep control with an alarm clock will increase the overall demand for sleep. This means that:
If you use an alarm clock, either:
- you will sleep longer, or
- you will feel more miserable.
Artificial sleep schedules will dramatically reduce your mental capacity. A healthy individual in normal conditions will find it difficult to fall asleep 4 hours after the main sleep episode unless that episode was unnaturally cut with an alarm clock resulting in sleep deprivation.
In free running sleep, once you know your average sleep time and your optimum wake time, try to stick to it religiously. Use SleepChart to find your optima in case your sleep is irregular. Plan your day in such a way so as to be sure that if sleepiness comes earlier, you can hop in to bed in a wink, and while sleepiness does not arrive in time, you can get busy with some low-priority sleep-conducive activities that will tire you until the right time for sleep comes.
Inefficiencies in the sleep control system
There are many factors that might increase the demand for sleep (e.g. learning, exercise, etc.), or shift the sleep control balance to favor sleep over wakefulness (e.g. brain injury, infection, poisoning, hypothermia, etc.). The impact of external factors can be used to illustrate a degree of inefficiency in the sleep control system. Since days are longer in the summer than in the winter we do tend to sleep a bit longer in winter. There does not seem to be an increase in the need for the neural function of sleep in winter. If we sleep more in winter and there is no biological need for more sleep, then it seems that we must be getting either more sleep than we needed in winter or less sleep than we need in summer. As sleep is primarily controlled by the circadian and homeostatic sleep propensity, and the circadian component is strongly influenced by light, variations in the levels of illumination will cause variations in sleep duration. It is conceivable then that we sleep less efficiently in winter (in terms of neural effects per unit time). Equally well, summer sleep might be less restorative. Eskimos cut off from civilizational influences sleep for a few hours more per day in winter. Dr Jim Horne is right saying that in some circumstances we might sleep more than we really need to. However, he goes a step too far when he compares sleep to eating, which makes some people believe that sleep restriction might be beneficial (by analogy to calorie restriction). In conclusion, we need to realize that sleep control mechanisms are not perfect, however, we have not yet come with any artificial and certified ways of improving upon what we were given by the biological evolution. Natural free running sleep is still the best way to accomplish healthy, refreshing and shortest-lasting sleep.
Length of sleep among users of SuperMemo
A survey of users of SuperMemo (SuperMemo World 1994) revealed that the average speed of learning was 243 items/year/minute. Users who sleep less than 7.5 hours learned at the speed of 240 items/year/minute. Those who sleep more than 7.5 hours learned at 256 items/year/minute. Amount of sleep, smoking, and exercise were poorly correlated with the speed of learning. Students aged 28 years old or younger learned at the speed of 264 items/year/minute, while those above 28 years old learned at the speed 179 items/year/minute. Remember, however, that in another study it has been shown that good students learn slower (Gorzelanczyk et al. 1998) because of their higher self-criticism in providing grades.
People who sleep less live longer?
"People Who Sleep Less Live Longer" screamed news headlines in February 2002. The reason for the uproar was a large-scale study by researchers from the University of California at San Diego who found that people averaging 8 or more hours of sleep per night were 15% more likely to die within the 6-year period of the study than those who slept seven hours. The study makes a valuable contribution to our knowledge of sleep habits but conclusions amplified by mass media are not only wrong, they are dangerous! If you decide to cut down your sleep today to live longer, you will certainly achieve the effect opposite to the one desired. It is your body (actually the brain) which knows best how much sleep you need. This might be five or it might be nine hours. We differ a lot in that respect. There are no noteworthy benefits of cutting down your sleep with an alarm clock, and the dangers are well documented. The erroneous conclusions media drew from Dr Daniel Kripke team study come from a typical cause-effect relationship confusion. It is not that long sleep is detrimental. It is more that poor health may increase the demand for sleep. Driven to extremes, comatose and bed-ridden patients will bias similar statistics. People with poor quality apneic sleep are more likely to linger in bed and report long nights. On the opposite side of this spectrum are people with healthy and sound sleep habits that often feel refreshed with as little as five hours of sleep, and wake up naturally before the alarm time. In addition to having adverse health effects, sleep deprivation is a major cause of traffic accident and causes immeasurable damage to nations' creative potential. Even a poorly designed alertness test is not likely to testify to your sleep's quality. This comes from the fact that stress hormones often mask sleep deprivation. However, if you try to learn with SuperMemo after an artificially shortened sleep, you will see that your recall gets worse and stress hormones may improve your sense of alertness, but they will do so at the cost of focus and memory. You will achieve best health by getting as much sleep as your body calls for in conditions that eliminate stress, stimulants, anti-depressants, sleeping pills and the like. Once more, the mass media amplifier is likely to produce confusion and negative ripples that will keep on reverberating for years to come!
As always, some research seems to make headlines, while more thorough meta-analyses don't. In this case, it is probably the self-comforting thought "even if I feel miserable in sleep deprivation, loss of sleep might actually prolong my life!" If we review the literature on the association between the length of sleep and longevity, we are likely to notice that very short sleep, as much as very long sleep, correlate with shorter lives (Cappuccio 2010, 2010). Professor Francesco Cappuccio puts it best saying: "while short sleep may represent a cause of ill-health, long sleep is believed to represent more an indicator of ill-health". If you take an average of the optimum amount of sleep for all members of the population, you will arrive at a specific number that is meaningless for a specific individual. In sub-populations that sleep longer or less than the average, longevity may be diminished. However, for each single individual, the optimum number is the one that is suggested by the body needs. If it is 4 hours or 10 hours, it matters less as long as the number comes from the natural sleep that is not controlled artificially. Moreover, that number will differ from day to day, it will be less before an exciting date, and it will be more after a day of heavy exercise. No one should worry about sleeping 4 hours per day or 10 hours per day, as long as he or she sleeps naturally, wakes up naturally, and feels refreshed.
Epidemiological studies that focus on morbidity and ask "how many hours per night do you sleep?" ask a wrong question! They should rather ask: "Do you artificially modify the timing and the length of your sleep?" Only that kind of question would tell everyone that artificial control of sleep increases morbidity, while the actual length of sleep is largely irrelevant.
Investigating links between sleep length and longevity is not much more useful than my own failed attempts to connect the sleep length with the quality of learning. In case of learning, short sleep produces poor results because of the impact of sleep deprivation on attention, recall, and consolidation. However, long sleep produces poor learning as well because it usually is an indication of something going wrong either with health or with the sleep control systems (e.g. sleeping in a wrong phase, or compensating for prior deficits). Asking about how much sleep we need is not different from asking how many calories we need: it depends on our size, our current fat level, our caloric expenditure, and many other factors.
Jim Horne and Daniel Kripke
Jim Horne and Daniel Kripke are two sleep researchers who seem to stand in opposition to the rest of the field in their prominent claim that sleep is like food and we can get too much of it. They even contemplate the concept that, as with caloric restriction, sleep restriction might prolong life! I mentioned Dr Kripke's research that is often erroneously interpreted as "short sleep prolongs life". It is not that long sleepers die earlier. The obvious interpretation of epidemiological studies is that sicker people often sleep longer. Drawing an analogy with calorie restriction is as weak as proposing a "wake restriction" that might have some unknown benefits, esp. that in the hormonal spectrum of glucose metabolism, wake restriction is more similar to caloric restriction. There is no evolutionary advantage to getting excess sleep due to the fact that sleep cannot be accumulated like fat can (see: Excessive sleeping). Metaphorically, if we compare sleep to garbage collection, there is no advantage in collecting garbage ahead of time. People on free running sleep schedule quickly reduce their total demand for sleep and sleep less than on various forms of regulated schedules. Their mental energy is naturally much higher despite sleeping less. I believe that comparing long sleep to overeating is particularly harmful. It sends wrong signals to teenagers and students for who their brain performance determines their future. Drs Horne and Kripke's main concern is that "scare tactics" employed by researchers who insist on the value of the proverbial 8 hours of sleep may worsen insomnia and stress related to not getting "enough" sleep. These are valid concerns, but these can easily swing the balance too far in the other direction, while there is a golden mean: free running sleep that helps people get exactly as much sleep as is needed. To seek some counterbalance, let me then nitpick at some of Drs. Horne and Kripke's statements and hypotheses to throw some light on sleep needs from the free running sleep perspective.
Here are some of Dr Horne's statements that keep detracting from the value and power of sleep:
- "The amount of sleep we require is what we need not to be sleepy in the daytime". It is hard to disagree. However, I would upgrade that statement to an Information Age status: "The amount of sleep we require is what our brain asks for". This is a higher standard than just "not to be sleepy". After all, it does not take much sleep to feel well-energized for work in a field, or a walk in the park, or a basketball game. It takes more quality sleep to be at one's intellectual best for writing, programming, learning, research, etc.
- "Sleeping continuously through the night is quite a modern invention". That's plain false. Many species sleep for many uninterrupted hours. This most certainly has been the habit of early humans. Natural sleep discontinuities, like nocturnal awakenings or segmented sleep, are a far cry from the forced interruptions inflicted by alarm clocks. Waking up for a specific purpose might have been invented in pre-historic human societies, however, there has never been a compelling need for a frequent use of that "invention". For centuries, even candles were available only for the rich. Frederick the Great might have been woken up by the firing of cannons in childhood, but his contemporaries, thankfully, did not have madly authoritarian military-obsessed fathers. Our ancestors never paid much attention to sleep. They just had plenty of it and it was never a subject of discussions or anxieties that now flood the forums for insomniacs. The true advent of alarm clocks and shredded sleep schedules and sleep problems dates back to the early days of electric lighting that disconnected the modern world from the natural cycle of daylight. Even though alarm clocks are not new, they only entered massive use in the last century.
- "There's a lot of fearmongering about alarms. You hear some funny ideas - that we should be woken up "naturally" by light or at certain points in our sleep patterns". Being woken by light is by far healthier than being woken by an alarm clock. Still, morning light is not necessary to accomplish top-quality sleep. See a case of perfect sleep in the absence of morning light in an octogenarian. Being woken at the lightest phases of sleep is by far healthier than being woken just 15-25 min. later when deep sleep sets in. Most people have experienced the difference and can easily list a couple of symptoms that are indicative of the negative impact of sleep disruption on health. If I am a "fearmongerer" in reference to alarm clocks, it is because I understand the physiology of sleep well enough to know that life without alarms should be a basic human right esp. at younger ages.
- "Insomnia is not really a sleep disorder but a disorder of wakefulness intruding into sleep". A typical cause of insomnia in an otherwise healthy individual is bad sleep timing. People who need to get up early, go to sleep too early in reference to their body clock, and just cannot fall asleep. It is not wakefulness intruding into sleep, but a human wish to impose sleep onto wakefulness. These are modern schedules and expectations that are the root of the problem in a vast majority of cases.
- When criticizing psychological tests used in studying sleep deprivation, Horne says "tests simply measure tolerance to boredom, which shortens with sleep deprivation. Take the stimulation away and the animal will fall asleep". Creative work will not always provide sufficient excitement to combat sleep deprivation. It is not the boredom that is the enemy of a sleep deprived creative brain. It is the mental effort itself. No amount of boredom can put a well-rested brain to sleep. However, mental effort in sleep deprivation may be tantamount to torture. Millions of students in the industrialized nations experience that daily. If we want to measure the impact of sleep deprivation on cognition, we need tests that are both stimulating, and tests that eliminate the excitement variable. Tolerance to boredom does not shorten with sleep deprivation! It is a fresh creative mind that truly abhors mental nothingness! It is our ability to stay awake in the absence of arousing stimuli that really gets shortened in sleep deprivation. Boredom does not induce sleepiness. There is no known boredom nucleus in the brain that would start firing in response to lack of stimulation (other than via disinhibition of the natural sleep control system). Even if that nucleus was ever identified, it would rather form a part of an arousal system. Children, when deprived of stimulation, may become cranky or aggressive. Evolution made us smart because we seek action and low entropy states. Boredom unmasks the underlying sleep deprivation. Whoever nodes off at a boring lecture must have simply not gotten enough sleep in the night. A refreshed mind will instead drift to more interesting thoughts such as an exciting project, problems of the day, or an attractive classmate sitting nearby. My impression is that sleep researchers who bring up the effects of boredom on wakefulness must extrapolate from their own sleep deprived condition. Top quality creative brain does not need a whip of adrenaline or dopamine to stay focused on its creative pursuits. We should all aim at sharp and crisp minds, not just minds that are "not sleepy".
- Dr Horne brings up the Inuit as an example of human capacity to sleep in excess. The Inuit are able to sleep for up to 14 hours in winter. It is true that lifestyle and environment will affect the length of sleep. You can blame our biology for not doing its best of the available time, but nothing natural can be done about this, so we might as well enjoy a full night sleep. When irregular sleepers claim they need 8-9 hours of sleep per night, one might expect them clocking 10-11 on a free running schedule if the "wastefulness of sleep" theory was right. Instead, in free running sleep they often go down to 6-7 hours per night and claim feeling better than ever! As for the Inuit, part of those excess hours might come from the documented wish to minimize the exposure to cold in winter months. Much of that "sleep" might be Wehr's segmented sleep (Wehr et al. 1992)). So we would need to employ polysomnography to see how much of that bedtime actual sleep is. On the other hand, sleeping little in summer can easily be explained by the fact that it is not easy to fall asleep at midnight in bright light, and there is a pressure from the other end of sleep to get up early for a hunt with a stress of possible oversleeping. We know that the sleep control system is imperfect and the actual need for sleep may be masked by arousing stimulation. Inversely, Arctic night might be conducive to leisurely sleep that may exceed the actual needs of the organism. However, we cannot state with certainty that it is the lack of stimulation that makes Inuit wastefully sleep longer. We know for sure though that extra summer stimulation makes Inuit get less value from sleep than they otherwise get in winter days. To settle the issue we would need to apply appropriate cognitive tests in summer and in winter, and compare the two. In particular, I would be interested in comparing the circadian peaks of recall. This is more important than just the recall average as I believe that creative people change the world primarily in their moments of peak mental strength, i.e. shortly after waking up from a refreshing night of dream-rich sleep. In my own work, I did not find much correlation between the length of sleep and the quality of learning. Free running sleep might be slightly longer in winter, however, shorter sleep in summer is often compensated by longer naps. On average, cognitive performance seems to be comparable. SleepChart logs emphatically contradict Dr Horne's claim that we like to sleep wastefully. Free running sleep decreases the total sleep as it increases its efficiency!
- "Researchers in America tend to take a different view. They assert that because most of us can extend our daily sleep, we must need to do so. This would mean that people who seem content with seven and a half hours of sleep a day during the week but enjoy nine hours at the weekend are, unknowingly, chronically deprived, and actually need nine hours every day. Evidence for this is said to come from the many people who are sleepy in the daytime, and the numerous reports of people falling asleep at work and, worse, while driving". Dr Horne's annoyance with the 8-9 hour sleep recommendation is understandable. After all, people have different sleep needs, and the recommendation might compound insomnia. However, sleeping more on weekends than on working days is a clear indication of sleep deprivation. Stress or passion associated with work can easily mask deprivation. That mask falls off on weekends. It is hard to say if 7.5 hours on workdays and 9 hours on weekends equals to a 7.8 hour optimum, or perhaps an 8.5 hour optimum, esp. than those numbers will change from day to day even for very regular sleepers. Everyone should determine his or her needs on their own in free running condition. This condition cannot possibly be approximated by sleep on weekends due to the sleep debt carried from the working week.
Here are some statements from Dr Kripke which undervalue the importance of undisturbed sleep:
- "Nobody seems to know exactly where the idea that we should sleep eight hours came from. I guess it was just passed down from somebody's grandmother". The "8 hour myth" does not come from grandmothers but from surveys in which many people claim that they function best once they get their 8 hour sleep. The actual perfect number may be less because of a common misperception. People who do not get enough sleep during the working week often compensate on weekends when they do sleep more to make up for various sleep deficits. That weekend sleep may be 8 hours or more, but it does not imply that this is what is actually needed. The same people on a free running schedule may be surprised to discover than they actually need just 6-7 hours. It is not uncommon to go down to 4-5 hours per night on a biphasic schedule, where the remaining sleep needs are met with a well-timed siesta nap. My small-sample estimation seems to indicate that people overestimate their sleep needs by 10-15%. Dr Carskadon's research shows that 8 hours may be a good estimate for teenagers who clearly have increased sleep needs. My own analysis of SleepChart submissions also indicates that for teenagers, 8 hours is a solid recommendation! The figure also often shows in other publications (Dinges et al. 2000). When a software glitch in the first two days of 2011 kept iPhone alarms silent, thousands of people realized their capacity to oversleep. Not by just 10-30 min. Some angry bloggers clocked extra 6 hours of sleep illustrating the monstrous degree of sleep deprivation. When grannies suggest 8 hour sleep, they are not that far off the mark!
- Kripke in his editorial in Sleep (February 2004) wrote: "The results [of epidemiological studies] falsify the widely-circulated hypothesis that it is best to sleep at least 8 hours". All epidemiological studies contribute to our knowledge one way or another. However, a study that focuses on morbidity and mortality that asks "how many hours per night you sleep?" is a waste of time! Like a study of the impact of height on mortality, we know ahead of time that it will provide U-shaped results with no bearing on our lifestyle recommendations. The correct question would be "Do you cut your sleep short with an alarm clock?", or "Do you delay sleep despite being sleepy?" or "Do you try to sleep early even if you are not sleepy?" In short, those questions should ask: "Do you artificially modify the timing and the length of your sleep?" Those studies would tell everyone unequivocally that artificial control of sleep is deadly. Length of sleep is largely irrelevant! For more see: People who sleep less live longer?
- Kripke: "Recent results from the Hordaland study in Norway showed that working people who reported sleeping less than 6.5 hours a night did not suffer any significant increase in disability. Those who reported more than 8.5 hours were more than twice as likely to become disabled as those who slept 6.5-7.5 hours. The suggestion from this study was that, if anything, spending more time in bed might tend to increase disability". We already know that on average people live longest if they spend a certain number of hours asleep, or if their height is somewhat above average. But the statement seems to suggest a reverse causality. Trying to spend less time in bed would be as good for longevity as trying to shrink people who grew beyond a certain height. Healthy people sleep less, but those who shorten their sleep are less healthy!
- Kripke: "We know that we do not suffer some horrifying loss of intelligence or memory every time we get less than eight hours of shut-eye. We make deliberate choices to sleep seven hours or six hours, feeling that the practical benefits of having the extra time outweigh whatever sleepiness might result from sleeping less than eight or nine hours". It amazes me to hear a sleep researcher accepting a practice of trading sleep for "practical benefits"! Natural creativity cycle explains how moments of lost sleep may translate to lost insights worth a fortune. If loss of IQ is not horrifying, the death toll on the roads definitely is. There is a neural and health cost to trading sleep for "practical benefits". An irreplaceable physician or a firefighter might do the trade. However, they should always remember that they sacrifice their own creativity, brain and health for the benefit of others!
- In a debate with Dr Stickgold organized by the Economist, Kripke countered Stickgold's claim that insufficient sleep is the cause of traffic accidents: "People claim that daytime sleepiness causes road accidents, when the fact is that the sleep-related road accident rate goes up late at night. It is driving late at night, which causes most sleepiness on the road. People who stay up unusually late, such as physicians called to duty at night, have extra driving risk and risks of mistakes, but that does not prove that the regular sleep patterns of the general population are too short." When two great researchers disagree on a fact that can easily be verified, I can always suspect professional or personal prejudice. Could it be that early riser Kripke feels most impaired when driving in the evening, while owlish Stickgold feels out of sorts in the morning? I had a peek at the statistics and noticed that overwhelmingly the accident rates follow the average of the human circadian sleep propensity. This would mean that both doctors have a point. It is probably a chronotype that will determine the difference in accident chances at different times of the day on average. The circadian phase while driving would be even a better determinant. My guess is that whatever impairs recall or memory consolidation in the waking day will also impair driving. In other words, alertness graphs plotted in SuperMemo could easily be used to determine the probability of a traffic accident while driving at any time of the day. An interesting difference shows up when investigating accident rates across different ages. There is a huge spike in accidents at 6-7 am in middle-aged people. That would contradict Kripke's claim. However, for younger ages, the rates are very high throughout the night. For retirees, who do not need to get up early, and do not need to drive insanely long into the night, the peak aligns with their siesta time. Whatever the truth, Dr Kripke is definitely wrong when using his statement to defend short sleeping. According to National Sleep Foundation, people who sleep less than 5 hours per night are four times as likely to be involved in a crash than those who sleep 8 hours or more. Even those who sleep 6-7 hours double their risk. Obviously, as argued throughout this article, it is not the length of sleep but its quality that matter. Those who get 5 hours in free running sleep need not worry.
Early risers find it difficult to understand the problems of evening type people. They are prejudiced by their own condition. Scientists frequently divide into fiercely opposing camps that are often based on serious prejudices coming from trivial sources. That's good. Discoveries benefit from passions even if they often and inevitably cross rational boundaries. I am seriously prejudiced too. So are those magnificent scientists who I dare to criticise. I bet that when Drs. Horne or Siegel read about new ideas coming from Drs. Stickgold or Walker's camps, they might be mumbling to themselves "Oh no! Not again..." If they ever come to read the presented text, they might exclaim "Who's that [bleep] Wozniak [bleep]!?" That's healthy. I would be honored if they bothered to read.
I bet that some researchers who are short sleepers or early risers themselves tend to extrapolate from their own position to a wider population. Having seen hundreds of SleepChart logs, I know that some individuals definitely need 9-10 hours of sleep and feel bad when they do not get it. Could there be an underlying health problem? Perhaps. However, most of these are teenagers that seem otherwise pretty healthy and good students. I have also seen logs of those who need just 4 hours per night and occasionally feel great on just 2-3 hours! Both extremes are in minority. Most of people do well on 6.5-7.5 hours in free running regimen.
My own prejudice
When standing against alarm clocks and short sleep, I must then delineate my own prejudice.
My love of free sleep does not imply that I do not know how serious sleep deprivation feels. I recall a time at SuperMemo World, in the early 1990s, when working as a programmer against a deadline, I had to go for some 72 hours without sleep. I did not feel brain dead because of the stress and excitement of the job. However, the outcome was near-to disastrous. A CD-R with the finished product was on its way to Germany for mastering and production when a disastrous bug was spotted in testing. That sleep deprivation and the deadline could have been very costly.
Had I not tried free running sleep, I might have been pretty skeptical of Dr Stickgold hyperbole: "sleep deprivation makes you fat, sick and stupid". In the 1980s, when I was in a constant battle for quality sleep, pulling a dozen of all-nighters annually, and nearly always late to bed, I was actually pretty healthy, very skinny (mostly as a result of coming from a poor household), and I ended up graduating with honors. Neither fat, nor sick, nor certifiably stupid. However, my views changed drastically now that I have tried free running sleep for over a decade. I am now much healthier than 20 years ago, and I learn much faster (even though most of that acceleration is due to technology and experience). I might still be in a daily battle to maintain a healthy level of body fat, however, that battle was by far hardest in the early days of SuperMemo World, in the early 1990s, when my sleep hygiene was at its lowest. I believe strongly that free running sleep improved my health and creativity. I believe that it was free running sleep that helped me eliminate the problem of colds and influenza, even though winter swimming might also have been a strong contributor. Most of all, I love to have nearly forgotten how sleep deprivation feels. Nothing undermines creative work as effectively as a bad night sleep.
When looking at my own sleep length over the last two decades someone might see signs of aging:
I rather claim improved sleep quality and improved total compensated sleep value (i.e. when nighttime sleep is added to napping time multiplied by 3). See: Power nap
Figure: Average length of night time sleep decreasing in the course of two decades (Wozniak 2000-2017)
Value of good sleep
In the light of my own experience, let me then take liberties and reword/soften Dr Stickgold's claim:
Sleep deprivation will make you fatter, sicker and dumber!
Robert Stickgold, PhD, Associate Professor of Psychiatry, Harvard Medical School (paraphrased)
I admit! I am severely prejudiced! Having tried good sleep, I cannot possibly think of sleep restriction or artificial sleep control. You do not need to trust my judgement though. If you are not sure, apply Pascal's Wager and treat sleep like God. Your intellectual strength is at stake!
Effects of sleep duration and sleep phase on learning
I started my investigations of the impact of sleep on learning in the early 2000 from the simple intuition that short-night sleep is bad for learning on the next day. After collecting two years of data with SleepChart, I tried to show the link between the length of sleep and the quality of learning. However, that attempt was not successful. My problem was that I used my own sleep and learning data. I am a religious adherent of free running sleep (i.e. sleep where all forms of sleep control, esp. the alarm clock, are forbidden). Upon closer inspection, it appears that in free running sleep, short night sleep is often an indicator of hitting the optimum sleep phase, while long sleep may result from going to sleep too early, heavy exercise, ill health, and other factors. The interpretation is analogous to Dr Kripke's research showing that people who sleep less live longer. That research led many to a wrong conclusion that keeping one's sleep short is healthy. In Kripke's and my own investigations, the confusion comes from the fact that it is the naturally short-night sleep that is an indicator of good health, correct sleep phase, or good prospects for long life. Using other people's data, I could later show that short sleep caused by the use of alarm clock has a negative impact on learning. One can expect the same effect of alarms on longevity. It is now obvious that the length of sleep cannot be used as an indicator of sleep quality in free running sleep.
The relationship of bedtime and the sleep phase is more important than the total amount of sleep. We should always sleep at the time when the body clock says it is the beginning of the subjective night. There is more benefit in 2-3 hours of sleep at the right time (subjective night), than in 8 hours of sleep at a wrong time (e.g. when jetlagged in Japan). Obviously, it may be pretty hard to get 8 hours during the subjective day without a serious prior sleep deprivation.
While doing my preliminary investigations with my own sleep data, I concluded that I could use sleep phase as a much better indicator of sleep quality. SleepChart makes it possible to analyze the data and approximate the optimum time of bedtime. Those predictions are very rudimentary and can be explained by the following reasoning: if you went to sleep at 5 am yesterday, and the lateness was natural, not forced, do not hope that you can fall into quality sleep at 2 am today. In free running sleep, tiredness is always the ultimate judge that tells you when to go to sleep. However, SleepChart can warn you when the tiredness is likely to be homeostatic and an "unreliable predictor" of the optimum sleep time. If you got to sleep too early, your sleep will be excessively long, not fully refreshing, and carrying a risk of premature awakening. If you go to sleep too late, your sleep will be unnaturally short and carry a risk of shifting the sleep phase (i.e. going to sleep even later on the next day). Those observations provide a solid suspicion that the sleep phase could affect the quality of sleep and the quality of learning on the next day. The sleep phase here is the difference between the optimum bedtime (e.g. as predicted by SleepChart) and the actual bedtime. However, when trying to correlate the sleep phase with the quality of learning, I was to be disappointed again. I could not find a correlation between the sleep phase and the quality of learning (e.g. as expressed by grades in SuperMemo). There were two major weaknesses in that preliminary effort:
- SuperMemo uses the concept of midnight shift that allows of registering repetitions executed after midnight with the previous learning day. In other words, in older SuperMemos it was not possible to say if the repetition executed on May 11 took place on May 11 or in the early hours of May 12
- SleepChart used to use simple statistics in predicating on the optimum sleep phase. It did not use a two-process model of sleep regulation. Just plain old averages. It did not even use the phase response curve to register block shifts that can affect the sleep phase. In other words, SleepChart was strong only when free running sleep was not disturbed by factors such as forced delays, exercise, health problems, sunlight, etc.
Both SuperMemo and SleepChart have been vastly improved since. SuperMemo registers the timing of each repetition, while SleepChart relies on a phase response to predict the circadian acrophase. With improved data gathering, I was able to have a preliminary peek at the relationship between the bedtime phase and the learning performance:
Relationship between the bedtime phase and the learning performance. The bedtime phase is defined as the difference between the actual and the optimum bedtime. Learning performance is measured by the average grade obtained while learning with SuperMemo.
As expected, delaying sleep resulted in a gradual decrease in performance. There is far less data on the "advance" side due to the fact that in free running sleep, early bedtime hardly ever results in early sleep, and is more likely to simply entail some unproductive wake time in bed.
SleepChart has become an integral part of SuperMemo as of SuperMemo 14.0 (2008). Some of the findings based on the data collected with those two applications are listed in later sections of the present article.
Sleep block length distribution
NREM-REM sleep cycles take roughly 90 min. A popular myth says that the length of a healthy night-time sleep episode will therefore always be a multiple of 90 min. Another myth says that it is ok to interrupt sleep after a multiple of 90 min. A variant of both myths says that sleep is supposed to last a multiple of a period that is specific to a given individual.
SleepChart displays the distribution of the length of all sleep episodes. That distribution can be used to invalidate the claim that sleep blocks cluster in multiples of 90 min. In the presented example, sleep block length distribution in a monophasic sleeper indeed shows clusters at: 1, 2, 3, 3.5, 4, 5, 6, 7, 8, 9, and 10 hours. However, upon closer scrutiny, this clustering comes only from inaccurate logging by the subject (it is easier to mark 3.0 hour block than 2.95 hour block). There is no 90 min. trend discernible, however, one might be tempted to notice a multiple of 60 minutes.
Here is then yet another example that uses a semi-log scale, which is better for visualizing short sleep blocks of a habitual napper. In this case, a biphasic sleeper shows only one significant cluster at 7 hours of sleep. This cluster was again caused by imprecise logging.
Finally, a sleep block distribution of a regular 7-hours-per-night monophasic sleeper. There are a few peaks discernible, however, no regular sleep length multiple. In particular, no peaks around the expected 5.5 and 8.5 hours.
How to make babies sleep well?
People often say "I slept like a baby". A joke says that it means that you kept waking up every 2 hours to scream. Indeed, babies tend to wake up in the night, and seem unhappy unless immediately soothed with mama's breast. This seems unnatural, unnecessary, and worrying. And yet babies have been designed to wake up many times during the night to feed.
The net is jam-packed with an assortment of advice from and for young parents who seek good sleep for their babies and for themselves. A great deal of that advice is based on myth and/or pseudoscience. If the advice includes the word "train" or "schedule", you need to triple your skepticism! Even world renowned pediatricians overemphasize the "routine" over the actual natural sleep mechanisms. As much as adult's, baby sleep is ruled by homeostatic and circadian mechanisms, and any attempt to override those is futile and potentially harmful. All routines such as rocking the baby, quiet room, feeding, quiet talking, music, etc. are welcome as long as they are not attempts to enforce a sleeping schedule on a baby. These routines are little more efficient in inducing sleep as all the grandma's advice against insomnia. Neither homeostatic nor circadian mechanism is trainable (beyond natural phase shifts, etc.). Babies should sleep on demand (ad libitum), i.e. only then when they are sleepy and want to sleep. Nevertheless, understanding their circadian patterns can be very helpful in assisting the routine. As baby sleep is more complex than adult sleep, you can use SleepChart to see through the chaos. This can help as guidance. Nevertheless, observing the symptoms of sleepiness is the oldest and the best practice.
Some moms claim proudly "my baby sleeps through the night". However, when actual sleep logs or hypnograms are analyzed, this appears not to be true. A great proportion of parents will go to any length to make their baby sleep through the night. All too often, parental convenience and comfort take precedence to baby's health. Many pediatricians are pretty ignorant in reference to the rules of chronobiology, which is not prominent enough in school curricula. Some acclaimed methods are plain scary.
The cry-it-out method must have been inspired by Pavlov's methods in conditioning dogs. There is little doubt that prolonged crying and stress will inhibit baby's development. For example, if prolonged crying correlates with later cognitive deficits, it is, at least to a degree, related to the impact of stress on development (Rao et al. 2004). In the end, Ferber's method seems to serve the parent, not the child. No parent's heart should stand baby's cry, esp. that it is entirely unnecessary.
Sleeping throughout the night
It takes roughly 1-2 months for the baby sleep to align into a rudimentary circadian pattern.
Consolidation of the fasting-associated wakefulness precedes that of the breastfeeding rhythm due to high feeding demands in the first few weeks of life (Odaa et al. 2008).
Figure: An exemplary circadian graph of homeostatic and circadian sleep preferences in the first 2 months of life. The average length of unconsolidated sleep episode (red line, left vertical number line) varies from 0.5 hour at 4 pm to 1.5 hours at 9 pm. There is a slight circadian preference for initiating sleep in the early evening hours (6.2% at 5 pm)(blue line, right vertical number line) as opposed to the morning (below 3% at 6-7 am). Using various sleep consolidation methods (i.e. methods for adding up episodes that follow in a short succession), the preference for the evening sleep may be shown to be more pronounced. The preference for night sleep keeps increasing in the first months of life in line with brain development
Your baby may redistribute its sleep episodes equally in the day and in the night. For an ever-sleepy mother, it can lead to the illusion that the baby stubbornly tends to sleep during the day, and just keeps crying throughout the night. No wonder that many moms keep asking: What am I doing wrong?, How can I make my baby sleep in the night? The short answer is: Nothing! (unless there is an organic cause disrupting sleep)! Sleep in short 1-4h bursts throughout the 24 hour period is normal in newborns, and nothing can be done about it! Moreover, whatever parents try to accomplish that goal is likely to be harmful for the baby.
In the exemplary sleep log below, we can see how the chaos of the first months slowly consolidates into a sleep pattern with a major nocturnal sleep episode and several naps during the day. Around five months of age, a pretty consistent pattern emerges with two daytime naps on most of days. Finally, at around one year old, an adult-like biphasic rhythm develops. The breakthrough usually comes when parents, unaware of the consolidation process, realize that the baby does not want to take the early nap and soon put the kid to sleep only once per day. Multiple naps during the day, at this stage, are often a result of health problems, missing some of the night sleep (e.g. due to early waking for infant nursery), bad "baby sleep management" (i.e. mostly not responding to baby sleep signals), or temporary variations resulting from lifestyle changes (e.g. travel, exhausting play, meeting people, etc.). Unless the infant clearly demands multiple naps, a single siesta nap after passing the age of 12-16 months is probably a pretty safe bet. Some parents try to push the kid to stay awake throughout the day to ensure a more solid nighttime sleep. However, sticking to child's natural preference is always a safer option.
Interestingly, in the presented graph there are 3 months, in which the infant tends to go to sleep very late. Such a sleep pattern may be a worrying prelude to future developmental, psychological or sleep problems. However, in this case it might have as well been explained by lesser resetting impact of morning sunlight in winter months. Spectral analysis of sleep in the first months shows that baby circadian cycle might possibly be quadriphasic with constituent frequencies getting damped over time to develop a typical biphasic rhythm with a major nighttime and a minor daytime crests.
For another example of the crystallization of the circadian cycle, see this one father's effort to map the regularities in his baby's sleep. This particular chart begins at a stage when the baby is primarily nocturnal, but still diurnally polyphasic (4th month). Around the 10th month, daytime napping consolidates showing a quadriphasic mode with nocturnal "naps" consolidated into a single long night-time sleep episode. Finally, around the 16th month, the child develops a crisp biphasic pattern. That transition to the biphasic mode might have been delayed somewhat by parental decisions that often determine infant's sleep slots.
Development of a healthy circadian cycle
There are two main factors that will affect the development of a healthy circadian pattern in a baby:
- Exposure to the natural 24-hour cycle of daylight and darkness, and all the associated daily routines.
- Exposure to mother's circadian routines, amplified by co-sleeping
In many animals, the development and the initial entrainment of the circadian cycle is primarily dependent on the interaction with the feeding mother (Rivkees et al. 1988). Co-sleeping should assist in the development of a healthy circadian cycle. Mother's presence in bed as well as breastfeeding can both act as powerful cues. They act as both PRC-related and PRC-independent zeitgebers (see: How do we fall asleep?). This means that co-sleeping will affect the sleep phase as well as the degree of nocturnal awakening and total sleep. In addition to sensory cues, breastfeeding plays also a hormonal role as the circadian cycle of tryptophan in breast milk correlates with the levels of melatonin in the child's blood (as evidenced by 6-sulfatoxymelatonin in urine; Cubero et al. 2005). This naturally calls into question the practise of collecting expressed milk for later use. The development of a healthy sleep-wake cycle will naturally also depend on the fact whether the mom herself applies the adequate principles of sleep hygiene. Millions of children are forced to sleep alone in their cots. This practise is so widespread in the industrialized nations that we may safely conclude that it does not irreversibly ruin the baby's circadian cycle, but, theoretically, it might underlie the epidemic of sleep disorders in modern societies.
In addition to the postnatal period, mother's circadian cycles exert their impact on the baby's brain already in pregnancy. This adds to the utmost importance of sleep hygiene in gestation.
It remains controversial if nighttime exposure to artificial light can slow down or disrupt the process of the circadian cycle development. Research on the impact of light on the development of the SCN suggests that it is possible to change rhythmicity or sensitivity to light of the body clock. The changes occurring in the course of development might affect the properties of the clock for a lifetime. However, it is also possible that lifestyle can reverse or magnify those changes. The development of the SCN has been studied in many animals and results differ. For example, rat SCN periodicity develops in utero (Altman and Bayer 1978), while the sleep-wake cycle in the SCN in an opossum develops in the first 3 postnatal weeks (Rivkees et al. 1988). The shape of the phase response curve, which lays at the root of sleep phase disorders, may actually be influenced by illumination conditions during the development, at least in cockroaches (Page 1991). Whatever the impact of light and locomotor activity in babies, until we know more, we should always aim at minimizing nighttime exposure to artificial light, and to minimize its luminance.
Despite the usual claims to the contrary, nighttime play might actually accelerate the return to sleep as long as the emphasis is put on physical as opposed to the emotional. However, as motor activity is also able to phase shift the circadian cycle, nighttime play on demand should probably be minimized.
For more more see: Polyphasic sleep in babies
Co-sleeping as a circadian solution
The process of maturing the circadian rhythm is neural and largely beyond parental control. However, the entrainment of mom's and baby's cycles is essential, and may determine the ultimate outcome of the process. Newborns are driven to sleep homeostatically, and woken up primarily by their feeding needs. Factors such as temperature, hunger, play, lighting, social interaction, etc. only add complexity to the picture. With a number of hard-to-predict factors that affect sleep needs, babies should always sleep on demand. If they want to play, the play should not be denied. All scheduling in their life should be done around their sleep. This basically means there are only two practicable solutions to newborn's good sleep:
- Recommended (McKenna et al. 2005): Mom and the baby sleep together. A mom needs to learn to adapt to periodic breastfeeding throughout the night. Mom's sleep will suffer a bit. However, with a bit of practice, breastfeeding can be minimally disruptive, and actually act as a soporific for both the baby and the mom. In other words, after a short breastfeeding session, both the mom and the baby should quickly fall asleep contented with the job well-done. Even though the development of the circadian cycle should proceed regardless, co-sleeping should assist that vital biological process. Baby safety issues related to sleeping together must be addressed beforehand (you can google for that in minutes: example). Even a strong one year old can be suffocated easily. If you believe your power baby will scream or wriggle, it is not true! When its chest is compressed or breathing passages get obstructed, a baby will nearly instantly lose strength and be unable to defend itself! This is why safety is paramount. For a healthy and vigilant mother, statistics are good. A bit of anxiety at the start is unavoidable and perhaps good to develop healthy and safe sleeping habits. Psychologists, behaviorists and all open-minded pediatricians will all admit that the night-time bonding is great for baby's brain development. Perfect formula for both the mom and the child.
- Emergency: Parental care in shifts with the help of breast milk. It may happen that for health reasons, the mom and the baby should sleep separately for a while. In such cases both parents can still get excellent sleep if they adapt to a variant of shift-work with the help of chronotherapy. For example, one of the parents chronobiologically adapts to sleeping in the 7 pm - 1 am bracket, while the other takes the 1 am - 7 am shift. In the period when the mom is asleep, breast milk, extracted in advance (or formula) can be used instead of breastfeeding to avoid major disruptions to mom's sleep (remember that milk properties also change along the sleep-wake cycle). If that solution is not feasible for any reason parents will probably need to choose between:
- their own good sleep, and
- employing a nighttime nanny.
Needless to say, baby's sleep should never be at stake here. Sleep is vital for adult health. However, for a baby it is literally a matter of life and death as sleep disruption will have an effect on many causes of newborn mortality.
Best timing for feeding
Unless specifically indicated by a qualified pediatrician for specific health reasons, feeding "on the clock" should be banished.
One of the cardinal sins of parenthood is overfeeding! Here are two scenarios I saw in real life:
- It is easy to observe that after a hefty dose of formula sucked down in a hurry, babies get drowsy for reasons that have little to do with actual sleepiness. It may then be tempting to feed the baby to sleep. Such a practice is bad for baby's health and will not help him or her sleep overnight. It may only assist a short-lasting sleep that will clear the homeostatic sleepiness and make baby even less sleepy than before feeding.
- Another reason for overfeeding is mom's convenience. In hope of getting some more uninterrupted sleep, mom will feed a baby extra formula before the night or during a nighttime feeding session. Again, parent's convenience may be harmful for the child!
Child's own bed
The American Academy of Pediatrics has amazingly come up against co-sleeping! Many moms will swear their babies can sleep alone in a cot without any distress. And yet most behaviorists and anthropologists will agree: the mom and the baby should sleep in close contact as it has been practised by the human race for millennia, and by nesting mammals for millions of years. There could be exceptions on health or safety grounds. However, for an average mom, sleeping with the baby should be a pleasure, a privilege and a duty. The young mom only needs to read about basic safety measures. For a baby, sleeping with the mom should be a basic human right! Many pediatricians, nurses, midwives and "old school" grandmas will still insist that the baby should sleep it its own bed for safety, discipline or convenience reasons. You may hear a medical professional advise: "Don't reward the baby! You are making a rod for your own back". This is very surprising in the light of the fact that baby reward system is pretty well tuned to satisfying its biological needs. An average adult with a number of control mechanisms messed up by the modern lifestyle should often be denied its rewards (a fat doughnut, a morning shot of whiskey, etc.). However, all natural rewards should be considered biologically advantageous for a baby. As for the safety issues, babies do die when sleeping with moms in soft beds, due to alcohol, etc. They do so too when sleeping alone. Simple preventive measures dramatically reduce the risks of the dreaded mishap. The idea that we should train up a child to sleep alone from birth is hard to uproot. One needs to look closely at the biology of breastfeeding and baby sleep to quickly realize that training up at that stage amounts to little less than cruelty (except for cases and moments where the baby does not seem to object sleeping in a cot or in mom's absence from the common bed). A little baby is basically a feeding, growing and learning machine. All its inborn reflexes are targeted at ensuring safety, growth, and brain development. The reflexes involved in rooting, sucking, and breastfeeding belong to the strongest drives in a little baby. A whole series of brain centers is involved and there is a close relationship between these centers, sleep centers, and the sense of well-being and pleasure. Even though the complexity of the mechanisms involved is far from being revealed to our understanding, denying a baby mom's breast and closeness is bound to have long-term developmental consequences. Metaphorically, you could try to put yourself in baby's boots by trying to sleep naked on cold concrete with horror movies blasting loud throughout the night. I might be overly dramatic here. If a baby goes to sleep on its own without much ado, its proximity craving is definitely not as powerful as described here. However, when a baby's growth is at stake, you should always err on the safe side. This is why it makes sense to assume the worst case scenario. I would not be surprised, if over time researchers discovered a need to extend the two-process model of sleep propensity in babies by a factor involved in breastfeeding. It has been shown that a tit is a soporific. I would not be surprised if it worked as an integral contributor to baby's homeostatic sleep propensity, or even a homeostatic trigger, esp. at time where the circadian cycle is not yet fully expressed. Needless to say, babies need sleep even more than adults. The degree of neural growth, network remodelling and learning in a young brain is staggering. Both NREM and REM sleep components are essential in that process. Changing the sleep structure will affect neurogenesis (Stryker et al. 2001). In other words, any form of stress before or during sleep will affect baby's brain growth. This is why baby's sleep should be the zone of highest protection. Training up to sleep alone can wait. Piglets weaned early have been shown to suffer damage to their hippocampus that results in personality changes, fear of exploration, and low stress tolerance.
Many parents oppose attachment parenting as too expensive timewise. It is hard to argue with someone who needs to choose between feeding the family and behavioristically correct approaches. Here again, modern lifestyle encroaches on human biology in a vicious cycle of long-term consequences in which babies brought up using an assembly line approach are emotionally and intellectually less likely to stand up to challenges of reconciling technological and societal progress with the needs of the human body and brain. For more see: Is sleeping with my baby safe? (McKenna 1995) and The Science of Attachment Parenting.
What about the mom?
Is co-sleeping good for the mom? I believe everyone should taste the blessings of free running sleep. However, some of my good sleep advocacy needs to take an exception here as mom's health takes a secondary importance when the baby sleep is at stake. Every mother is equipped with hidden or overt instincts that should make the experience of co-sleeping pleasurable. Naturally, in the modern world, stress, mobile phones, TV, Internet, rat race, and other factors can make it very hard. If a mom claims "I hate breastfeeding", or "I cannot sleep with my baby", she should start from a thorough examination of her own life. Breastfeeding and co-sleeping can be very rewarding if the household is sufficiently sheltered from the storms raging outdoors.
Why babies sleep so much?
Do babies sleep so much because they're learning so much or are they learning so much because they are getting so much sleep? Babies sleep so much because their brains have been designed to do so in the first months of their life. They do learn a lot, and learning does increase the demand for sleep, but this is not the main regulatory factor. Sleep control systems in babies simply work differently, and you probably would not be able to make babies sleep less by making them learn less. On the other hand, long bouts of sleep are used to reorganize neural networks in the brain. In short, sleep helps learning, learning induces sleep, but the whole sleep sequence is a direct outcome of genetically programmed properties of a young sleep control system. Considering the fact that babies spend around 50% of their sleep time in the REM phase (as compared to around 20% for adults), one of the theories says that even when babies do not learn much during the day while exploring their surroundings, that function is filled up by the exploratory function of the REM sleep which helps them discover new properties and rules in things they have learned thus far.
You will often hear that newborns sleep most of the time. Actual measurements may show that babies are more likely to sleep through just half of their days, while spending only a small proportion of that in deeper stages of NREM sleep.
An exemplary SleepChart log of sleep in the first month of life. Sum total of all sleep blocks is displayed on the right and averages to slightly above 10 hours per day with substantial day-to-day variations reflecting the impact of rich homeostatic input changes such as a walk, family visit, diaper rash episode, formula supplementation, etc.
Conclusion: Perfect formula for baby sleep
Babies sleep best if they sleep on demand and if they are fed on demand! All forms of artificial intervention in those homeostatically-regulated patterns should be considered potentially harmful. It is helpful to spot regularities and pay extra attention to baby signals at his or her preferred feeding and sleep times, however, it is the baby's needs that should determine the actual timing. As sleep is vital for the development of neural structures in the growing brain (Stryker et al. 2001), any form of intervention and artificial control should be considered potentially harmful in the long-term.
Problem of insomnia
Insomnia is a difficulty in falling asleep or in staying asleep. Psychophysiological insomnia can often persist for years, and result in untold damage to a person's life. Those who are desperate enough to visit a doctor are often prescribed slipping pills that are usually not much better than insomnia itself! The good news is that in most cases insomnia can be remedied easily with a sleep phase adjustment as described below. Bad news is that such an adjustment may be incompatible with one's desired work or school schedule.
Half of the population in the industrialized nations has problems with falling asleep! This is called a sleep onset insomnia. Except for various underlying organic reasons, the overwhelming majority of cases of sleep onset insomnia result from the inability to entrain one's sleep hours to match the desired waking time.
In other words, most of otherwise healthy people who cannot fall asleep in the evening suffer from the combination of two chief factors:
- going to sleep too early in reference to their natural circadian cycle
- suffering from the associated stress: if I do not fall asleep immediately, I will be totally wasted in the morning
If the same people were allowed to sleep as much as they wanted and go to sleep only then when they are really tired (perhaps 2-5 hours later), in a vast majority of cases, the problem would not exist! Some scientists speak of insomnia as the inhibition of de-arousal processes. In sleep phase problems, the problem of de-arousal does not exist! De-arousal proceeds correctly. It simply proceeds at a later phase.
For the young studying population, the sleep phase problem is the most frequent cause of insomnia. For students who need to get up for school early, their sleep phase is often positioned too late in reference to the desired waking hour. In other words, the optimum sleep time comes too late. Sleepiness arrives too late, and natural waking comes later by the same degree. Such a student will always battle with sleep deprivation when going to sleep late, or a degree of insomnia when going to sleep early. In that sense, there is a physical/biological underlying cause. However, as sleep deprivation is pretty unpleasant, a student may try to go to sleep early (to ensure the night is long enough), but be unable to fall asleep due to the early circadian hour. If this occurs again and again, a psychological component may compound the original problem of insomnia. The recurring sleep deprivation will produce a fear of not falling asleep in time and making things even worse. In other words, in a vast majority of cases the problem is both biological and psychological. The only true remedy is to go to sleep later and wake up later thus being late for school (almost certainly a lesser evil given some understanding on the part of the educators). The only natural half-remedy is to measure as precisely as possible the optimum time of going to sleep, and sticking to that time religiously every day. That optimum time is the earliest time that roughly provides 95% or more certainty that sleep latency will be less than 10-15 min. (i.e. no more than a quarter of an hour of tossing and turning). Very often, this optimum time will provide for a mere 4-6 hours of sleep. However, this sleep is most likely to be the best quality sleep achievable in such conditions. Naturally, affected individuals will suffer a degree of sleep deprivation on a daily basis. This is still better than futile tossing and turning, waste of time, and fitful sleep associated with insomnia. If you suffer from sleep onset insomnia, and you suspect it could be caused by DSPS, you could research additional remedies such as morning sports, strong morning lights, evening melatonin, and radical solutions such as ... giving up electricity after 19:00.
Another type of insomnia, nocturnal awakening, is also often related to going to sleep at a wrong time. People who need to get up earlier than indicated by their body clock, often try to compensate for the short night by going to sleep early. If they succeed in falling asleep, they will often experience premature awakening that is nearly always accompanied by a difficulty of re-initiating sleep. If the same people were allowed to go to sleep only then when they were really tired (perhaps 2-5 hours later), the problem would likely not exist!
If you wake up often during the night, you should identify and eliminate possible reasons, esp. if you appear to wake up tired. The reasons and the way to diagnose them are too many to describe. However, you should always start from the simplest one: problem with the circadian phase. In simple words, the timing of your sleep may be wrong. Partitioning of sleep is a typical symptom of going to sleep too early. If you are healthy, in free running sleep, you will rarely wake up during the night, and if you do, you will fall back asleep fast, and if you won't be able to, the reasons will be quite obvious such as: stress, noise, thirst, cold, full bladder, etc. However, if you attempt to regulate the timing of your sleep, the partitioning of sleep (i.e. interrupted sleep) will be a frequent result. It is possible to push your sleep slightly ahead or back (e.g. 15-25 minutes per day) without this negative outcome. However, once you try to push too hard (e.g. more than an hour per day), partitioning is almost inevitable. If you push backwards (i.e. going to sleep earlier and earlier), you will likely wake up early in the night, i.e. before your circadian low ensures deep sleep. On the other hand, if you push forward (i.e. going to sleep later and later), your circadian low will end before you complete your sleep cycle. As a result, you will often wake up earlier than expected. If this waking up happens very early (when you push ahead very hard), you will be tired enough to fall asleep again. In other words, whichever way you push your sleep, it will not be properly aligned with your circadian rhythm. You will then wake up early or late in the sleep cycle depending on at which end the misalignment occurs. In a vast majority of cases, waking up problem can be resolved by going to sleep at the time when your body calls for it.
The solution for most of cases of sleep onset insomnia and nocturnal awakening is: Go to sleep only when you are truly sleepy! Amazingly, most people do not care to listen to their body. Many struggle with sleepiness to get more life in the evening. Others force themselves to bed long before their optimum bedtime and then toss and turn for hours. This premature landing in bed is at the root of the epidemic of insomnia (even though the official figures put circadian disorders at only 10% amongst the causes of insomnia). The only sensible and healthy time to go to sleep is when you feel you start getting sleepy. If this natural time is outrageously late, see Curing DSPS and insomnia.
Early waking is also a problem for a large number of people. Those people may suffer from the ASPS syndrome. In their case, going to sleep earlier will often be a sufficient remedy. If you happen to wake up early in the morning, your further sleep decision should probably be made on the basis of how fast you believe you would be able to fall asleep. If you do not think the sleep is coming soon, it is definitely better to get up and do some work. This way you will gain in three ways:
- get tired and sleepy faster (work is more likely to make you tired again)
- you will not get anxious about falling back to sleep as soon as possible
- you will not waste time on futile tossing and turning
Insomnia vs. electricity
Insomnia has reached epidemic proportions since the advent of electric lighting. See how lifetime costs of insomnia match the degree of industrialization.
less than 25
more than 80
How to fall asleep faster?
There are tons of lengthy books written about sleep onset insomnia and there are a zillion tricks that people use to be sure they fall asleep "in time". The sad truth is that all those tricks only fight the inevitable: the natural sleep mechanism. They are based on slowing down the brain at the time when the brain simply does not want to slow down. Yet these tricks rather tend to blow the problem of insomnia out of proportion by adding to the sleeper's stress: so much effort, so many tricks in use, and it still does not work... I will probably just have to live with this scourge for ever!
If you follow a conventional insomnia advice (see an example), you will quickly realize that most measures work great at the beginning, and then, when the placebo effect wears off, you are back to square one. Some hotels offer $1000 per night services in curing insomnia in jetlaggers. All those services are a big waste of time and money. Without a phase adjustment, insomnia will persist. It can only be masked for a while.
Here is some typical unworkable advice that you may get from your sleep "expert" or from your grandma:
- Ineffective! Count sheep - this is nothing else than trying to slow neural firing in the brain. This will often work but there is an increased risk of waking up after 20-60 minutes of sleep. As a result, the chances for early slumber may be gone for good. The slowdown in firing can come naturally. However, it can only come at the right circadian time which may be 2-5 hours later than you would want
- Ineffective! Thought dispersion - by trying to "think about nothing" you can indeed increase the chances of falling asleep; however, this may be of value only in a short period that precedes the ascending circadian sleep propensity slope. If you try it early, you will likely hover in a superficial sleep with substantial chances of awakening spontaneously. It is the circadian sleepiness that stands for your ability to maintain sleep
- Ineffective! Cut down on sleep - this has some power to reset the circadian cycle, however, you will feel more tired, your cognitive powers will drop, your insomniac's stress factor will triple, and most of all... your creative brain will not benefit from the last REM-rich hours of sleep! Cutting down on sleep may be an unavoidable solution among those who experience DSPS and cannot free run their sleep
- Harmful! Sleeping pills (esp. benzodiazepines) - pills change the sleep physiology and may affect the quality of sleep. As they often have little impact on modifying the actual circadian cycle, they can quickly become a serious addiction. Additionally, they affect short-term memory and the reaction time. Sleeping pills are a major enemy of a creative individual!
- Ineffective! Warm bath, quiet room, rituals, cup of cocoa, etc. - all these work to reduce the stress factor and slow down the brain. However, again that won't work well if you try it too early in reference to the circadian cycle
- Ineffective! Drink milk or eat turkey (tryptophan is used in the synthesis of melatonin) - this is again a method for a mild slow down with negligible effectiveness
- Ineffective! Take vitamins - vitamins or minerals are a standard plug for whenever something goes wrong with health. For many people, the first question is "perhaps I do not get enough vitamins?" Doubling up on the recommended dose is what often follows. A healthy diet should rarely require supplementation!
- Ineffective! Do not nap - well-timed short naps have little influence on the circadian cycle and not napping may only have a residual homeostatic influence at the cost of evening alertness. It may help slightly but will not solve the problem. Naps should be avoided only at times of major circadian misalignment when the body starts losing track of time and circadian variables start oscillating out of synchrony
- Ineffective! Avoid caffeine - when taken in the morning in reasonable quantities, caffeine should have negligible effect on the quality of sleep. When you are really tired, it is really hard to prevent sleep with coffee. Naturally, giving up the addiction (gradually!) will help a bit and won't do any harm. However, it will never be a solution!
- Ineffective! Herbs, meditation, yoga, aromatherapy, acupuncture, chamomile, relaxation, massage, magnesium, ... - ... the list is virtually endless!
The question posed in the present headline was intentionally provocative. You cannot fall asleep faster, but you can fall asleep fast. All you need to do is to wait for the right time. Instead of trying to fall asleep faster, go to sleep later, and fall asleep fast.
Phase-shift based insomnia
In most cases, the real culprit in insomnia is the relationship of your working hours vs. the circadian rhythm! This is magnified manifold by the associated stress factor. For many, insomnia produces an unsolvable vicious circle that just has to be lived with. However, everyone with a chance for a flex-time work system or telecommuting should realize that the greatest benefit of these may come from increased productivity as a result of better sleep that complies with natural body rhythms.
A very specific degree of morning misery is needed to reset the clock sufficiently in people with DSPS. In the equilibrium state in which misery is sufficient to keep a regular schedule, the whole night sleep is cut substantially. Daily sleep deficit and daily struggle with tiredness result. In such circumstances, it is best to go to sleep only shortly before the expected sleep hour! This way you can reduce stress, on one hand, and help your homeostatic component on the other (by making yourself tired for sleep).
Dr Kripke (see a critical chapter on short sleep) says: "The false belief that people generally need eight hours of sleep is one of the common causes of insomnia. Spending less time in bed is an important solution for many with insomnia". That statement is only partly true. Indeed, trying to get 8 hours of sleep by going to sleep earlier will backfire. It is better to get less sleep when it is initiated in the right phase than to force extra hours prematurely. However, the key is not in sleeping less, but in sleeping at the right time! If you sleep in the right phase and do not need to get up early in the morning, you might actually get your eight hours with zero risk of insomnia.
If you cannot free run your sleep make your morning misery as regular as possible to reach the equilibrium state. Once you know the equilibrium, stick to your standard bedtime hour. Morning misery solution should only be used as a last resort!
There remains the question of weekends. Many people catch up on lost sleep during weekends. This naturally unbalances the system and results in the Monday Morning Blues. Sleeping it out on weekends, you should weigh up your pros and cons:
- on one hand, you entrain your sleeping cycle to later hours and make it harder to stick to your misery equilibrium
- on the other, this is your only chance for quality sleep
There is no simple answer to the weekend dilemma! If you want to maximize the effects of sleep on learning, skills and experience, you would need to quantify how much you lose as a result on never actually getting enough sleep (the losses could be dramatic!) and how much you lose as a result of departing from the misery equilibrium on weekends thus tripling sleep disturbances early in the week.
Free running sleep is simple to define, but a bit harder to execute for beginners. It will often conflict with one's expectations and needs as to the timing of sleep. You will know that free running sleep worked for you if you replace insomnia with no more than 3-5 min. in bed before you fall asleep (without medication).
If you succeed with free running sleep and discover that you can fall asleep in 5 min. as long as you go to sleep at your natural hour, you may discover a new problem. You might show a tendency to wake up later each day. If this is the case read: Curing DSPS and insomnia.
What is sleep inertia?
Sleep inertia is the feeling of grogginess that may follow sleep. There are different types of sleep inertia and there is a monstrous confusion in terminology, as well as a great deal of confusion between different types of sleep inertia in scientific literature. An example of a confusing definition of sleep inertia: "Sleep inertia refers to the feeling of grogginess most people experience after awakening". A more appropriate definition would say "Sleep inertia refers to the feeling of grogginess that is a result of interrupted sleep or other violations of sleep hygiene". Most of all, sleep inertia is not an inevitable part of sleep in humans. In healthy individuals, sleep inertia is a direct result of errors in the art of sleeping. With a religious adherence to the principles of sleep hygiene, you need not ever experience sleep inertia and its negative consequences for learning, attention, health, etc.
All research into sleep inertia should clearly distinguish between its different types:
- interrupted deep sleep - in the exemplary hypnogram, we can see Stage 4 NREM setting in after some 30 min. of sleep. Waking up a sleeping subject at this stage is particularly difficult. When woken from deep sleep, we experience an overwhelming need to get back to sleep. The feeling can be compared to being hit on the head with a heavy object. The brain is in a state entirely unsuitable for processing waking information. This is the torture that many polyphasic sleepers impose on themselves by trying to interrupt "naps" taken in their subjective night phase.
- interrupted REM sleep - in the hypnogram, REM sleep is marked in blue and its occurrence increases towards the end of night sleep. Interrupting fully-blown REM sleep is equally unpleasant. Yet the neurohormonal state of the brain is entirely different than that when the deep sleep is interrupted. One of the easily recognizable hallmarks is muscle weakness. Due to the function of REM, the motor system is turned off in this phase. If you were to test your strength on a hand gripper, you might score less than your kids. Interrupted REM is also frequently associated with dream recall. As soon as the brain returns to its typical waking mode, REM dream memories dissipate very fast. If you do not write down your dream instantly, it may be unrecoverable from memory as soon as 5 min. later.
- waking in a wrong phase - even natural waking can result in sleep inertia. If you wake up naturally in the period of your subjective night sleep, you may feel pretty groggy for a while. The main cause of premature waking is an early bedtime. Sleeping in an early phase is pretty widespread. Many people need to wake up early against their body clock. They will often use various remedies to fall asleep early (from alcohol to sleeping pills). As a result, they will fall asleep early, wake up early, and seemingly get "sufficient" sleep while still feeling tired and unrefreshed. This is because they wake up while their body is still, in circadian terms, in the period of the subjective night. Early bedtime will often result in insomnia. However, when sleep is initiated successfully, the sleep control system can launch an equivalent of night sleep ahead of time. Such sleep may run its course and even last a bit longer. However, its structure will differ, and the morning circadian sleep propensity will still not be fully cleared on waking. As a result, morning grogginess will result as a combination of circadian sleepiness and various sleep deficits caused by a change in sleep structure (e.g. REM deficit). Inertia caused by early waking is far easier to combat than interrupted NREM or REM sleep as it largely dissipates with the expiration of the circadian sleep propensity. However, minor sleep variable deficits may last for the course of the day. For some people, sleeping in a wrong phase is so much of a daily reality that they tend to forget what a crisply alert mind is, and, as in the mis-definition quoted above, they tend to think this is the "type of sleep inertia most people experience", as if it was part of normal human physiology. They might dismiss it by saying "I am just perpetually tired. It is just me". Technically, the definition of sleep inertia should also be extended to the brain state caused by a major delay in bedtime. That brain state is similar to the inertia caused by early waking. It may feel more natural as it combines both components of sleep propensity: homeostatic and circadian sleepiness. Instead of feeling like inertia, it will feel like severe sleepiness that might verge onto nausea. That type of inertia is particularly dangerous for drivers as it only gets worse in time and may results in moments of microsleep when portions of the brain cortex simply enter the sleep mode with the appearance of theta waves in the EEG.
Does sleep inertia show a circadian rhythm?
This question does not have a straight answer. Whatever you read on the subject, make sure you deconvolve the all-encompassing term "sleep inertia" and ask the same question for each of the types of sleep inertia. If you interrupt deep sleep, it will always feel bad. The degree of that feeling will likely depend on the depth of sleep, your homeostatic status and, to a lesser degree, your circadian status (only because deep sleep is largely homeostatic). However, if you interrupt REM sleep, it is more likely to have a more profound effect at the times of the circadian REM peak. Finally, the wrong-phase inertia is purely circadian. It will hit you only in the periods of your subjective night, and it will dissipate on its own at time of your subjective day.
How can I recover from sleep inertia?
You can google out dozens of remedies against sleep inertia (example), and you might be amazed that there is a big wide hole in reasoning behind all that "Internet advice", which often fails to notice that: well-timed sleep is the best remedy against all forms of sleep inertia!
For interrupted sleep inertia, NREM or REM, the simple remedy is: go back to sleep. The more powerful the inertia, the greater your chances of quickly falling back asleep. Remedies like coffee or exercise might make you feel better (or not), but they can do their own damage. If your profession calls for waking up in the middle of the night, remember that you are doing the service at the cost of your own health and longevity.
Wrong-phase inertia is a bit harder to combat. In many cases you won't be able to fall asleep. Even worse, trying to sleep can sometimes make things worse. The best solution is to suffer through the discomfort, avoid napping till your next subjective night period, and go to sleep in the right phase. Most of the time, sufficiently long wakefulness and hitting the right phase will help you instantly synchronize all sleep variables. However, in some cases, circadian ripples may drag for days, esp. if you are not too fluent in computing your correct sleep phase. If you do lots of shift-work or intercontinental flying, it is very easy to be confused about when your subjective night time occurs. In such cases, you could use SleepChart Freeware to get some visual support that makes a guess easier.
Can sleep impair learning?
Amazingly, the confusion into the types of sleep inertia has been responsible for yet another myth: sleep before learning increases forgetting! Well-timed sleep will not cause sleep inertia and will not contribute to a decline in learning. Just the opposite, it is 20-60 min. after natural waking when the learning results are best. Naturally, this is only true in free running sleep. All too often, alarm clocks are used to interrupt the night sleep and the early morning is pretty unconducive for learning.
Why naps cause sleep inertia?
Naps will cause sleep inertia only if they are taken:
- too late, or
- in conditions of severe sleep deprivation, or
- in conditions of REM sleep deficit.
All those three conditions can fool the sleep control systems into thinking that the nap is the opportune time for launching a full-night sleep episode. If an attempt to launch full-blown sleep takes place long before the main circadian low (nighttime acrophase), you may wake up prematurely with the sense that you got an incomplete and unrefreshing nighttime sleep. Such sleep will leave you groggy and will make it harder to initiate proper sleep during the subjective night. To avoid sleep inertia associated with napping then, avoid sleep deprivation in the first place, and read about the optimum time window for napping.
Long sleep and sleep inertia
Many people believe that long sleep causes sleep inertia, headaches, etc. The root cause of problems that follow long sleep is prior sleep deprivation or sleeping in a wrong phase. Unusually long sleep is simply not possible in a healthy individual on a free running schedule. It is usually a severe sleep deprivation that makes it possible to fall asleep well ahead of the optimum circadian bedtime. The unusually long sleep will then carry through the subjective evening and the entire subjective night, adding up to some highly unusual sleep totals (12-18 hours). Such sleep is often followed by a state that is reminiscent of sleep inertia (the "worn-out" syndrome). No wonder it is easy to build a wrong association between long sleep and sleep inertia. It is very difficult to persist in a long-sleep routine, since the sleep-regulating mechanism will quickly bring the length of sleep to a more typical range. On one hand, the "worn-out" syndrome might seem to persist if the sleep period is wrongly adjusted to the circadian cycle. On the other hand, the "worn-out" observation is usually produced by those who cannot get enough sleep during the week and then sleep long on the weekend. In the latter case, follow-up observation is often impossible due to the next week's obligations. This deepens the wrong conviction that too much sleep is harmful. Healthy individuals cannot get "too much sleep"! Their brain will simply produce natural waking up at the right time. Drs Jim Horne and Daniel Kripke may claim otherwise. Perhaps they never tried to nod off at a peak alertness window?
Shift work and jet lag
Health effects of shift-work and jetlag
Nearly 20% of the population in the industrialized nations is involved in shift-work! Surveys show that only 10% of the shift-working population have no complaints about the negative impact of their sleep schedules on their health and life. With well-designed shiftwork, those numbers could look much better. This would not, naturally, change the fact that all forms of sleep regulation are risky and potentially unhealthy. Research shows that shift-workers suffer from various gastrointestinal and cardiovascular problems. Cardiovascular changes might be mediated by inflammatory markers such as C-reactive protein. Many have problems with achieving refreshing sleep. After many days of chronic sleep restriction, a significant degree of cognitive decline accumulates. This decline leads to levels that in the end approach those found in severe acute total sleep deprivation. Substance abuse among shift-workers is also much higher than average. Seemingly minor problems such as headache, inattention, decline in libido, fatigue, irritability, etc. all add up to pretty miserable life for a vast majority of workers on a poorly designed shift schedules. The set of problems affecting shift-workers is pretty familiar to researchers studying jetlag. Separate medical terms have been coined for the two related sets of symptoms: shift work disorder (SWD) and jet lag disorder (JLD). The most dramatic finding in reference to jetlag was the loss of cells in the hippocampus in flight attendants who were employed for longer periods in jobs involving intercontinental flights (Cho 2001; Cho et al. 2000). We can surmise that the exactly same health issues (times ten) would affect polyphasic sleep adepts if they could only last on their schedule long enough.
In addition to the direct effects of sleep phase misalignment, there is also a degree of sleep deprivation in shift-work and jetlag. Sufficient sleep is important for proper glucose metabolism and prevention of obesity and type II diabetes. Sleep restriction decreases the levels of leptin and has an opposite effect on ghrelin. Those two appetite hormones, as a result, make sleep deprived individual feel hungrier than well-rested individuals and shift upwards the set point of body fat weight in the caloric balance homeostat. Restricting sleep to 5 hours per night causes some 20% change in the levels of these appetite control hormones. This change corresponds to some extra 1000 kcal in free running feeding, or over 3 kg of fat per month in energy terms. Sleep restriction can easily halve insulin sensitivity leading to type 2 diabetes. It also significantly increases the risk of hypertension, stroke, heart attack or kidney failure (Van Cauter et al. 2007). Other hormonal changes include increase in thyroid hormone levels (Allan and Czeisler 1994), prolactin, LH, and estradiol (Baumgartner et al. 1993). Finally, the root cause of many phase shift problems is a complex impact of shift-work and jetlag on the circadian changes in the level of the stress hormone cortisol. The net effect of the impact of cortisol level changes is the hypercatabolic state that effectively results in the body "eating itself up" in the long run. This way, when neglecting your body clock, you can become obese and biologically "wasted" at the same time.
In 2007, the International Agency for Research on Cancer issued a statement saying "Shiftwork that involves circadian disruption is probably carcinogenic to humans". Using the term "carcinogenic" is probably slightly misleading as the actual cause of increased cancer in shift-workers is probably related to the decline in the immune function and the body's natural ability to fight off mutating cancer cells. However, the statement is important as it seals the fate of shift-work and jetlag, which should ultimately fall into the category of long-term health risk factors that cause wide ranging and serious systemic health problems.
For more about the tiny and delicate structure of the body clock, see the section devoted to the suprachiasmatic nucleus.
Properly designed shift work
I often qualify shift-work as a health risk with the designation "poorly designed". This is because it is possible to design schedules for a group of people where the circadian disruption is minimum. Using chronotherapy it is possible to gradually phase in employees into working through the night. The chief principle of such a therapy is that phase shifts should not exceed one hour per day and should, with few exceptions, be forward shifts (i.e. shifts where the days are longer than 24 hours, not shorter). All therapies that depart that principle and involve leisure time, napping, bright light, melatonin, sleeping pills, modafinil, etc. are a pure waste of time as they keep fighting the inevitable: a misalignment between the work time and the subjective night period. This misalignment can only be remedied by a gradual properly timed phase-shift-based adjustment.
Even though many shift workers will disagree with me (mostly for psychological and convenience reasons), I insist that it should be easier and healthier to maintain a night shift for a longer period (e.g. a month) than to do regular cycling between night and day without the body clock having any chance for adjustment. Some cancer researchers also oppose long periods on night shifts due to the documented decline in melatonin that is believed to have cancer protective properties. However, those need to be weighted up against an even more serious problem of the circadian disruption.
One of the most persistent myths about sleep is that our body is programmed to get as much sleep as possible. Even some reputable researchers subscribe to this idea! They compare sleep to overeating. Some note how long Inuit sleep in winter. Others note that people allowed to sleep freely often binge heavily and clock up an indecent number of sleeping hours. As if conservation of energy was the main function of sleep. As if all animals were made as lazy as they are perpetually hungry.
Some scientists even contemplate sleep restriction analogous to calorie restriction. It is conceivable that sleep restriction might be helpful in some rare cases in sick people (e.g. "wake up to get your medicine"). However, it's analogy to calorie restriction is as weak as the reverse proposal: wake restriction. The myth was probably born from epidemiological studies that show that people who sleep 7 hours per night live longer than those who sleep 9 hours per night. However, the suggestion to restrict sleep to live longer is as smart as an effort to shrink or stretch people just because those who are very short or very tall do not live as long as an average man in the street.
We can't demonstrate any evolutionary advantage to getting more sleep than neurally necessary. The harmful myth of excessive sleeping might make you think that free-running sleep will make you sleep longer in the same way as free access to the kitchen will make you overeat. Considering the known functions of sleep, there is no specific benefit to sleeping beyond the standard 6-8 hours. Sleep is a neurophysiological consumer of benefits accumulated in waking (such as learning, exercise, etc.). Its healthy homeostatic and circadian control roughly ensures the optimum proportion of sleep to waking. People who binge on sleep in free-running conditions usually come from a period of long-lasting sleep deprivation or initiate sleep too early in reference to their circadian phase. Their total sleep time quickly drops to their natural average after a couple of days on a free schedule. A study showed that to get over 8 hours of uninterrupted sleep, the sleep should be initiated some 6 hours before the temperature nadir (shortly after the alertness acrophase)(Dijik and Lockley 2002). The same can be seen in SleepChart data submissions. For example, in the presented graph, the maximum length of sleep is obtained when sleep is initiated 3 hours ahead of the most favored bedtime (merely an hour after the evening "forbidden sleep zone"). Those observations have put paid to the idea that we have a tendency to sleep excessively.
Circadian graph that shows that "excessive sleeping" occurs when sleep is initiated too early. In the graph, sleep initiated in the 16th hour is longer than average, while the sleep-wake cycle does not add up to 24 hours (unbalanced cycle with phase advance). In contrast, sleep initiated in the preferred 19th hour is nearly an hour shorter and produces a perfectly balanced 24-hour sleep-wake cycle.
If your main concern is time, you can survive on less sleep and get more time at the cost of your mental acuity. If your main concern is the brain power, you should live by the motto: Maximum efficiency of sleep is accomplished when sleeping without artificial sleep regulation (i.e. without alarm clocks, pills, designer schedules, substances, etc.). Free-running sleep schedule will make you sleep less on average. It will make you sleep much less than on any artificial sleep schedule that forces you to catch up with the accumulated sleep debt. Irregular schedule is bound to produce deficits because you can accomplish irregular sleep only by interfering with it. To read more about excessive sleeping see: Jim Horne and Daniel Kripke.
Sleep apnea is a problem with breathing during sleep. In all cases of getting unrefreshing sleep despite adhering to all the rules of sleep hygiene (esp. in free running sleep), sleep apnea needs to be ruled out. An initial home-made diagnosis may be made by a bed partner or by video-taping one's own sleep. All pauses in breathing or heavy snoring should be worrisome and consulted with a sleep expert. The most frequent type of sleep apnea is the obstructive sleep apnea(OSA), which affects up to 10% of male population (it is about half as frequent in women). OSA involves a loss of muscle tone in the throat and tongue areas. These structures tend to collapse during sleep and block the flow of air. As a result, the patient will wake up temporarily (often a hundred times in a single night) without completing the natural NREM-REM cycle. Patients with OSA wake up feeling unrefreshed. You can also videotape yourself when sleeping. Most often, OSA affects obese and heavily-snoring males. There are multiple support sites for OSA on the web (including recordings of snoring patients and typical signs of interrupted breathing). Sleep apnea needs to be treated with urgency. It undermines one's cognitive powers in a short-run (due to its effects on the quality of sleep) and in a long-run (due to its negative effects on brain growth and aging). If anyone tells you that you snore heavily, do not treat is as a natural thing: "my uncle also snores like a tractor and seems to be ok". All cases of snoring should be investigated. Snoring may be a first sign that your brain is not getting what it needs during the night sleep! Sleep apnea can also lead to cardiovascular disorders, depression and a whole host of negative health consequences. One of the best natural weapons against sleep apnea is weight loss by combining a healthy diet and exercise. Exercise on its own may also be helpful as long as it is chosen carefully to make sure it does not exacerbate snoring (e.g. through nasal congestion).
Hypersomnia is excessive sleepiness in conditions of getting physiologically sufficient sleep. Hypersomnia may be related to serious health problems. However, if you keep battling drowsiness, your problem does not need to have a serious organic cause. There is a simple home-grown diagnostic method that can help you eliminate a frequent and less severe cause: a phase shift disorder. Try to free run your sleep for a few weeks. Very often, the phase adjustment will resolve perpetual tiredness! Quite frequently, sleep initiated too early in reference to the circadian sleepiness will last very long and paradoxically result in the feeling of not being refreshed in the morning. If the subjective circadian night period overlaps with the actual waking time, you may experience overwhelming drowsiness, yet you will not be able to fall asleep for longer than 20-30 minutes and you will still wake up unrefreshed. Even buckets of coffee may not help in such circumstances. If you do not notice a significant improvement in the quality of sleep after 1-2 weeks of free running sleep, you may suffer from an underlying health problem that will require a professional consultation. See: Sleep and Hypersomnia at WebMD. A frequent cause of poor quality sleep is Obstructive Sleep Apnea (OSA). OSA is caused by breathing difficulties during sleep.
Delayed Sleep Phase Syndrome
When a tendency to go to sleep later each day is strongly pronounced, it may become a serious problem. People with a particularly long circadian cycle or with an insufficient sensitivity to zeitgebers are classified as suffering from Delayed Sleep Phase Syndrome (DSPS for short). Sometimes the abbreviation DSPD is used where syndrome is replaced with disorder. The terms non 24-hour sleep/wake syndrome (N24, N-24, Non-24) or hypernychthemeral syndrome (with a few spelling variants) are occasionally used to refer to the most severe cases. I will consistently stick with the label DSPS to emphasize that these are all variants of the same problem expressed differently in different circumstances. This quarrels with the established definitions used by other authors, which I will disrespectfully ignore due to the fact that the established terminology leads to a harmful confusion and the sense of disabling inevitability.
In DSPS, an individual finds it difficult to fall asleep late in the night, and sleeps well into the afternoon if not awakened. DSPS has only been characterized in 1982, but increasing data indicates that various degrees of DSPS occur with epidemic frequency, esp. among high school and university students. DSPS individuals often like to keep on learning late into the night, go to sleep very late (for example, 4-6 am), and find it very hard to wake up early on a regular basis. For example, regular getting up at 7 am is a pure torture for individuals affected with DSPS. They often fail to keep jobs that require them to perform early in the morning. Very often, they tend to split the day sleep into two components. For example, DSPS students often get a short sleep in the night, wake up early with an alarm clock, go to school where they are semi-conscious and perform poorly, get a solid nap after school and only late in the evening they regain vigor and their full mental powers. DSPS students feel best after midnight when everyone else is asleep and they can focus on learning or other activities (reading, Internet, watching TV, computer games, etc.).
The main factors contributing to DSPS:
- increased period of the body clock (well above 25 hours)
- reduced or increased sensitivity to factors that reset or advance body clock (e.g. light, activity, stress, exercise, etc.)
- electric lighting, 24-hour economy and the resulting "want to do more" lifestyle
A normal individual has a body clock running with a period slightly longer than 24 hours. The clock is reset in the morning with activity and bright light. Thus a normal individual easily adjusts to the standard day-night cycle. However, DSPS individuals may have their clocks running periods long enough to find it hard to fit to 24 hours. They also push their clocks ahead by activity late in the evening (the process opposite to the morning reset synchronization). DSPS individuals, when given a chance to sleep when they want, will tend to go to sleep later and later. They will also wake up later and later. DSPS people do not have problems with sleep if they sleep in their favorite hours. Most mild DSPS cases can be remedied by changes in lifestyle, but rarely are those changes painless to individuals affected by the condition. If this description fits your problem, you may diagnose the degree of your DSPS with SleepChart freeware.
DSPS in teenagers
Research shows that DSPS is very frequent in adolescence (Carskadon 1995). Teenagers with DSPS will often find it difficult to adapt to normal school time. They will experience maximum daytime sleepiness at 10 am (in the middle of the school day) and a peak in alertness right after the school. For many teenagers with a natural tendency to go to sleep late, school becomes a torture and a true waste of time! Educators have already taken on this subject; however, students dozing off during classes are still a norm! Sleepy students learn little, and may naturally develop strong negative feelings for the school in general. This is a problem of colossal proportions! If you are a parent of a teenager who finds it difficult to wake up for school, you will need to act now! Otherwise young man's school years will be a period of monumentally wasted time! It won't be enough to demand an early hour for going to bed. If you ban the late evening Internet surfing, you will just swap a dose of evening education for an idle tossing and turning in bed. Actually, there is only one simple solution, let the kids get up at their natural time but... this may not be realistic in most cases. Your sleep therapist may not be able to help either. The whole school system might need to be changed to accommodate the prevalence of DSPS among adolescents. There have been positive results noted in schools that decided to start classes 1-2 hours later. However, long circadian cycles may result in students staying up yet later in the long-run. Researchers suggest schedule stabilization and gradual realignment. Those measures may still be largely ineffective. Homeschooling and free running sleep could be a great option for those kids.
Solution to the DSPS problem
Free running sleep is usually an instant solution to sleep problems in DSPS, however, it inevitably results in a "shifting" sleep pattern (see below). Other than free running sleep, the best known remedy is to:
- cycle the sleep phase to alignment with the desired hour bracket and
- battle to reset the cycle (see: Curing DSPS and insomnia)
In other words, if possible, you could use your natural tendency to go to sleep 1-2 hours later, until you align with the desired sleep rhythm. At that point, the real battle begins by efforts to provide strong morning resetting stimuli (e.g. bright light, stress, exhaustive exercise, etc.). Those can be enhanced by evening measures such as melatonin and the avoidance of phase delaying factors such as light, stimulation, stress, Internet, etc. In general, you need to provide resetting stimuli in the morning, and avoid evening delay factors such as computers, TV, artificial lighting, etc. For most people, a degree of sleep deprivation is more acceptable than several futile inactive hours in the evening in a dark room.
Is DSPS a disease?
Probably, most of the cases of DSPS can be explained by a lack of compatibility between the genetically determined sleep control system and the lifestyle. For some people, the degree of the problem may be greater than for others (see: Clock genes and mutations affecting the clock period (Golombek and Rosenstein 2010)). Everyone can easily cure the disorder with a decision to drastically change one's habits (e.g. a return to a farmer's lifestyle). However, such a change is usually not feasible due to the type of employment or family life conditions. This means that DSPS sufferers are probably, for a while, sentenced to wage a constant battle with their body clock.
Officially, 0.2% of adults suffer from DSPS. Using numerous SleepChart data submissions, I see a true epidemic of DSPS. Moreover, there is a large hidden DSPS population. I have seen cases where people started showing DSPS sleep patterns as soon as they gave up an alarm clock after years on a normal schedule with a seemingly normal life contaminated somewhat with a degree of sleep deprivation.
Admittedly, people who write to me are already a pre-selected population, but the numbers are really staggering. I am pretty sure that most of those DSPS cases are lifestyle related. As the term "syndrome" might suggest DSPS is a disease, I keep emphasizing that DSPS is rather a reflection of our modern electricity-based lifestyle than an actual disorder. Interestingly, I received very few ASPS submissions. It seems that it is DSPS people that hang out late in the night googling on their PC for solutions to their sleep problem. In the end, they arrive to SleepChart and the concept of free running sleep that can be their magic cure (if ever truly affordable).
DSPS epidemic can be considered a civilizational disorder in which the pressure of a modern lifestyle stands in disagreement with millions of years of evolution. In the long run, once we fully understand all biochemical and hormonal processes underlying sleep, it is possible that mild pharmacological intervention will make it possible to regulate the circadian cycle.
Combating phase shift
People suffering from DSPS find it difficult to synchronize with the 24-hour clock. In the picture below, an adolescent female with a mild DSPS suffers disintegration of the sleeping rhythm due to the failed efforts to synchronize with "the rest of the world":
After the vacation period, she begins in early September well-synchronized with the "rest of the world". She wakes up between 6:30 and 10:00. However, her body clock experiences continuous shift in her subjective night period. Soon she wakes up at 12:00 and begins a struggle against the further shift. This results in the disintegration of the sleep cycle, short sleep periods below her preferred average (e.g. 4 hours) and frequent bouts of tiredness. SleepChart attempts to plot the extent of the subjective night (i.e. the hours of maximum natural sleepiness). The statistically predicted subjective night is bracketed between the red and blue lines. Circadian acrophase (middle-of-the-night) is plotted in yellow. Circadian sleep propensity is expressed by the shades of red. Sleep blocks terminated with an alarm clock are marked in aqua. Clearly, the greatest disruption in the sleep pattern occurs at the point where the "natural" rhythm departs furthest from the "desired" rhythm. Mild DSPS cases are able to force the body clock to remain more or less in the desired bracket at the cost of a constant struggle with sleepiness. In more severe cases, the circadian variables will run a 24 hour cycle and the individual will experience return to "good sleep" when free running variables align again with the "desired" sleep period.
The average sleep length is 6.8 hours but total sleep changes widely from day to day. The average DSPS shift is difficult to determine due to the battle against the natural rhythm. However, it is likely that the shift is around 60 minutes as evidenced by the average progression of the circadian acrophase estimate (in yellow). Without the use of an alarm clock, the advancing sleep phase would likely complete a full 24h turnaround in 3-4 weeks.
Resynchronizing the cycle
In the next example, a middle-aged female with a severe case of DSPS experienced a similar struggle in stabilizing her sleep rhythm within socially acceptable limits:
Subjective sleepiness was minimum when the body succumbed to the progression of the sleep phase (Sep 16 - Sep 23), daytime tiredness increased markedly at the time of the battle with the progression (Oct 2 - Oct 11) where light aqua blue sleep blocks were blocks artificially terminated with an alarm clock. Finally, daytime drowsiness peaked in the period of lost synchrony between sleep periods and the circadian phase (Oct 19 - Oct 22). The breakthrough came with the religious adherence to free running sleep. The next log shows the same female on a well-managed free running schedule:
A perfect alignment of sleep periods with the circadian acrophase (yellow line in the middle of the subjective night) resulted in tripled energy and a sense of well-being.
Social life in DSPS
Naturally, only people who are telecommuting, self-employed, or working from home office can afford to let their sleep run free in DSPS. Even then, the shifting sleep phase is a serious predicament. A legally blind DSPS sufferer from the Netherlands wrote about the pain of the shifting sleep pattern: "I am free running my sleep. I had an appointment at 17:30. I expected to wake up around 15:00 as in the previous three days. Instead I woke up around 17:00 still a bit tired. I had to skip my morning routine (meditation, breakfast, SuperMemo, etc.). FRS works really well for me. But today sucked. It was really stressing having to run due to waking up later than expected". After a medical consultation, this subject was prescribed evening melatonin and was able to stabilize his cycle (for at least a few weeks at the moment of writing these words). The torturous battle of the same subject with phase shifts before running free and before administration of melatonin is shown in this graph:
This example illustrates the major dilemma of all more severe DSPS cases. Free running sleep will often produce a phase shift. Anyone who tends to wake up very late is also highly likely to tend to wake up later each day in free running sleep. This is a hallmark symptom of the DSPS. DSPS, however severe, is never a health problem on its own if the sleep is run free. It is the scheduling problems that are most bothersome. The choice is between the two extremes:
- either make one's life less dependent on meetings and appointments that can collide with your sleep schedule, or
- study DSPS remedies that can stabilize your sleep-wake cycle.
If you happen to always wake up late, waking up always at the same time makes scheduling much easier. If you do not opt for one of the above extremes (free schedule vs. stabilization battle), you will risk collisions that will make life pretty hard. What is even more dangerous, if one disrupts a circadian rhythm on a free running schedule, there can be a loss of synchrony between various circadian variables. This will result in a situation in which for a day or even a few days one is not sure of the optimum bedtime. Even SleepChart may be unable to make a good prediction. This will inevitably result in poor quality sleep, and a few days of low productivity.
DSPS: genes or lifestyle?
Even though I keep repeating that the DSPS epidemic is a reflection of a modern lifestyle, genetic factors clearly play a role and the "creative personality" can also be at the root of the problem. Here is an interesting story of a writer mom whose parents and two kids do not show any signs of sleep problems, however, she suffers from a severe case of DSPS and so does her 20-year-old son:
My mother claims I have had sleep issues from the day I was born. In those days "rooming in" was new as most babies were kept in the nursery and it was the norm for both baby and mother to stay in the hospitals for a week after a normal birth. She enjoys telling the story of how the nurses forced her to have me "room in" with her during hospital stay because I was "keeping up all the other babies in the nursery all night long" because I refused to sleep during nighttime hours. She said that after she and my father brought me home, I was afforded one night in the room with them but I was up all night so I had to "cry it out" alone from then on because I simply would not sleep at night. When I was a few years old, one night, exasperated, my parents said, "Fine. If you want to stay up, you can sit here and watch Johnny Carson." Apparently I was happy to do so! I did, however, have very unhappy memories dating back to the beginnings of my recall of being put to bed at around 20:00 every night and lying up awake and with nothing to do for hours and hours in my darkened, boring room. Back in those days, there was no Internet, no cable TV, no video games, no cell phones, etc. I simply could not get myself to sleep at a decent hour ever in my entire life! I'd often sing myself to sleep and it would take hours to do so. Sometimes I would run out of songs to sing and have to repeat a few until I passed out."
Today, this self-employed female is experimenting with free running sleep and claims that, except for her social life that suffers as a result, the new schedule brought her back from the "hell of perpetual drowsiness".
Chronotherapy and its perils
Chronotherapy makes it easy to fit the circadian phase into a desired time bracket, e.g. after an intercontinental flight, in circadian disorders, or for the sake of shiftwork. For most people, shifting the cycle forward by inducing phase delays is easier. It is possible to go to sleep 40-70 min. later each day and to cycle throughout the day until the desired sleep phase is reached. Pushing one's circadian cycle should always be the last resort. All artificial forms of sleep control should be avoided if possible as they are not healthwise neutral. However, some reports in the literature suggest that chronotherapy may have serious long-term cycle synchronization consequences. Wehr reports (Wehr et al. 1992): "In 1983, one of us described a 28-year-old man with DSPS who underwent chronotherapy and found himself unable to stop his sleep period from rotating around the clock or restore his rhythm to a 24-hour schedule. Instead, hypernyctohemeral syndrome developed, with a persisting 25-hour sleep-wake cycle. This rare syndrome is extremely debilitating in that it is incompatible with most social and professional obligations".
I have personally witnessed numerous cases of phase delays upon switching to free running sleep in seemingly normal people, and have a different interpretation. People differ in the degree of difficulty in sustaining a balanced 24h cycle in free running sleep. That difficulty is well expressed in circadian graphs by the angle of the sleep maintenance curve in reference to the breakeven line that determine a balanced 24h cycle. Most adolescents will experience a degree of difficulty in maintaining the balance if they are allowed to engage in their hobbies and passions late into the night. Once they are allowed to do as they wish, they will often induce phase delays by a simple unwillingness to go to bed "in time". Even though the progressive 24.5 hour cycle may seem unsettling at first, resulting in sleeping in unusual hours, the convenience of running free with extended waking days may outweigh the negative side effects. For people suffering from DSPS, running free may provide an unusual degree of comfort that is difficult to forfeit. No wonder that many DSPS sufferers, who enjoy a progressively shifting circadian cycle in free running sleep, often give up the battle to reset the cycle, or even discover that their natural phase shift is far larger than originally diagnosed.
As the medical and psychiatric terminology of severe cases of DSPS is very confusing, I need to yet attempt to explain cases of "irregular Non-24-hour sleep-wake syndrome", by which I mean a severe DSPS where sleep episodes do not fall into a regular pattern. I have presented some cases in the Asynchronous DSPS section. I have little doubt that most of such irregularities come from subject's own ignorance of his or her sleep preferences, as well as the rules of a healthy free running sleep regimen in circadian disorders. Once a sufferer is instructed on the rules of healthy free running sleep, perhaps with some assistance from SleepChart, the sleep pattern becomes regular.
My personal stance on chronotherapy is therefore as follows:
Chronotherapy is the best approach to repositioning the phase of the circadian cycle. It should always be the last resort as it is not neutral for the quality of sleep. However, long term consequences of occasional use of moderate chronotherapy are probably negligible.
When a DSPS sufferer attempts free running sleep, sleep phase delays are inevitable. There have been cases in literature that documented people living along such a shifting DSPS schedule for decades without major health side effects (Neubauer 2000). For an exemplary report see this blog. Some authors claim that a shifting schedule may increase the incidence of depression, alcoholism, or dependence on sedatives (as a result of attempts to induce sleep at the "appropriate" time).
The following graph presents a sleep pattern of a free running middle-aged self-employed male:
A very clear and regular DSPS pattern visible in the graph with a daily phase shift of 64-68 minutes. Although sleeping in "unnatural" hours is certainly less beneficial healthwise than normal sleep, for a DSPS subject, free running sleep rhythm may by far less stressful and disruptive than any attempt to fit to "standard" lifestyle. A very reliable determinant of synchronous DSPS is the loss of the link between the sleep onset hour and sleep duration (see: Preference for night sleep). As the duration of sleep is determined by the circadian phase, well-synchronized sleep schedule shows little variability in the sleep length (6.6 hours in the presented graph). In particular, the sleep length is independent of the sleep onset hour. Whenever the subject makes any attempt at synchronization with daylight or daylight-related activities, the link between sleep length and the onset hour will be reconstituted. Mistakenly, DSPS people are often called "owls" for their tendency to stay up late, while ASPS people are called "larks". The graph illustrates why this is a misnomer.
A similar graph shows a DSPS case with an even greater degree of phase shift (84-90 min):
Due to a better sleep efficiency in episodes well aligned with the circadian cycle, people on a regular free running DSPS schedule report a much higher subjective alertness and energy as compared with those on an irregular DSPS schedule. This difference also shows up in data collected with SuperMemo.
28 hour day schedule
28 hour day schedule was proposed for those who seek higher productivity and more hours in a day. An example of a 28h sleep pattern design shows a phase shift that needs to reach the daily extreme of 4 hours per day for anyone to be able to sustain that schedule for a longer period of time:
The advantages of a 28 hour schedule supposedly include longer working days, regular 6 day week, repeatability, long weekends, increased energy, unlimited sleep, etc. (for more see: 28 hour day). This proposition is the other extreme of a spectrum of propositions that begins with polyphasic sleep. However, it seems far easier to sustain as it does not need to involve an alarm clock. Sleep researches believe that this schedule is so extreme that nobody should be able to sustain it in a long run. 4 hour phase shifts are so unlikely that researchers choose it for their experimental forced desynchrony protocols. These are experimental protocols in which the body is supposed to fail to adapt to the timing of zeitgebers. Such an entrainment failure is beneficial in studying free running circadian variables. 4 hours shifts have been used in both advancing and delaying protocols (20 hour days (Wyatt et al. 1999) and 28 hour days (Carskadon et al. 1999)). All research to-date seems to indicate that the circadian cycle keeps running free in the background in forced desynchrony protocols due to the fact that resetting stimuli cancel each other out and sleep episode intersect with the circadian variables in unpredictable patterns that result in segmented sleep, premature awakening, shortened sleep, reduced REM, and other symptoms of asynchrony. In short, 28 hour day is considered extreme enough to cause perpetual lack of synchrony between the timing of sleep and the circadian cycle.
28 hour day in DSPS
Some sufferers from DSPS report feeling better on the 28 hour schedule than on a conventional 24 hour sleep schedule. I do not think it is likely there are individuals out there with an innate ~28 hour circadian cycle, however, it is conceivable that the effort to squeeze a DSPS cycle into 24 hours is more painful than the alternative in the form of stretching the cycle to 28 hours. The main difference is that the shortening of the cycle usually involves the painful use of an alarm clock, while stretching the cycle requires "only" extra 2-3 hours of zombified wakefulness. Even in severe DSPS, it should be pretty hard to adapt one circadian cycle to the 28 hour schedule as the phase response curve indicates that the sleep phase does not respond strongly enough to strongly delayed bedtime, which may, in extreme cases, cause a phase advance. Phase delays beyond 2 hours should be extremely rare.
For most people, it is pretty hard to tolerate even minor deviations from one's optimum cycle period. For this reason, all designer schedules should be avoided unless they come from a strict analysis of one's own sleep preferences. Again, free running sleep is a better option, even though it may be less predictable and less convenient in planning one's social or professional life.
Let us consider an exemplary case of Subject S, and compare her sleep efficiency on conventional, 28 hour, and free running schedules.
Conventional schedule in severe DSPS
When S attempts to adhere to a conventional sleep schedule, under medical supervision, with the help of sleep medication (incl. melatonin), the sleep is strongly fragmented, short, unrefreshing, and the schedule is unsustainable:
This type of sleep is tantamount to mental torture, and all individuals with a similar degree of entrainment failure should always be allowed to let their sleep run free on the grounds of severe disability.
In the presented chart, a pattern of possible free running circadian cycle can be noticed in the chart with the subjective night leaving the conventional night bracket around April 6, 2011. This is more noticeable upon sleep episode consolidation:
Needless to say, the conventional schedule, if maintained for longer, may lead to serious health problems due to the state of persistent sleep deprivation compounded by medication. Even though the presented case is pretty drastic, the number of people suffering from similar sleep problems is constantly increasing and is definitely affecting overall population health and productivity.
28 hour schedule in severe DSPS
When S attempts to adhere to a 28h day schedule, her subjective sleep quality increases dramatically along with the total sleep achieved. Detailed analysis of the sleep chart, however, shows that sleep fragmentation is still substantial showing strains in the sleep control system:
Segmented sleep starts showing after two cycles which might indicate that the actual phase shift lags behind the planned phase delays. Segmented sleep is often a sign of premature bedtime and shows up when the 28h schedule bedtime falls ahead of the presumed free running subjective night. When the sleep schedule undergoes an eventual collapse, the positioning of lengthy recovery sleep episodes seems to indicate that the average daily phase shift might have actually been much less than 4 hours. In an extreme case, large disparity between the subjective night and the planned nighttime might result in self-cancelling phase shifts that might paradoxically stabilize the sleep cycle.
Circadian graph for the same 28h day schedule illustrates the degree of chaos in the sleep control system:
Free running sleep in severe DSPS
When the sleep chart of Subject S running free just a few months earlier is inspected, the sleep phase shift is closer to a mere 41 min per day (as opposed to 240 minutes needed to smoothly sustain the 28h day schedule):
However, the chart shows that even in that period sleep was strongly fragmented and irregular. Similar analyses are often misleading due to compounding circumstances such as a disease, family problems, medication, and even a wrong choice of bedtime (e.g. in an attempt to stabilize or accelerate the cycle).
In similar cases, it is paramount to chart one's precise circadian preferences. For this reason, a few weeks of uninterrupted free running sleep would be precious to determine one's natural innate daily phase shift. Once this is done, more can be said about the sustainability of a 28h day schedule for a given individual. However, continual free running sleep is always the best option for those who are absolutely unable to balance the cycle and those who can afford the sleep schedule that is hard to reconcile with the rhythm of the outside world. See: Curing DSPS and insomnia.
Advanced Sleep Phase Syndrome
Problems with phase advance
Advanced Sleep Phase Syndrome (ASPS) is the opposite of DSPS. People suffering from ASPS get very sleepy early in the evening and wake up very early in the night. Their circadian clock runs at less than 24 hour period or get easily reset in the morning (e.g. by stress). ASPS people constantly struggle to survive awake to a reasonable evening hour, sleep less, wake up early, and experience increased tiredness during the day.
Genes and lifestyle in ASPS
While a typical DSPS person is an adolescent student, a typical ASPS person is a retiree or a middle-aged woman with low stress tolerance. The link between the age and sleep phase disorders may be related to aging itself, however, it may also be a result of lifestyle changes that come with age.
Remarkably, while I have received dozens of SleepChart submissions showing a free running DSPS pattern, I had to actively seek submissions that would illustrate ASPS. This alone can serve as an illustration of personality and lifestyle differences between the two groups. It is the DSPS group that keeps surfing the net till the early morning hours in search for a solution to their sleep problem. In the end, they often arrive at super-memory.com, download SleepChart, and begin logging their sleep in an effort to understand their own sleep patterns. The ASPS group is usually in bed early and often not refreshed enough during the day to seek a solution on the web. I have not received even a single reverse ASPS pattern with sleep starting progressively earlier in the day!
Example 1: ASPS and substance abuse
The presented SleepChart log illustrates a stabilized ASPS sleep pattern of a postmenopausal unemployed female with a lifelong history of substance abuse, currently in a period of abstention and recovery. Without medication, the subject struggles to stay up past 5 pm. She often wakes up at 1-3 am and finds it impossible to fall back asleep. She reports a perpetual tiredness. The only solution to her sleep problems seems to be sleeping pills regularly prescribed by her GP and/or psychiatrist(s). Those pills have also been a part of a vicious cycle of addiction to benzodiazepines and alcohol.
Superficially, the log seems to look like a picture of a perfectly healthy sleep. However, the entire schedule and the sleep phase are kept in check with a cocktail of psychoactive drugs. The main difference between this ASPS case and a similarly-looking perfect sleep case is the said persistent tiredness throughout the day. The subject reports that her chief preoccupation is to "somehow get through the day" when combating tiredness, and struggling with an ever present threat of a fallback into addiction.
The difference in sleep length on individual days (8-9 hours on good days, 0-3 hours on bad days) comes from the fact that the subject sleeps at different family locations on different days. Some of those are considered better (resulting in better sleep), others are considered more stressful. One of the nights was sleepless due to family stress. This illustrates again how lifestyle determines sleep patterns.
Sleep maintained with drugs always yields fractional cognitive benefits. In this case though, the effect is truly dramatic with cognitive performance comparable with that encountered in a state of severe intoxication. The drugs schedule, which changes periodically for various reasons, is invariably composed of pick me ups in the morning, and put me downs in the evening, as well as some "extras" for controlling various neural side effects of the "sleep control cocktail". Individual drugs interfere with each other producing a constellation of side effects that result in a horrendous chaos in the system, and long-term consequences that in turn result in an inevitable spiral towards a psychiatric decline and dramatically reduced well-being, ability to function in society, and longevity. The half-life of opposing drugs results in their effects cancelling each other and producing unpredictable resultant consequences. Why is then this pharmacological horror tolerated? For an unemployed individual with a history of substance abuse, for his or her family, and for the doctors involved, anything that resembles normality today takes precedence over the long term consequences. Naturally, for nearly everyone involved, this zombified status quo is preferred to actual intoxication even though that both are bound to destroy the brain in the long run. EEG findings indicate long-term and largely irreversible changes in the function of the central nervous system caused by substance abuse and/or psychoactive medication.
As with all medical intervention in general, psychiatry is particularly troubled with tunnel vision that fails to see the big picture of individual's life and population health in general. New drugs pop up too fast to effectively study their long-term consequences. They are subject to prescription fashions that wax and wane. As barbiturates gave way to benzodiazepines, and benzodiazepines to Prozac, a well-meaning psychiatrist is often confused by a welter of contradictory data, never-ending lists of contradictory side-effects and the scourge of scientific observation: guaranteed false data coming from patients who always have multiple reasons for lying to their doctor. Making all patient history records near-to useless. Patients often change doctors to suit their dream prescription, or seek parallel advice and contradictory prescription from different sources. They rarely stick to the drug timing and dosage.
For the record, at the moment of writing, the drug array in use in the presented example was:
- antidepressant venlafaxine that is able to lengthen the period of the circadian cycle (8 am, half-life 10 hours)
- antipsychotic aripiprazole (8 am, half-life 75 hours)
- anxiolytic benzodiazapine lorazepam (6 pm, 2.5 mg, half-life 15 hours): in use for years instead of the recommended weeks, despite various side effects including severe sleep apnea; possibly the prescribing physician was not aware of the fact
- antipsychotic olanzapine (8 pm, 20 mg)
- in addition: anticonvulsant lamotrigine (50 mg)
As of the moment of writing, I was not able to ascertain if these have been prescribed by a single psychiatrist and if the prescribing physician(s) had an insight into the patient's full medical history.
Example 2: Overmedication and daytime drowsiness
The second example shows another severely medicated case. 56-year-old male retiree carries on on an equally potent cocktail of drugs. In this case, poorly-planned irregular free running sleep helps reveal the degree of daytime sleepiness with sleep episodes initiated regularly starting with the 3rd hour of wakefulness, short and early forbidden zone in the hours 7-9, and preference for short waking day of 12-18 hours:
Perhaps due to the impact of the sedatives, the length of sleep episodes may reach an equivalent of a full night's sleep at practically any time of the day. Needless to say, the subject is hardly able to function cognitively and complains of never-ending tiredness. The drugs used in this case:
- between 7-8 am antidepressants: aripiprazole (5 mg) and bupropion (150 mg)
- around 11:30 am: an antipsychotic quetiapine (25 mg)
- around 9 pm: benzodiazepine clonazepam (1 mg), antidepressant escitalopram (40 mg), lamotrigine (200 mg), quetiapine (50 mg)
- in addition: tamsulosin to avoid nighttime urinary incontinence
Curing DSPS and insomnia
What is DSPS?
If you cannot live without an alarm clock to wake up in time for school or work, you might be suffering from a delayed sleep phase syndrome (DSPS). DSPS is also associated with problems with falling asleep if you try to keep an earlier bedtime. In other words, any cure for DSPS is also likely to solve the problem of sleep onset insomnia. If you go to a sleep expert with your DSPS problem, you will likely be prescribed melatonin or light therapy only to discover their limited impact on the quality of your sleep. If you are an insomniac, you may additionally be prescribed sleeping pills that might help you sleep without achieving the desired effect: a refreshed mind. This chapter should help you solve the problem. Using the properties of the human sleep control system, it can be proven mathematically that the problem of DSPS, and the associated insomnia, is always solvable, however, the solution does not need to imply the crispiest mind or the highest intellectual productivity. Moreover, many people will still fail due to lack of self-discipline! Where the modern world encroaches upon human biology, it is still possible to withstand the tide with the rules of reason. However, these imply a religious adherence to the decalogue of healthy living. Life shows that humans find all decalogues difficult to abide by!
Solution for insomnia, DSPS and N-24
This algorithm should help you in all the following cases:
- if you are an insomniac, your body most likely tries to sleep in a different phase. If you fix the phase, you will fix the insomnia
- if you are a DSPS case, esp. a long-term sufferer, you will be very skeptical of the algorithm. You need to give it at least a few weeks of well-disciplined try to see that your sleep cycle can be modified with the right tools
- if you are a Non-24 and you believe there is no pattern to your sleep, you are most likely wrong. There are rare mutations that can totally mess up your sleep, but a more likely explanation is that you are a DSPS case who just introduces enough chaos into your sleep pattern to make it seem unpredictable. If you are otherwise healthy and if you try free running sleep, you will notice the pattern. For the pattern to emerge your "free" sleep must truly be free. No regulation. No artificial control! After that, you can try the algorithm presented below to see if you can show enough self-discipline to stabilize your cycle.
DSPS and lifestyle
Even though there are various genetic influences that play a role in DSPS, the problem is, for most people, largely a matter of lifestyle. I claim that due to the fact that a return to a farmer's lifestyle provides a guaranteed disappearance of the DSPS problem. Below, I have compiled a simple algorithm that should resolve DSPS in a vast majority of cases given sufficient self-discipline. Until now, I have been far more successful in showing people how to cope with DSPS using free running sleep than with the prescription listed below, which is a derivative of free running sleep with some limitations targeted at preventing a phase delay. The presented algorithm fails primarily because of one issue: violation of the rules! There are true hardcore DSPS cases with some psychiatric overtones or other health issues that might be particularly intractable. However, those should form a rare minority in the ever-increasing mass of people struggling with DSPS. Among young users of SuperMemo, I literally know dozens of cases. That mass now includes a countless population of insomniacs who have never heard of DSPS and never even arrived to the problem of phase shift due to the employment of the alarm clock. Weitzman hypothesized that a significant number of patients with sleep onset insomnia might be suffering from undiagnosed DSPS (Weitzman et al. 1981). Now we know that hypothesis certainly holds true, which can be demonstrated by letting insomniacs free run their sleep. A significant phase delay may be observed within the first few days of such a release from the restrictions on the timing of sleep. At the same time, there is an accompanying and nearly instant disappearance of sleep-onset insomnia.
People who suffer from DSPS often resort to their own implausible solutions that include:
- (poorly managed) free running sleep with inevitable phase delays that ultimately result in sleeping through the day
- skipping a night of sleep at a point when the bedtime reaches an outrageously late hour (i.e. usually after the dawn)
- stabilization of the sleep pattern with medication (incl. melatonin) and artificial zeitgebers such as an alarm clock or a mom who pulls a sleepy student out from his or her bed for school
- messy sleep that occurs at unpredictable hours with the loss of sense of the nighttime and long bouts of drowsiness, low energy, or poor health. This sleep pattern is often labeled Non-24 or N-24, however, in most cases it differs from DSPS only in the lack of self-discipline or knowledge needed to adhere to a natural sleep cycle, even if its period is longer than 24 hours
Only a well-managed free running sleep can produce healthy sleep in DSPS with a minimum risk of negative health outcomes. However, very few practitioners do really adhere to the rules of their own body clock as there are always excuses or inescapable reasons to violate the subjective night when it collides with daytime obligations or diversions. Sufferers of DSPS who try to "free run" their sleep for many years are at risk of messing up their sleep control system. I conclude that from the fact that their sleep patterns often become less and less regular, and the quality of their sleep often decreases. This effect would almost certainly be minimized if the sleep was truly free without medication, light therapy, artificial delays, or the use of the alarm clock. They may even falsely claim that their cycle started getting longer and longer, while it is the lifestyle demands that keep stretching the waking time. The culprit here, naturally, is not free running sleep per se, but various violations thereof that are inevitable due to a conflict of sleep with daytime activities.
If we exclude a healthy farmer's lifestyle and the renunciation of evening electricity, we arrive at only two reasonable lifestyle solutions to the DSPS problem:
- (well managed) free running sleep with phase delays, high creativity, high productivity, good health, and schedules that are nearly impossible to reconcile with the outside world
- stable 24h sleep cycle, with monastic self-discipline, good health, and slightly lower productivity
I write about free-running sleep throughout this article. I worship free sleep so much that I have been accused of labeling DSPS with the stamp of incurability. Here I would like to present a plausible algorithm for sustaining a 24h sleep pattern in DSPS with minimum artificial intervention into the fabric of sleep.
Is the battle for a 24-hour cycle worth having for those who have no work or family obligations? There is no good research to make comparisons. However, when in doubt, I always fall back on natural answers stemming from biological evolution. The biggest problem with non-24 cycles is that social life will always interfere. Nobody is hermetic to those influences. I have seen too many cases of free-running sleep falling apart for a minor temptation of an evening party with friends. Last but not least, we cannot discount the power of sunlight. It has been with us for millions of years of evolution. It is a personal bias, but jogging via woods in sunlight cannot be beat. Those who run in the evening or in the night miss out on a great blessing of nature. In the end, we do not need to choose between nature and modernity, my algorithm should provide a compromise.
The algorithm: 24h lifestyle in DSPS
The only reasonable 24h solution to the problem of insomnia and DSPS is the change to the sleep phase. We can advance the sleep phase using evening measures (pulling the sleep backwards) and morning measures (pushing the sleep backwards). That takes care of the circadian component of sleep. In addition, all measures that boost homeostatic sleepiness in the evening are also welcome. However, without the circadian components those might actually compound insomnia. This is why only a comprehensive approach, as presented below, provides a solid chance for you to leave your DSPS and/or insomnia behind:
- Determine the length of the waking day: Use SleepChart to determine the optimum length of your waking day. You may need a few weeks of free running sleep data to make a good estimate. If you cannot free run, and/or the procedure seems too complex, you could just make an educated guess. For example, if you believe you need 8 hours per night, in free running sleep it might be closer to 7 hours, and your optimum duration of waking would be 24-7=17 hours. However, due to the problem of possible insomnia, it makes sense to start from a slightly longer day and shorter sleep, e.g. from 17.5 hours of waking.
- Determine the preferred waking time: This should be the same time every day! If you have to get up early only once per week, your entire cycle should be positioned to accommodate that early hour, or you will risk never-ending ripples in your circadian system and a possible failure. If the waking hour is late enough to ensure bright sunlight in the morning, your chances will increase greatly.
- Religious bedtime: By adding your optimum length of the waking day to your preferred waking time, you will get your optimum bedtime that must be respected religiously. For example, if your preferred waking time is 9 am, in 17.5h waking day, your bedtime should be 9+17.5=26.5, i.e. 2:30 am. Respect for your bedtime is one of the keys to success. You will encounter many obstacles in the process. One of the most confusing ones is the change to your optimum bedtime depending on the contents of your day, your activities, your stress, weather, season, social interaction, and so on. For most people those changes are less than one hour. However, you are more likely to be in the wild variations category as this is one of the underlying problems in DSPS. Start adhering to your optimum bedtime as if your life depended on it. If you are very excited or stressed, and you are not sleepy at your bedtime, you can try again 20-30 min. later. You need to be sure you won't toss and turn, and that you do not wake up prematurely. If you see any signs of insomnia, you will know your bedtime comes too early and you allocated too much time for sleep. If you are very sleepy ahead of your bedtime and you have no doubts you will sleep like a log, go to sleep earlier. Your optimum bedtime is the guidance, your actual sleepiness is the ultimate deciding factor.
- Start the algorithm. Ideally, you should start the algorithm at a time when your waking time precedes your desired waking time. Otherwise, you may need to temporarily resort to using an alarm clock. Each use of the alarm clock will add to your stress and anxieties related to early bedtime. If you have to use an alarm clock, and if you struggle with getting up in the morning, you should consider taking a full free running cycle with a daily delay of 20-40 min. until you arrive at getting up comfortably 2-3 hours before your chosen waking time. You could then try to re-start the algorithm.
- Protected zone: As you suffer from DSPS, your primary objective is to prevent phase delay. This means that, in your last 2-4 waking hours, you need to avoid light, electricity, excitement, stress, intense sports, TV, computers, social interaction, e-mailing, web surfing, etc. If the presented algorithm does not work for you for a longer time, you may have to extend this "protected zone" up to a point where the resulting "inaction" results in more stress or frustration than it actually eliminates. Extending the protected zone beyond 2-3 hours of dark silence will probably be counterproductive. See for yourself. Remember, the protected zone is not a time for lying in bed! Unproductive time in bed will add to your stress, and if you fall asleep early, you can mess up your sleep on that night and actually delay the phase! Obviously there is a very limited range of activities you can do in the protected zone. I leave it up to your imagination. Perhaps moderate exercise in dim light. Or sex? Or walking and thinking? Seth Roberts swears by one-legged standing before sleep.
- Morning light: One of the most powerful zeitgebers is light. If sunlight can stream into your room at the time when you should wake up, you might eliminate half of the difficulty in achieving a 24h balanced cycle. If this is impossible due to the season, or your early waking hour, or your living arrangements, you might consider using one of the commercial devices used in the "bright light therapy", esp. if you turn on the lights with the timer (if your device has no timer, you can use a timer that cuts off the supply of electricity for the night). Please be sure you read all the relevant safety instructions to prevent any long-term impact on your health.
- Morning exercise: Another powerful zeitgeber is outdoor exercise. If possible, you should schedule that exercise early in the morning, or at least in the first 5 hours of the day. Morning exercise will affect your intellectual performance. You will most likely tire faster. However, this is also one of the reasons why exercise should help you sleep better. Intense exercise late in the day may have the opposite effect and might delay your phase. However, some forms of exercise will not have this effect as the phase-shifting stimuli will be counterbalanced by an increase in homeostatic sleepiness and the "fasting effect" that will help you sleep early. Those evening exercises should be free of stress, injury, emotion, etc. Gentle calisthenics, yoga, or stretching would be an example of harmless evening exercise. Perhaps even body building, if not too exciting or strenuous. This late exercise would best be performed in dim light and in absence of other rousing stimuli. If you can swear that evening exercise helps you fall asleep faster, even if strenuous, remember that sleep itself has a phase-shifting power and can actually outweigh the opposite effect of adrenaline or locomotor activity. If you are not sure, experiment on your own.
- Evening fasting: Unless you are a ravenous type that cannot sleep without a snack, give up meals in the last couple of hours of the day. As explained in the DMH section, food may have an additional impact on the positioning of your circadian cycle, and you do not want your brain to think that evening is an opportune time to get food. A healthy breakfast in the morning should have the opposite effect on the sleep phase and is always recommended by nutritionists even if sleep phase delays are not in the cards. Seth Roberts recommends skipping breakfast, but this should apply to those who suffer from early waking or ASPS, not for onset insomniacs or DSPS. Don't be discouraged by research that may claim that evening fasting will not affect your circadian cycle. For example, Sensi found that morning meals seem to favor carbohydrate metabolism, and meal timing, within the studied range, did not impact circadian cycle (Sensi and Capani 1987). Fasting has many other added benefits, incl. slimmer waistline. Try it for yourself and see the impact of evening fasting on your sleep and health within a week! Read more: Fasting.
- Adding pressure: If you keep failing by waking up too late, or not being sleepy at the desired bedtime, keep adding up pressure at both ends of your night sleep. More exercise in the morning, brighter lights in the morning, longer protected zone in the evening (free of stress, computers, lights, TV, etc.). The harder your case, the more you need to add. Choose a day free from other obligation for an exhaustive exercise marathon (which, for you, may be just a few hours of brisk walking), and spend the evening on undemanding relaxing activities that will help you advance your bedtime. You will need to figure out for yourself what exercise intensity ensures you do not wake up prematurely and what advance you can actually afford without making the situation worse.
- Emergency: Melatonin: If your "protected zone" is stretched to the limit. You may consider an occasional pill of melatonin (e.g. 2 hours before your optimum bedtime). Remember that melatonin will affect your creativity and alertness, and should not be used on a regular basis. You could resort to melatonin on days when you are particularly wound up or when your waking time was particularly late. Remember also that large doses of melatonin may backfire. I guess you should not ever exceed 3 mg, however, you would better consult your sleep expert to make the final decision as to the dosage and timing. If you keep failing with the algorithm, you should rather increase the frequency of taking melatonin. You should not increase the dosage! Do not use marijuana as a "melatonin substitute"! If you think that your sleep and your brain performance do not suffer on MJ, you are wrong!
- Emergency: Radio timer: As the algorithm is supposed to let you sleep without an alarm clock, you cannot expect to wake up at the same time every day. Actually, the larger your DSPS problem, the larger the variations you may expect. Achieving a very regular waking time is a matter of practice that some good sleepers develop over many years of repeated habits. However, if you regularly oversleep by more than an hour, or even still experience the dreaded phase shift, you may consider some occasional mild form of artificial sleep regulation such as turning on your alarm radio at the minimum sound level. Such a gentle alarm is only to help you wake early from the shallowest stages of sleep early in the morning. It should not deprive you of valuable REM sleep, and it should absolutely be set below the level that could rouse you from deep sleep. Your goal will always be to get rid of any interruption or intervention in the morning. Consider this only as a temporary measure that may help you in the initial adaptation period that is bound to include minor failures. You have probably heard of alarm clocks that read your sleep phase and help you minimize the pain of waking. I am not a great enthusiast of such solutions. The concept makes sense, however, it only minimizes the pain of possibly cutting a vital portion of your sleep. Set up an On-timer in your TV, on your favorite channel, at a minimum volume. If it fails to wake you up, you will know you need to work on other points of this algorithm. Trying to wake up for a specific hour is stressful enough to act as a substitute alarm clock that works for many people pretty well. Some scientists believe that ACTH secretion is set to ensure timely waking. I believe that this type of compacted sleep is still better than the one interrupted by alarm clock. At the very least, natural waking after short sleep definitely feels better than sleep interrupted at its deeper stages with an alarm clock. In conclusion, you should remember that your determination to succeed is also an important component of the algorithm. Determination may be unhealthy, it might affect your hormonal profile in sleep, it may weaken your immune system, however, balancing DSPS without giving up modern lifestyle will always have some residual negative health effects. Only a return to an ancient lifestyle would provide a hermetic solution.
- Napping: A very controversial issue in DSPS is napping. Many experts will tell you that you should avoid napping altogether. I disagree. Naps may cause a phase shift if they are taken too late. Early naps, e.g. taken in the 6th hour of your waking day, will not have this effect. Naps will help you get your evening productivity, esp. if early waking in bright sunlight or early exercise aren't too good for your morning alertness. If you keep failing to achieve your 24h balanced cycle, you can gradually shift your naps to earlier hours. This will reduce their length and their efficiency, however, this will also minimize their phase shifting effect. Only if all other suggestions fail, you should drop napping altogether. If you bemoan the loss of mental acuity, remember that we have not evolved to be alert and productive all day. 100% sharp mind is only our desire that has, among others, contributed to the DSPS problem in the first place. See: Best time for napping
- Caffeine: Sleep experts will often tell you to wean yourself off caffeine. However, I beg to disagree. A cup of early morning coffee or tea will likely have a beneficial effect. Its impact on the homeostatic sleep drive will increase your alertness, which on its own has a phase shifting power. Early caffeine will help you advance your phase! You should avoid caffeine in the later parts of the day though. Before sleep you will already be in mild withdrawal, which, theoretically, should boost your homeostatic sleep drive and allow for a slightly earlier bedtime. This way, caffeine may help you achieve phase advances at both ends of your night sleep.
- Alcohol: Drinking alcohol before sleep may have disastrous effects on your effort to balance the cycle. One of your major enemies will be premature waking after premature bedtime. Alcohol will increase that risk manifold. Premature awakening will result in insomnia, in powerful phase shifts, in difficulty in waking, in sleep deprivation, and in an early collapse of all your efforts! Western culture is generally tolerant of moderate drinking in the evening as opposed to the morning when we are supposed to be productive. I think this tolerance should change. As much as early drinking undermines the thinking, evening alcohol might do even more damage by destroying sleep.
- Stress: Stress can ruin all efforts presented in this algorithm. Wherever possible, try to pile up stress in the morning, and let the second half of the day be always free from worries and anxieties. Naturally, stress that lasts for days is a general enemy of healthy and productive living. If this is your problem, please have a peek at the stress section of this article to see if you can find any useful hints there.
- Myopia: If you are shortsighted, you should consider getting a correct prescription. Some theories of myopia favor underprescription (Rehm 1981), however, you will also find evidence to the contrary (Coghlan and Le Page 2002). If you lean to the underprescription side, consider using the correct prescription only in the morning. Only the first 2-4 hours matter. You may recall from your physics class that the amount of light reaching the retina will actually be reduced with the increase to the focal power. However, well-focused light will be more likely to produce strong maxima with a phase-shifting power. Analogously, you might also consider using dimmed spectacles in the evening. Naturally, these are not recommended for "close world" applications (such as reading the computer screen) as these might accelerate the progression of myopia.
- Computer screen: There are computer screen filters and applications that can help you filter phase-shifting light frequencies that can dim your monitor in the late evening. I hear from users of those applications that they are helpful in their DSPS battles. As always there is a risk this might be a placebo effect. However, the cost of trying isn't high.
- Difficulties: If you keep experiencing phase shifts, move your siesta to an earlier time, or give it up altogether (at last until you regain the balance). Use emergency measures (melatonin, radio news) only as a last resort. If those last resort solutions keep being necessary, it may appear that a shifting free running sleep cycle does less damage than trying to stick to a "healthy" stable rhythm. Review all the recommendations on this list. Did you employ them all? Did you show self-discipline? If so, it should work in most cases. All the failures I have seen came from rather flimsy excuses and minor violations. All the success I have seen came from treating sleep regimen as a religious routine. If all else fails, write to me with your story.
- Target: In the initial period, lack of experience, misalignment of sleep control variables, sleep debt, lack of conviction, etc. will make it a bit harder to stick to the presented algorithm. This is particularly difficult for those who do not need to get up early for work. Their mind might vacillate. Is early rising worth the effort? It may therefore be psychologically helpful to begin one's sleep reform 3-4 weeks before an important event that requires early rising. The thought of participating in an important event with a fresh mind will increase the motivation to adhere to the rules.
- Creativity dip: Once the presented algorithm works for you, you may be disappointed to notice that your creativity isn't as rampant as on those weekends when you get your solid 8 hours of sleep, or when you go to sleep at 5 am (whichever is your cup of tea). That's the unavoidable cost of sleep phase resetting and phase advance measures. We have evolved to live 24h lives with a primary concern for surviving, getting food, etc. Crisp or creative mind isn't that important when you are to run away from a predator. When you meet a lion on a street, you are bound to be in your top shape for flight. The degree of your creative decline may be proportional to the degree of the original problem as well as the length of the period in which you tinkered with your sleep control systems using the alarm clock, sleeping pills and other brain health enemies.
- It works! Remember that every healthy individual can stick to a 24 hour cycle on a farmer's lifestyle. Some disabilities, e.g. blindness, may make it impossible, but these should only be an exception to the rule. At worst you could try out the farmer's lifestyle to realize that it solves your sleep problem, and then try to reconcile that extreme with the presented algorithm. There is an optimum for you somewhere in-between and it does not need to be unhealthy.
- Ultimate cure? The title of this chapter should not make you think that you can cure DSPS for good. It is always there. You can temper your response to the temptations of modern life and return to a balanced sleep pattern as your reward. Your lifestyle is your cure.
Outcome: Balanced 24h sleep cycle!
Figure: Perfectly balanced 24h cycle in DSPS:
- the green line determines the conditions for the balanced 24 hour cycle (waking time + sleep time = 24 hours)
- the nap is taken in the 7th hour from arising (the left blue peak) and lasts 1.2 hours (the left purple vertical line). The nap is used to partly compensate for a short nighttime sleep
- the optimum length of the waking day is 18.3 hours and is determined by the crossing of the green line that determines the 24h sleep-wake cycle and the average sleep time depicted by the red curve. Shorter day is possible but entails a risk of sleep-onset insomnia. Longer day will result in phase delays (the area where the red line is located above the green line)
- the optimum length of the night sleep is 5.7 hour (horizontal orange line).
- the cycle implies 5.7 hours of sleep in the night and nearly 6.9 hours of total sleep, which might be equivalent to 9 hours of nighttime sleep in monophasic sleep (see: Optimum nap duration)
Summary: Curing DSPS
In the presented algorithm, you try to stick to your optimum bedtime and waking time every day. You establish a protected zone in the evening to favor phase advance (minimum light, computers, stress, excitement, etc.). You wake up to bright sunlight and use morning exercise to advance the phase in the morning. You ingest caffeine only in the morning. You avoid alcohol in the evening. If you nap, you nap early. If your phase keeps shifting, you add more light and exercise in the morning. You also extend your protected zone in the evening. In an emergency, when you fear falling out of synch, you could occasionally use melatonin in the evening or delicate sounds in the morning as the minimum effective departure from the free running sleep principle.
Charting phase shifts
Phase shift graph
The phase shift graph may be used by people in free running sleep suffering from ASPS or DSPS. This graph shows the degree of phase-shift as well as its dependence on the time of day. The graph can be used to see the expected bedtime given a specific natural waking time:
Blue line shows the bedtime (vertical axis) for days with a given waking hour (horizontal axis). Red line shows the next day's waking hours (which are shifted by 1-2 hours in DSPS). Fuchsia and gray lines indicate the siesta period. Even though the red waking line begins at the origin of the graph, it shows a substantial phase shift at later hours (DSPS). From the presented exemplary graph one can read that for the waking time equal to 7 am (horizontal axis), the expected time to go to bed, as indicated by the blue line, is 1 am (vertical axis), while optimum siesta time occurs between 15:00 and 16:00. However, if the wake time is 11 am, the bedtime is likely to come only at 5 am the next day.
Remember! Each individual will have a his or her own unique graph. Moreover, the graph will look differently if it is taken at times of work or at times of summer vacation. It will be affected by stresses at work and at home. It may even change when you move from one house to another, or when you change the climate zone. The graph will accurately reflect your rhythm only if you adhere to free running sleep. If you use an alarm clock, this graph will be meaningless!
Correlates of sleep phase syndromes
It is not known which are the predominant underlying physiological factors that result in sleep phase disorders. Family clusters show that genes may affect the length of the circadian period. The lifestyle will affect the levels of neurotransmitters and via their impact on the sleep phase will affect the period of the circadian clock as well. Lifestyle also affects the timing of zeitgebers (e.g. late night web surfing in DSPS). Conversely, the level of neurotransmitters may select for a specific lifestyle choices. Age may have a direct impact on the clock circuits, it may affect neurotransmitters, or it can affect the lifestyle. Last but not least, sleep phase disorders will affect the mood and the levels of neurotransmitters in varying ways depending on whether free running sleep is used to remedy the disorder, or whether the individual attempts to fit a predetermined desirable sleep schedule.
Subsets of circadian rhythm sleep disorders (CRSDs) are strongly correlated with certain personality characteristics, and may have a strong genetic background. DSPS is more prevalent among adolescents, while ASPS is more frequently observed in an aging population. Women prevail in ASPS, while a slightly larger proportion of males suffer from DSPS (Sack et al. 2007). Impaired vision often leads to DSPS due to a lesser impact of light on the circadian clock.
DSPS is by far more frequent among students, programmers, avid readers, passionate artists, writers, computer game addicts, etc. It is possible that the same characteristics that help individual's creativity may also lead to problems with falling asleep early. ASPS seems more likely in individuals whose life is deprived of intense stimulation (esp. in the evening), who meet fewer new challenges, who are less passionate about their job or hobbies, or who are not facing information overload and the related stress, etc. Perhaps this is why ASPS is more prevalent in the elderly. For hormonal reasons, its prevalence also shows a sharp increase around the time of menopause in women. ASPS tends to run in families. A number of genes have been identified to be involved in FASPS (familial ASPS)(Golombek and Rosenstein 2010). See: Clock genes.
There is a complex relationship between DSPS/ASPS and psychiatric disorders. 25% of people who could not maintain their 24h sleep-wake cycle were suffering from a psychiatric disorder (Hayakawa et al. 1998). Some psychiatric disorders or the prescribed medication may induce DSPS, while, at the same time, DSPS conversely may cause various psychiatric symptoms. On one hand, there may be a link between DSPS and manic personalities. Anti-depressants tend to increase the period of the body clock (e.g. clorgyline, imipramine). On the other, paradoxically, DSPS individuals may be more likely to suffer from depression (e.g. when suffering from persistent insomnia, sleep deprivation, and the resulting social problems, etc.). Dr Daniel Kripke concluded that DSPS phenotype is familial and is associated with unipolar depression (Kripke et al. 2008). However, the epidemic of DSPS in creative individuals suggests that those correlates need further investigation. Perhaps some contradictions can be explained by the fact that the state of mind of a DSPS sufferer depends largely on his or her ability to get sufficient and properly timed sleep? Thus more on a naturally manic side when sleep-satisfied, e.g. on a free running sleep schedule, and more on the depressed side when in circadian trouble (e.g. when forced to an early waking schedule)?
Similarly, low-stress tolerance depressed individuals are more likely to suffer from ASPS. Again, when they are forced to adapt to "normal" life, their symptoms of depression tend to weaken either due to a sense of higher productivity or due to the fact that mild sleep deprivation counteracts the depression. The cause-effect relationship between sleep phase disorders and mood disorders is complex. Understanding it will contribute substantially to mitigating the escalating epidemic of sleep problems.
In 1992, Dr Thomas Wehr published the results of his interesting experiment on sleep in periods of prolonged darkness (e.g. as seen in Inuit during the arctic winter)(Wehr 1992)). He divided the experimental day into 10 hours of daylight (photoperiod) and 14 hours of darkness (scotoperiod). This type of artificially controlled environment resulted in segmented sleep that was often composed of two 4-5 hour segments separated by an hour of wakefulness. Wehr found that the onset of sleep was associated with an increase in melatonin, which is released in the periods of darkness (in both diurnal and nocturnal animals). He also noticed that the release period of nocturnal melatonin lasts longer in shorter photoperiods (Wehr et al. 1993).
Interpretation of segmented sleep
When the results of the experiment were publicized, insomniacs rejoiced: Perhaps this is normal? Perhaps this is how we all should sleep? Those who tend to wake up in the night and spend an hour or so reading, watching TV or plundering the fridge, no longer had to feel abnormal. Indeed, the best criterion that should separate healthy sleep form unhealthy patterns is the refreshing power of sleep. Nocturnal awakenings should not matter as long as they did not contribute to morning misery.
A historian, Dr Roger Ekirch noticed that this prolonged two-part sleep is frequently mentioned in historical records that predate the advent of electricity: "Until the close of the early modern era, Western Europeans on most evenings experienced two major intervals of sleep bridged by up to an hour or more of quiet wakefulness. [...] The initial interval of slumber was usually referred to as "first sleep," or, less often, "first nap" or "dead sleep." (Ekirch 2001).
Sleep researchers speculated that this is perhaps how healthy sleep should look like and that our sleep control models with a single nighttime circadian peak are wrong. Evolutionists speculated that this could be an adaptation to nighttime sex, or breastfeeding, or periods of extra vigilance. Dr Horne likes to refer to segmented sleep as an example of the human propensity to excessive sleeping.
Segmented sleep and Borbely model
I happen to disagree with most of the interpretations put forward thus far except those that stand in agreement with the mainstream sleep research. We need to observe that most of human and pre-human evolution took place in tropical areas with far shorter nights than those that characterize winters in the north, and, mathematically speaking, there should be no preference for waking up in the middle of the night for 1-2 hour as opposed to entering shallow sleep or waking at the end of each full NREM-REM cycle. The segmented sleep observed in Wehr's experiment can easily be accounted for with Borbély model of sleep. Even though Borbely model provides very specific mathematical conditions needed for initiating sleep, we must remember that it is only an approximation of reality that does not necessarily account for the level of lighting or external arousing stimuli. In periods of prolonged darkness and silence, lesser overall sleep propensity will be needed to initiate sleep. I will try to illustrate my claim using SleepChart's two-component model and some real life examples.
Segmented sleep and two-component model
When Wehr's data are processed using SleepChart's two-component model, we see that sleep is characteristically initiated at periods of relatively low sleep propensity:
Wehr's segmented sleep as interpreted with the help of the two-component model of sleep employed in SleepChart.
In real life, the two-component model indicates that sleep is initiated when alertness levels drop to 33-40%. In segmented sleep, alertness at sleep onset is much higher: 40-50%.
Circadian graph shows that the favorite sleep initiation hour is the 15th from arising and it results in 5 hours of sleep on average. As waking comes close to the circadian bathyphase, wakefulness cannot last long due to a rapidly ascending circadian sleepiness. The second bout of sleep then follows in the 21st hour and lasts 3 hours on average. Thus the sleep is segmented into a 5 hour long pre-sleep and 3 hour long "correction".
Sleep periodogram shows a typical frequency peak at period 24h, and two atypical peaks at 8 and 6 hours (instead of the usual 12h siesta peak).
Examples of segmented sleep
I scanned years of sleep logs in search for natural segmented sleep examples. I did not found that many. Without doubt, I can blame the modern lifestyle that rarely permits a leisurely early bedtime. Below I list three very different examples from real-life logs. Several characteristics seem to be associated with segmented sleep:
- this sleep often results in high alertness, e.g. as testified by the absence of daytime napping
- as predicted, this sleep is easier to find in winter periods
- intense exercise may be a factor that helps induce segmented sleep (perhaps due to earlier bedtimes)
- as in Wehr's data, SleepChart's two-component model of sleep propensity shows a very rapid decline in sleep propensity early in segmented sleep due to the fact that the sleep is initiated very early in reference to the circadian bathyphase
Example: Premature bedtime
Typical long darkness premature bedtime segmented sleep. Sleep initiated too early, again with a very marked decline in sleep propensity resulting in a nocturnal awakening:
Example: Nocturnal awakening caused by stress
Premature awakening caused by stress. Segmented night with a "correction":
Example: Intense exercise
Prolonged sleep induced by intense exercise with increased demand for sleep. Over 12 hours of segmented sleep are initiated early with multiple harmless awakenings, fast decline in sleep propensity (inverse of the red line) in the first 3-4 hours of sleep that results in shallow prolonged sleep:
Application of segmented sleep
This type of sleep results in very refreshing nights, however, it would be pretty hard to implement in agreement with the modern lifestyle. Certainly, it would not optimize the time use. I can only guess that matching sleep well with the circadian bathyphase should also increase the efficiency of sleep. This should be verifiable with SuperMemo data, however, as of the moment of writing, I have not done the necessary computation. Considering the nice effect of this ancient sleep on mood and alertness, I would love to subject myself to an experiment with 14 hours of darkness, however, 10 hour working day would be pretty hard to stomach even in a short term. I also doubt I would be able to extinguish the thoughts of the day and initiate sleep early. Even the mere fact of collecting exciting data for the experiment would keep me up with my thoughts racing. I might try this in retirement when my vital powers decline sufficiently enough to make it truly enjoyable.
Function of sleep
Origins of sleep
Human brain is the highest achievement of the biological evolution. It all started from a simple ability to conduct impulses. Then the amazing concept of a neural network was developed. The brain of primitive vertebrates started adding new structures as well as new mechanisms for optimizing the jungle of neural connections. Sleep is a relatively old invention used to re-organize memories via molecular and neural mechanisms. Circadian rhythms are known in plants and in animals independent of the need for sleep. The process of evolution has, however, conveniently hooked up sleep to circadian rhythms to efficiently alternate between the explorative state (i.e. the use of the brain for learning new things) and the consolidation state (i.e. sleep). The circadian cycle has been associated with around a hundred known physiological functions and parameters that change in concert during the day (this number now increases rapidly, e.g. with circadian analysis of gene activation). Closely related to sleep are cycles in the levels of hormones such as serotonin and melatonin, ACTH and cortisol, acetylcholine, adenosine, and growth hormone. There is a circadian function that we can observe on our own without complex measurements: changes in the body temperature (see the figure Temperature changes in the course of the day in Biphasic nature of human sleep).
As we spend a third of our lives sleeping, there is little wonder that sleep has attracted lots of attention from neurophysiologists. Given the enormous complexity of the brain and its functions, there have been literally hundreds of theories that attempted to explain the role of sleep. Only recently, with the arrival of new research technologies, have we been able to see the big picture in the sea of detail.
Over the last two centuries, dozens of theories of sleep have been proposed. Some scientists believe that we sleep to remember. Others believe that we sleep to forget. Yet others believe that sleep has nothing to do with memory. There are also theories that come from philosophers, religious figures, ideological movements, etc. For example, "sleep maximizes positivity" is a very vague "theory" that is actually largely correct.
For a biologist, the best ground for determining a validity of a theory is its evolutionary perspective. The main question to ask is: what vital function is subserved by sleep that demands turning off cognitive functions for a third of our lives! Each sleep theory must past this primary litmus test. Let's call it a shutdown test. Allan Rechtschaffen put it best saying "If sleep does not serve an absolutely vital function, then it is the biggest mistake the evolutionary process has ever made". The shutdown test is the best sieve for eliminating implausible theories of sleep, however, as an exercise, you can also check which theories explain the fact that if we do not get sleep on one day, we need more on another. For example, if sleep was to help avoid predation, we would not incur the predation avoidance debt that needs to be repaid. If we survive predation despite roaming around, our chances of survival on the following night are exactly the same.
We cannot forget, naturally, that some variables related to sleep do not need to be an expression of its primary function. In evolutionary terms, sleep is a very old phenomenon, and all species learned to attach dozens of neural and non-neural functions to this state. For example, sleep is the main anabolic state for the body. For this multifunction reason, it is hard to determine the primary function of sleep by just studying variables such as the sizes of animals, sizes of their brains, their habits, types of food, longevity, levels of activity, proportion of sleep spent in REM, etc. These all studies are very interesting but seem to explain little because of the huge number of overlapping variables which effectively obscure the main function of sleep.
Below I will quickly list theories that attempt to explain the role of sleep. I will divide them into those which answer and those which fail to answer that main question: why the conscious mind needs to be turned off. Let me start with a subset of countless theories that fail to pass the shutdown test (as suggested above).
Theories that fail the shutdown test
Here are just a few of innumerous theories that may be based on true facts, true models, or partially explain the function of sleep. However, they do not explain the essential need to "lose consciousness" for many hours each day:
- energy conservation - If sleep was just a form of rest, why would we not just lie down in a dark place, while staying vigilant to predation. The brain is actually buzzing through the night as much as in waking, so turning it off would not offer any extra advantage. Sleep is no hibernation.
- repair and restoration - If sleep was just to rebuild the body used up during the day, why does it not employ a simple metabolic reversal without a loss of consciousness? Many tissues can restore and repair in waking. Dedicated anabolic state is useful, but it could easily be enacted with the brain still on.
- reactivation of infrequently used circuits - If activation of infrequently used memories was necessary to prevent forgetting, evolution might have simply employed molecular mechanisms in which once synapses were potentiated, they would spontaneously build up synaptic strength in time without secondary activation. Naturally, this would abolish the valuable evolutionary mechanism of forgetting. Similarly, random REM activation for the purpose of memory fixation would simply be wasteful. Forgetting plays a specific evolutionary function and the increase in synaptic strength should only result from a repeated exposure to a specific experience (for example, a repetition in SuperMemo serves the exactly this purpose)
- recovery from slow-wave sleep - REM sleep was postulated as a recovery stage from NREM sleep. If REM was just a recovery stage, why should it happen several times during the night? Would it not just better come at the arrival of zeitgebers indicating the end of the sleep period? Why would we enter NREM sleep in the first place? NREM and REM clearly play a complementary function, but the theory says little about the need to enter the unconscious state of sleep.
- avoidance of predation - Inactivity during sleep is supposed to minimize exposure to predators? However, why would we need turn off all the senses and extend reaction times to a near-useless degree? Moreover, carnivores tend to sleep far more than omnivores. This would hint that vigilance might be better than slumber in eluding the predator.
- reversing oxidative stress damage - How does a loss of consciousness help the process of cleaning up free radicals? Carnivores sleep more than omnivores? Slow and big elephants need 3 hours of sleep while fast-living tiny rats may sleep for 18. Not all sleep variables need to be an expression of its neural function.
- Siegel theory - Dr Jerome Siegel, a famous opponent of the link between sleep and memory, proposed his own theory of sleep: "sleep is merely a good way to increase an animal's efficiency, by hunting only when the hunting is good, for example, and minimize its risk" (Siegel 2009). A tiny subset of the complex sleep control machinery would be sufficient to meet Dr Siegel's proposed function of sleep. For example, it would be enough to keep the animal awake and paralyzed as in REM sleep. All the rest of sleep control would be a simple waste of time and resources. For more on this theory see: Robert Vertes and Jerome Siegel
- corneal anoxia - a theory says that REM sleep nourishes the corneas. Again, would not just waking up do the job if eyeball movement is all that is needed? If that function needs the anabolic state of sleep then it is only secondary to the main purpose of sleep. That eyes move in REM might simply be an expression of the fact that this is a movement that can do animal no harm. Possibly, eyes move just because their muscles do not need to be turned off.
- promoting longevity - sleep is vital for survival, as such it contributes to longevity indeed. However, so does the liver, and no one claims that the main function of liver is to promote longevity. Moreover, the evolutionary process does not optimize for longevity, but for the perpetuation of a gene pool.
- thermoregulation - some scientists believe that sleep serves thermoregulation while others attribute a large role for the body core temperature in generating sleep propensity. Dr Kurt Kräuchi summarizes his reasoning as follows: "all thermoregulatory effects which lead to an increase in the core/shell ratio (e.g. a reduced shell by increased distal skin temperature) lead to increased sleepiness and, as a consequence, to increased sleep propensity. However, the sleepiness/sleep regulatory system feeds back onto the thermoregulatory system only indirectly via sleep-related behaviors (e.g. relaxation, lying down)" (Kräuchi 2007). That theory speaks a lot about causality, but very little about the purpose. It flunks the shutdown test.
- detoxification - One of the fathers of sleep science, Romanian psychiatrist Constantin von Economo asserted that sleep was a detoxification process, in which the hypnotoxins were removed from the blood. However, some New Age authors who attempt to revive this view seem to commit a simple logical fallacy in which the brain shuts down for the purpose of preventing a hypnotoxic shutdown. The hypothesized function could well be done by the kidneys, the liver and other mechanisms that do not even come close to passing the shutdown test.
- other theories: other theories of sleep such as psychoanalytical theories, Cayce theory, etc. border on paranormal and pseudoscience. They all may carry a grain of truth (e.g. neural network optimization will indeed serve self-edification), however, listing them all goes well beyond the scope of this article.
Theories that pass the shutdown test
If we consider the current status of knowledge about the function of sleep, many of the older theories start making sense. They all seem to converge into a central theme and all carry lots of inspiration. Here are some of these:
- Krueger and Obal theory: the theory speaks of a natural propensity of neural networks to enter sleep states. However, the theory probably goes too far when calling sleep a network-emergent phenomenon that requires little or no central control (compare: Why we fall asleep?).
- dreams create wisdom: yes! Dreams are an expression of a process that serves reorganization of memories. As a result of dreaming, knowledge becomes more coherent, more abstract, more applicable, etc. In sleep, skills get honed and fine-tuned. This means that we are wiser and more skillful in the morning than before going to sleep!
- garbage collection: an early theory by Evans (1965), derived from computer garbage collection metaphor, says that sleep is needed to clean up the brain from useless memories. Indeed, there is a lot of evidence on synaptic weakening in sleep. Lots of garbage and chaos evaporates from the brain overnight.
- reverse learning: inspired by the ideas of Evans, reverse learning was proposed in 1983 in Nature by Crick and Mitchison. Reverse learning theory, a neural equivalent of garbage collection, says that dreams sift away memories that are not needed, irrelevant or incongruent. Born in 1916, Francis H.C. Crick together with James Watson unraveled the mystery of the DNA double-helix for which they were awarded a Nobel Prize in 1963. In 1983, Crick and his colleague from Salk Institute, Graeme Mitchison argued in Nature (Crick and Mitchison 1983) that human memory can get overloaded and REM sleep is used to run a garbage collection process on memories. They called this process reverse learning, and originally attributed it only one purpose: forgetting the unnecessary memory ballast. Crick named those ballast memories "parasitic memories", saying that "REM sleep allows us to have a smaller brain for a greater purpose". Crick's seminal 1983 article was taken further by many researchers who have ultimately concluded that REM sleep must serve optimization of memories expressed by synaptic weights of the neural network of the brain! If REM sleep was only to be used for forgetting the excess information, Crick's theories would run into the same evolutionary trouble as other theories listed above. After all, spontaneous loss of synaptic information with passing time would be a cheap solution that could be easily implemented metabolically. The value of the network optimization goes far beyond just forgetting.
- consolidation of memory - some researchers posited that memories get consolidated in REM sleep. What is the purpose of NREM then? How can this be that antidepressants do not damage learning even if they are strong REM suppressants? Why does the REM brain behave like a waking brain? The term consolidation is a bit ambiguous. It may refer to processes occurring at the molecular as well as at the neural levels. Only neural consolidation might require turning off the brain. In reference to procedural learning a distinction has been made between acquisition, consolidation (post acquisition) and enhancement (sleep dependent). Of these, only the enhancement would depend on sleep. For a number of procedural tasks, sleep-dependent enhancement has been demonstrated (Stickgold and Walker 2007). For more see: Neural optimization in sleep
- psychotherapeutic theory - The theory says that in sleep we dream up situations that would be emotionally difficult to confront in real life. This is supposed to help us learn how to react in emergencies without actually living through those emergencies. This interpretation is plausible due to the fact that it involves an actual need to turn off the consciousness. Moreover, when dream contents are analyzed it is indeed found to be dominated by threatening situations. The psychotherapeutic theory, however, should rather be considered a narrow interpretation of a larger and more encompassing function of sleep: optimization of the neural circuitry. In this case, known information is played back in various constellations to compile new behavioral strategies that might be useful for survival. As I try to argue throughout this article, creative and optimizing function of sleep is more general and needed also for other neural functions, not only those related to the threats encountered in waking life.
- activation and synthesis theory - Proposed by Hobson and McCarley in 1977 (Hobson and McCarley 1977), Activation-Synthesis theory says that during sleep there is a sensory inhibition (input attenuation), and, during REM, there is a motor inhibition (output inhibition). Pontine PGO waves characteristic of the REM sleep travel up to the cortex that tries to interpret that stimulation resulting in dreaming. Visual, auditory and motor areas of the cortex are active and responsible for whatever we experience in dreams. Activation-Synthesis theory inspired further research into the neural function of REM sleep.
- AIM model - the AIM model is a Hobson's improvement upon the activation-synthesis model that has dominated this field of research for two decades (Hobson et al. 2000)(picture). In the AIM model, brain states are described by 3 variables: (1) Activation, (2) Input source, and (3) Modulation. A healthy brain moves in selected trajectories in the AIM space. Various psychiatric and sleep disorders can be characterized as the departures from those trajectories. The model does not explain the function of sleep, but helps describe sleep in simple neurohormonal terms. For example, REM sleep is characterized by high activation, minimal sensory input, and cholinergic modulation.
- brain development - In brain development theories, dreaming is supposed to provide internal stimulation of brain circuitry to ensure growth and development. As babies dream far more than adults, this theory finds inspiration in the correlation between the degree of immaturity in infancy and the proportion of REM sleep in animals. Human babies are particularly limited in their ability to interact with their environment and dreaming is said to act as a substitute for the waking experience. REM-induced muscle atonia combined with the brain activation would enable brain growth by activating the synapses without the negative consequences of acting out. The role of REM sleep in developing the visual system has been demonstrated by Stryker (Stryker et al. 2001). REM deprivation results in developmental abnormalities later in life. Recently, the role of NREM sleep has also been confirmed.
Sleep and learning
How sleep affects learning?
Why is sleep important for learning?
If I was to bet on the top two factors that hinder learning in industrialized nations, these would be:
- Stress which takes away your focus, stifles creativity, saps motivation, and which can contribute to poor sleep
- Sleep which is needed for optimizing memories. Without sleep, you cannot even experience the sense of a "good day"
Health is important too, but, statistically, it is stress and bad sleep that affect nearly everyone, and take the largest toll. Reduce stress and improve sleep, and you might see a society changed beyond recognition!
For healthy people, all other factors in learning seem to be somewhat secondary. Self-discipline improves greatly if you are rested and happy. The fun of learning follows. The way you approach learning, tools and techniques, the way you represent knowledge in your mind, and other factors can all be improved gradually and consistently. If you are on a steady path ahead, success is nearly guaranteed. Metaphorically speaking, your brain comes with a solid warranty of progress that you can easily void with stress and/or poor sleep.
Given the importance of sleep, unless you are a "natural" and rarely get a bad night sleep, you should understand the basics of sleep physiology and the impact of your sleep habits on learning. Moreover, even if you sleep well today, you are always in danger of ruining your sleep patterns through the use of computers, Internet, mobile phones, SuperMemo, etc. In short, the human brain has not yet got enough time to evolve and adapt to the stimuli of the modern lifestyle. That's why we witness an epidemic of sleep disorders in industrialized nations.
In the following sections, I will try to show that the impact of sleep on learning goes far beyond the simplistic concept of "rested mind".
Sleep and learning research
Everyone knows that without a good night in bed, the next day can be ruined. When sleepy, you can easily shovel the garden in fresh air, but if you try some creative work in front of your computer in a warm room, your brain will tend to switch off and stifle any creative progress.
It is quite evident that cognitive functions and learning are the prime victims of sleep deprivation. Scientists have for long suspected that the main function of sleep is related to learning and memory. Even in the 17th century, John Locke campaigned for good sleep for kids for those reasons. However, only recent decades and years brought an exponential increase in evidence demonstrating the role of sleep in memory. There are still prominent sleep researchers that dispute the link. Some insist that only a conscious brain can be involved in memory. Others claim that sleep is like eating, if you can get more, you will always want to get more. Outside the scientific community, sleep is held in an amazing disregard. Many people do not want to waste time on sleep to economize more time for work and "creativity". Others try to get "best" sleep in minimum time (see: Polyphasic sleep).
The worst part of that disregard is that little kids worldwide are woken up early in the morning to go to school to "learn". Not only does their learning suffer, or even becomes futile; not only do those kids get stressed and cranky; their health can be affected. Their immune systems undermined. Their long-term development stunted. Some sleep researchers try to battle the establishment for more rational school schedules (hats off to Dr Mary Carskadon and Dr Amy Wolfson; see interview). At the same time, the ever-present rat race produces forces in the US, in Europe, and beyond, that insist on even earlier school hours. That comes from both parents and from the authorities. They all bring up a spurious and biologically untenable excuse: the kids can just go to sleep earlier.
In this gloom and doom scenario, there is still a ray of hope though. Science is slow to percolate into social awareness; however, in the end, it wins most of the time (except where it needs to combat stronger forces; e.g. intelligent design theories still keep doing well with the backing of religious doctrinaires). My optimistic prediction is that, sooner or later, governments, school authorities, and parents will realize that the use of an alarm clock to rip kids from their beds contradicts the goals of education!
Studying sleep and learning with SuperMemo
For three decades now, I have been interested in the negative impact of modern lifestyle on sleep and learning. I have suggested that a large proportion of sleep disorders can be remedied with simple techniques such as chronotherapy, free running sleep, etc. The first step towards a solution to a sleep problem is the understanding of one's own sleep patterns. For that reason, I have encouraged people with sleep problems to collect their sleep data with SleepChart freeware that was released in 2003 (download). When SleepChart was created it was not clear what benefits it would bring. I have suggested that SleepChart might in the future be used to investigate the links between sleep and learning, and that SleepChart could become a tool for the optimization of learning, esp. when used in conjunction with SuperMemo. One of beautiful things about SuperMemo is that it keeps a detailed record of memory performance while you learn. If that record could be combined with measurements of sleep quality before and after learning, an ocean of research opportunities would emerge. It was the SleepChart application that provided the missing link. With SuperMemo and SleepChart, we can collect data that can provide answers to a virtually infinite set of questions about sleep and learning. However, my suggestion that SleepChart and SuperMemo be integrated, raised a lot of opposition, primarily from users of SuperMemo who have always complained that the program fell into an endless spiral of mounting complexity and that few users will ever need or make use of the new functionality.
Long sleep results in poor learning?
As of 1996, SuperMemo makes it possible to keep a detailed record of all repetitions. You can check which piece of knowledge was reviewed, when, and with what outcome. As of January 2000, I kept a detailed record of my own sleep timing. I was always curious how sleep affects learning and how learning affects sleep. With learning and sleep data at hand, I could look for correlations between the two. My first, most atavistic and raw intuition was that it should be easy to show that short sleep produces poor learning. "Does more sleep help learning?" I took my own sleep-and-learning data to quickly investigate such a correlation. However, nearly a reverse relationship could be demonstrated. In retrospect, the paradox is very easy to explain: in free running sleep, which I practise religiously, there is a correlation between the quality of sleep and its length: the better the alignment of the sleep episode with the circadian rhythm, the shorter the sleep, and the better its quality. Unless they are sleep deprived, healthy people sleep long only if they sleep in a wrong phase. Optimum sleep is usually very short. In other words, length of sleep is no measure of sleep quality.
Learning reduces the demand for sleep?
An analogous question to ask was "Does learning increase the demand for sleep?" When I tried to investigate this mirror question, I was equally unsuccessful. Again an inverse correlation could be noticed. This time, the reason for that surprise was that insufficient sleep discourages learning. This way, less sleep means less learning, and longer sleep on the following night to repay the sleep debt. In other words, lots of learning would paradoxically be followed by little sleep! For more details see: Impact of learning on sleep.
Those failure made it apparent that little evidence can be garnered on the relationship between sleep and learning without considering the circadian timing, i.e. the time in which learning takes place in reference to the sleep phase (e.g. as determined by the natural waking hour).
Approximating the sleep phase
In the next step, I was hoping to see a correlation between learning and the disparity between sleep time and sleep phase. However, for this correlation to be computable, one needs a good estimation of a circadian rhythm phase. SleepChart uses a rough heuristic algorithm that attempts to do just that. However, this algorithm was too weak to interpret major disturbances in the sleep rhythm caused by delayed sleep, stress, exhausting exercise, etc. That algorithm was replaced in SleepChart 2.0, which uses a recursive phase response curve (rPRC) to estimate the circadian acrophase. rPRC is a variant of a phase response curve that is based solely on the outward expression of the circadian rhythm as documented by sleep logs, and whose only phase shifting stimulus is the delay in bedtime (in reference to the optimum bedtime).
Timing of repetitions
Another stumbling block in further research was a feature used in SuperMemo called Midnight clock shift. It makes it possible to use circadian time for repetition record as opposed to clock time. For example, if the student keeps working after midnight, repetitions are recorded for the previous day, not for the new calendar day. That could cause misalignment of sleep and learning data by an entire day. Sadly, earlier versions of SuperMemo kept only the date of the repetition, not its precise time. This was changed only in SuperMemo 13 (2006), in which the clock time of each repetition is recorded. This makes it possible to compute the exact circadian timing of each memory recall act. At last, it was possible to correlate sleep data with learning in precise time frames! I had the alpha release of SuperMemo 13 for Windows available as of July 17, 2006. The data set is getting bigger and more meaningful with each passing day. The circle of people logging their sleep in SuperMemo is increasing.
The impact of SleepChart
The application of SleepChart in SuperMemo surpassed all expectations in its value. It is now a unique tool for investigating sleep and learning. At the time of the first major results publication in 2012, this was the only tool in the world that made similar investigations possible. The employment of SuperMemo in this research is essential as it effectively aims at the same level of knowledge retention at each review. This provides for a steadier comparison platform between different levels of circadian and homeostatic sleep propensity. Developers of other spaced repetition applications have never expressed much interest in investigating sleep. Moreover, the extra accuracy of the newest SuperMemo algorithm provides for extra sensitivity that should yield faster clarification of trends and correlations even for smaller datasets.
You can also join the research effort! In SuperMemo 15 Freeware, you only need to log in your sleep and send the data with just one button push. More details on the functionality of SleepChart in SuperMemo can be found here.
Recall vs. Consolidation
In studying the impact of sleep on learning, we have to separate two important measures of memory: recall and consolidation.
Recall measures the proportion of pieces of information that can be recalled from memory at any given circadian time. In SuperMemo, recall can be simply measured as the average grade received in learning within a selected subperiod of circadian time. Grades can be converted to percent recall, or can be used as an equivalent measure of recall. The conversion to recall may be of all-or-nothing type (successful recall is treated as 100% recall, while a recall failure is treated as 0% recall). The conversion can also rely on the expected and/or estimated forgetting index in SuperMemo to provide a more precise reflection of recall difficulty. The conversion that uses the forgetting index may be based on the correlation between grades and the expected forgetting index, or can use a heuristic based on the subjective estimated forgetting index assessment (note that the estimated forgetting index, unlike the expected forgetting index, is not part of repetition history in SuperMemo). That latter, seemingly less precise approach, provides sharper contrast between recall levels and is accomplished by depressing the Exp FI button in alertness graphs in SleepChart.
Consolidation measures how well we consolidate or re-consolidate memories with repetitions executed at any given circadian time. Recall measurements are fast. We get our data on the day of learning. We instantly know if we can or cannot answer questions at the selected time. However, memory consolidation data may take years to collect. We may review an item today, and need to wait several years before the outcome of the review (consolidation) can be verified. As sleep-and-learning options in SuperMemo are relatively new (timing of repetitions is collected as of 2006), only very large sets of data collected over the periods of many years provide a basis for meaningful memory consolidation measurements.
Exemplary illustration of the speed in which recall drops during a waking day. In this example, the average grade drops from 3.3 early in the day to less than 3.0 after 16 hours of waking.
Interestingly, even a short nap seems to bring the recall back to the baseline level. In other words, there seems to be a direct link between recall and alertness. recall seems to be inversely correlated with the homeostatic drive to sleep. A slight increase in recall around the 12th hour of wakefulness is a reflection of the circadian component of alertness. The waviness at later waking hours seen in the graph comes from the scarcity of data as learning at later hours makes less sense (of total 31,000 repetitions used to plot the graph, only 684 fell beyond the 10th hour of waking).
An exemplary recall graph displayed by SleepChart shows the decline in grades scored in learning during a waking day. This graph also shows a slight increase in the grades in the second half of the day due to circadian reasons.
Note that both graphs above show a similar time constant of 178 and 172 respectively (half-life of 124 and 119 hours). For calibration reasons, half-life becomes meaningful only when actual recall percentage data is used (in SuperMemo, grade 3.0 is a sharp border between recall success and recall failure).
The decline in the ability to consolidate memories during the waking day follows a curve that mirrors the decline in the ability to recall things from memory!
Exemplary relationship between the circadian time (hours from waking) and the ability to consolidate memories (expressed by an average grade scored in the next repetition)
As the day goes on, the ability to store facts in memory declines. A repetition in SuperMemo is a single effort to recall previously learned information from memory. The graph has been constructed by correlating the circadian time of one repetition (in reference to waking time), and the grade scored in the successive repetition of the same piece of information. The successive repetition often takes place months or years after the repetition for which the memory consolidation time was registered. Again, short naps seem to restore memory consolidation power to baseline. As much as recall, memory consolidation seems to be inversely correlated with the homeostatic drive to sleep. A slight increase in the quality of learning can also be seen around the 12th hour since natural waking (in the presented case).
The conclusion is that in free running sleep (i.e. primarily in the absence of an alarm clock), we can get best learning results if we learn early in the morning. The same holds for exams. The recall and exam results will be best if the exam is held in the morning even though some time for pre-exam cramming may skew the outcome.
Correlation between recall and consolidation
The fact that both recall and memory consolidation curves seem to follow a very similar course during a waking day seems to indicate that they both may depend on the same underlying mechanism. This conclusion is amplified by the fact that recall is a passive process, while memory consolidation is an active process of forming new or reconsolidating old memories. We can hypothesize that the underlying mechanism is therefore not molecular. The decline in recall and memory consolidation might simply be caused by a decline in operational efficiency of the neural networks involved in learning. That efficiency, expressed as alertness (see: Alertness in SuperMemo), depends on both homeostatic and circadian components of the sleep drive. The homeostatic component determines an overall decline in network efficiency over the course of a waking day, while the circadian component allows of a small bump in the second half of the waking day, presumably due to a neurohormonal impact of the circadian cycle on the overall function of the central nervous system.
Good learning days
The correlation between recall and memory consolidation can also be seen in abstraction from the circadian phase. If the overall recall and memory consolidation data are taken from individual days of learning process, they correlate pretty well too:
Exemplary graph that shows that learning days that are good for recall are also good for memory consolidation. recall is expressed as a fraction of correct answers on a given day. Consolidation is expressed as a fraction of correct answers on the day of the next repetition that follows the one on the day for which the consolidation is measured.
We can conclude that good learning days are equally good for recall as they are for memory consolidation. A more general conclusion is that successful recall is essential for memory consolidation of memories.
Figure: Exemplary graph showing the average memory consolidation for days producing a given level of recall. The relationship between consolidation and recall is nearly linear. The graph was plotted using over 1.1 million repetitions in SuperMemo. Over 600,000 of those repetitions contributed to consolidation data. Consolidation levels with fewer than 3,000 data points have been omitted from the graph. The Deviation parameter says how well the linear fit matches the data (the less the deviation, the better the fit). The deviation is computed as a square root of the average of squared differences between the approximation and the data.
Exemplary graph showing the average recall for days producing a given level of memory consolidation. The relationship between consolidation and recall is nearly linear. The graph was plotted using over 800,000 repetitions in SuperMemo. Consolidation levels with fewer than 3,000 data points have been omitted from the graph. Lowered recall for consolidation of 100% comes from the fact that this consolidation level is overrepresented by small sample days where lucky perfect recall in just a few items may result in perfect consolidation reading without actually saying anything about the recall on the day the consolidating repetition took place. Sufficiently large number of such cases will let consolidation category of 100% pass the 3,000 data points outlier limit set for this graph, and result in a recall level that is much closer to the average level.
Alarm clock vs. learning
There is an urgent need to collect sleep data from subjects who disrespect healthy sleep in various ways. The most interesting area for further investigation is how poor sleep hygiene affects learning. As an example, let's have a peek at an interesting graph showing the average recall of a teenager who often needs to get up early for school, far ahead of his natural waking time. If grades are converted to the forgetting index, we can see that this student forgets 53% more on schooldays when he needs to get up early. This is a very preliminary sample that should not be used to draw far-reaching conclusion (for example, more learning occurred in earlier hours on days free from school), however, it is my hope that with more data pouring in, we can tangibly demonstrate the disastrous impact of early school times on learning. In other data sets, it has also be found that later waking time (after 11 am) often correlates with lower grades as well (perhaps as a result of weekend late "partying" that results in poorer sleep and later awakening).
Learning in free running sleep
Everyone has his or her own optimum learning hours that depend on the circadian rhythm. For most people, optimum learning occurs in the morning and after a siesta. Non-nappers also improve their learning in the evening due to a circadian upswing. However, the exact timing of those optimum periods can only be determined on an individual basis. The disconnect between the optimum learning time and the absolute clock can be seen in a regular free running sleep rhythm as in the analogous graph below that does not show any hours (on the clock) in which learning is more efficient:
However, when the free running sleep data presented in the graph above is processed using the circadian time rather than the clock time, a typical two-peak circadian pattern re-emerges with good grades in the morning, siesta dip, and an evening upswing. The circadian phase estimations have been generated with SleepChart. The peak learning times are usually separated by 10-13 hours:
It is obvious that alertness improves learning. However, it is worth noting that even marginal improvements to high alertness can yield major benefits to learning. In other words, it is not enough to be alert. Crisp alertness might substantially improve learning as compared with just being ok. In the presented graph, sleep propensity has been estimated with SleepChart using the two-component model.
The more time we spend learning on a given day, the lower our learning capacity is. recall decreases along a homeostatic increase in sleepiness. However, it decreases much faster when the learning process continues. In other words, learning increases sleep propensity. That observation agrees nicely with the complementary encoding theories that explain how the brain copes with catastrophic forgetting that occurs in artificial neural networks. Those theories speak of secondary memory systems used to redistribute knowledge originally stored in low-interference short-term networks. The act of storage redistribution is hypothesized to occur during sleep. In other words, as you keep loading your memory with knowledge, your brain turns on a defense mechanism, makes you drowsy, and sends you to an earlier sleep. This is why, against conventional advice of sleep experts, I recommend SuperMemo to insomniacs (if they must go to sleep early). Except where the circadian component of sleepiness is missing, learning is a good tool for boosting homeostatic sleepiness. Obviously, it will not work in cases like learning before an exam, which may subconsciously be associated with stress.
Average grade in learning with SuperMemo depends on the position of the tested item in the learning queue. Later items receive lower grades. To eliminate the impact of the homeostatic sleep propensity, all repetitions studied took place in the hours 5-7 of the waking day.
Sleep might be the chief anti-overload protection mechanism. The hypothesis says that sleep helps unload separated neural representations from the hippocampus. It optimizes the long-term neocortical overlapping representation. Learning with a fresh mind after a good night sleep will then be recommended. Learning in condition of sleep deprivation or mental fatigue would then be a mistake (unless employed as an anti-insomnia tactic).
Robin Clarke who hypothesized that too much learning can cause Alzheimer's (Wozniak 2002) writes: "Natural selection will favor further mechanisms, which enable local matrixes nearing overload, to signal their lack of spare capacity, thus activating diversion to other locations". This sounds exactly like the job of NREM-REM sleep interplay. Optimizing the storage is the simplest defense against memory interference. Sleep may act as an anti-overload and anti-interference mechanism that does not show the same destructive powers as forgetting. The signal on the "lack of spare capacity" might simply be adenosine-based homeostatic component in the two-process sleep model. See also: Neural optimization in sleep
Alertness vs. learning
As shown in the preceding sections, in healthy individuals who are not sleep deprived and who sleep in the correct phase, the best learning results are obtained early in the morning. This easily reproducible observation was an incentive to introduce two options in SuperMemo that help users of the program study their alertness throughout the learning day. The term alertness, in SuperMemo, is used interchangeably to describe two different measures of cognitive function: inverse of sleep propensity (or sleep drive) as derived from the two component model, and the average grade in learning with SuperMemo which corresponds with memory recall. Both expressions of alertness are closely correlated. SuperMemo measures alertness as well as attempts to predict changes in alertness in two different time frames intended to separate the homeostatic and circadian components of sleep propensity. Both approaches require a sleep log for the measurements and for the predictions to be possible. To demonstrate the homeostatic changes to alertness, SuperMemo measures the learning performance since the last sleep episode. To demonstrate the circadian changes to alertness, SuperMemo measures the learning performance in reference to the circadian time (i.e. time measured since the optimum natural waking hour) in periods that may or may not include intervening sleep episodes. As it can be seen in the enclosed pictures, it is not possible to fully deconvolve the impact of homeostatic and circadian sleep propensity on learning. homeostatic graphs will always include a small circadian bump related to post-siesta learning, while circadian graphs will be affected by sleep habits that are closely correlated with the circadian cycle, esp. in free running sleep.
If you have already collected your sleep data with SleepChart, you can see your wake-recall correlations with the newest SuperMemo. Note that only repetitions executed with SuperMemo 13.0 (2006) or later will be included in the graphs as earlier SuperMemos did not store precise time of repetitions in repetition history.
You can see how fast your alertness, recall and grades drop during the day by inspecting the Alertness (H) graph in SuperMemo. In this graph, you can see the time that has passed since the last sleep block, and how your recall changes in waking:
Alertness (H) graph makes it possible to visually inspect how recall decreases during a waking day. It also shows the impact of circadian factors with grades slightly lower immediately after waking and slightly higher in the post-siesta period (i.e. in the 10-13 hour bracket). The Deviation parameter displayed at the top tells you how well the chosen approximation curve fits the data (in the picture: negatively exponential recall curve). The lesser the deviation, the better the fit. The deviation is computed as a square root of the average of squared differences (as used in the method of least squares).
In Alertness (H), the minimum length of a sleep episode in consideration is determined by Min. sleep block (h) box (0.2 hours, or 12 min. is the default minimum). Shorter sleep blocks are disregarded in plotting this graph. homeostatic alertness half-life (in hours) tells you when your learning capacity drops by half after waking. You can modify this parameter to look for a better curve fit in your case (the Model button must be depressed). See Deviation to evaluate the fit. This half-life can differ between individuals. Notably, it is very short in narcoleptics, and very long in natural non-nappers.
The circadian changes in alertness can be seen in the Alertness (C) graph, which plots alertness throughout the day in reference to the circadian time measured from the actual waking time or from the optimum natural waking time:
Alertness (C) graph showing the powerfully biphasic nature of the human circadian cycle. The horizontal axis shows the circadian time, i.e. the time that elapses from phase 0, i.e. the predicted "end of the night" time (if Model is depressed). The prediction comes from the circadian model employed in SleepChart, and is derived from the sleep log data. The yellow line is the predicted circadian alertness derived from the same sleep log data using the two component model of sleep propensity developed for the purpose of sleep optimization in SuperMemo (inspired by similar work by Alexander A. Borbely and Peter Achermann). The overall alertness, not shown in the graph, is the resultant of the status of the two components of sleep propensity: the homeostatic component and the circadian component. The blue dots are recall data taken from the learning process in SuperMemo that correlate well with overall alertness
How learning affects sleep?
Impact of learning on sleep
In many of my older articles I often mention the fact that learning should increase the demand for the total sleep time. I read about the impact of learning on sleep yet in the 1980s. I have since lived with the conviction that this is a science fact that is as obvious as the fact that sleep is essential for learning. However, when I tried to prove the claim with data collected with SuperMemo, I discovered that it was not as easy as I thought.
When I tried to see if prior learning increases the length of sleep, I found the opposite. Again I started with my own sleep and learning data, which is rare in its size and the fact that the free running condition applies to both sleep and learning. I explained free running sleep earlier in the article. By "learning at libitum" I mean learning that, for the sake of efficiency, is more intense and long lasting on good learning days, and less intense on worse learning days. Good and bad learning days are primarily determined by the quality of sleep, and not, for example, availability of time. I thought that the free running condition is essential for such investigations, esp. sleep should not be controlled artificially so that to make sure that increased demand for sleep is reflected in total sleep obtained.
It appears that in a free running condition, the days with lots of learning were followed by less sleep in the night!
The amount of sleep obtained in the first 11 hours from bedtime as a function of the amount of learning in the last 8 hours preceding the bedtime.
Upon a closer inspection, it appears that the reason for this surprising outcome is that if learning is done on demand, i.e. more learning on good learning days, prior quality of sleep determines both the amount of learning and as well as the total sleep on the following night.
I tried to correct for prior total sleep to get a better picture. You may recall from the section devoted to napping that the amount of napping correlates well with the prior night's total sleep (the less sleep, the more napping). If I could find a similar neat relationship between the sleep on two successive nights, I could perhaps correct for sleep debt and reveal if more learning entails more sleep.
However, the relationship between total sleep on two successive nights is also pretty surprising. For example, the following U-shaped relationship shows the amount of night-time sleep depending on the total sleep in the preceding 20 hours.
Exemplary U-shaped relationship of total sleep and sleep on the preceding night. Total sleep on the vertical axis is taken as the consolidated night-time sleep (i.e. sleep in which short-lived nighttime awakenings are ignored). The horizontal axis represents total sleep whose termination point is embraced by the 20 hour margin preceding the bedtime in consideration. This margin was chosen to capture the preceding night sleep as well as follow-up naps without reaching into areas of sleep that should be considered two nights away from the period of interest.
The U-shaped graph shows that a simple sleep debt formula cannot be used to correct for sleep demand after a day of learning. However, a subset of normal-length nights could be used to filter out for varying sleep debt conditions.
As for the explanation of the U-shape obtained, it might be a combination of three main causes:
- Most obviously, short sleep on one night will often result in longer sleep on the successive night (e.g. as with total sleep from 0-3 hours on the preceding night).
- Some factors that determine the length of free running sleep may span over periods longer than a single day (e.g. health status, season, humidity, availability of sunlight, etc.). Those factors will result in a positive correlation between total sleep on successive nights (e.g. as seen in the graph in total sleep spanning 5-8 hours).
- Extremely long days, with more than 20 hours of wakefulness, will result in failing to register the preceding sleep on the graph (i.e. the preceding night sleep will equal zero).
Using data on the relationship between the length of sleep on two successive nights, we can apply a "band filter" on the data used to generate the first learning-vs-sleep graph. If we eliminate short-sleep nights by choosing only data points with total preceding sleep equal to five or more hours, we can reverse the downward trend and produce a nearly flat linear relationship between learning and the follow-up sleep:
If sleep on the preceding night is above 5 hours, then the amount of learning has nearly no impact on the follow-up sleep
If the "bandwidth" is narrowed to 5.0-6.5 hours, we get a perfectly flat line (slope=0.00). This data seems to indicate that an increase in learning does not increase the total sleep on the follow-up night.
As there are many lines of evidence that learning does affect the follow-up sleep, there could be many explanations of that conclusion. Sleep density might change instead of the length of the night sleep episode (as it is the case with REM density (Smith et al. 2004). In an active lifestyle, learning may not increase the demand for sleep much above the baseline. Last but not least, the result may differ between students. Some students can swear that more learning requires more sleep in their case. I am yet to receive an appropriately large set of data that could demonstrate this fact. As much as free running sleep makes it impossible to prove that short sleep is bad for learning, learning on demand may make it impossible to prove that lots of learning increased the demand for sleep. As much as alarm clocks can be helpful in showing their own bad impact on learning, forced learning may also be a more grateful research subject. Forced learning may be more costly for the brain and show a more pronounced impact on the density and length of sleep. Perhaps learning needs to be heavy enough to notice the effect due to the fact that all our waking experience is a form of learning, even if we do boring repetitive activities. A mere thought process, e.g. recalling a relative, will form new memory traces in the brain. These will be processed in sleep. For sleep demand to come well above the baseline, learning must come above its own baseline as well.
Sleep and school
Schools have changed the world for the better. Literacy is on the increase worldwide. However, there is one huge factor that holds schools back: sleepy kids!
Sleepy kids learn little!
Modern lifestyle results in an epidemic of delayed sleep phase disorder in the adolescent population. Millions of families nowadays struggle with putting their kids to sleep early enough, and to have them wake up fresh in time for school. It seems like we are losing this battle worldwide. Kids seem to be getting less and less quality sleep! Drs Amy Wolfson and Mary Carskadon study sleep in teenagers. They were horrified to find out that sleep latency during school hours was lowest for 10th graders and was a shocking 1.8 minutes! This latency is lower than the value a good sleeper usually achieves at bedtime! In other words, kids are more ready for sleep at school than a normal individual is ready for sleep in the night!
Learning in such sleep deprived state is worth little more than zero!
This is an alarming situation that can undermine the future of education as well as the physical and mental health of the next generation! Some sleep researchers ring the alarm bells, others look for remedies. I do not have a prescription for the problem. Hereby I would only like to appeal for more tolerance and understanding on the part of parents and schools. All my life I have worked for the purpose of better education for everyone. However, there is no learning without sleep. Sleep is important enough to often take precedence over the education itself! My appeal is:
The premise of this appeal is very simple. Waking up a semi-conscious kid for school implies a day that is practically wasted for learning, or literally crossed out from a young life's calendar. Adding those extra 2-3 hours of sleep means that the kid will only miss a class or two, with many additional productive hours left in the day! It is by far better to spend an hour on productive learning than to spend 8 hours on comatose "survival through the class". It amazes me how little this simple truth is appreciated! When I speak to parents, they always excuse early waking with "there would be consequences for missing the class"! There must not be any consequences! Sleep deprivation shrivels the brain! Sleep is the fundamental human right of a developing brain. If someone threatens the kid with "consequences", you need to combat that attitude. Sadly, for many parents, the timing of the early morning schedule is determined by work and other obligations that cannot be worked around.
Excessive school workload
One of my favorite journalists, Fareed Zakaria, spoke in his GPS program about his prescription for better education: "Some elements of the solution seem obvious. The writer Malcolm Gladwell says it takes 10,000 hours to get really good at anything. It's really just another way of making Thomas Edison's famous point that genius is 1 percent inspiration and 99 percent perspiration. Now if our kids spent two years less in school than in many other countries, they will find themselves behind in many areas. We don't have to go to the lengths that South Korea has gone to lengthen the school day and the school year, but we can't do the least work and hope for the best results" (source).
The problem with this "Korean solution" is that it fails to account for a dramatic difference between good learning and bad learning. Given a shortage of good teachers, good funding, good methodology, etc. we might as well pump up school hours in hope of converting quantity into quality. However, a good hour of self-learning or a good hour of customized one-on-one tutoring is worth more than 10 hours of boredom in an average classroom. Perhaps Finnish schools with their reliance on excellent teachers would show a better ratio. If we added just two factors to our school systems: (1) good sleep and (2) spaced repetition, we could safely cut school hours to 1-2 classes per day and still get better results!
School hours and homeschooling
Due to a well-documented sleep phase shift at adolescence, teenagers find it more and more difficult to solve the problem of sleep deprivation by just going to bed earlier. Instead of providing for longer sleep, early bedtimes may result in insomnia and a multitude of psychogenic sleep and emotional problems. Teens are simply unable to fall asleep at designed early time, and trying to force them to do so may actually backfire. Even a mild degree of sleep deprivation might be better than hours of tossing and turning, or nocturnal awakenings. Return to a farmer's lifestyle would remedy the problem of teenage body clock, however, this would mean many hours of physical work in the field from the early morning. Sitting in a school bench just won't do. Heavy load of schoolwork on its own contributes to the late sleep phase lifestyle!
When schools experiment with later class hours to accommodate the adolescent body clock, they get better learning results (Wahistrom 2002). Traffic accidents among young drivers on the way to or from school also drop (around 25% for a mere one hour shift clockwise). However, it appears that kids just tend to adapt and stay up later in the night. Later school hours are an imperfect remedy, esp. that kids differ by chronotype and each will have its own optimum window for the best learning performance. Callan (1998) reported that in high school, less than 10% of kids preferred the early school hours, while 15% preferred evening hours. Reported preference is often confused by the misalignment of circadian cycle with the waking period, which often makes evening types claim evening is better for learning, while in free running sleep, the same kids would prefer the subjective morning hours (with "morning" coming as late as mid-day). Moreover, as the kids get older the predominance of eveningness starts becoming more pronounced.
I do not know a universal solution, however, all parents should consider homeschooling, which could make a world of difference. Not every parent is qualified, and not everyone can afford it. Amazingly, some modern and progressive countries banned homeschooling altogether. It is hard to believe, but two leaders in the adoption of rational and scientific social solutions, Germany and Sweden belong to that group! In fear of dangerous ideologies, some governments block a return to a tradition that is as old as the human race. A tradition that could remedy many weaknesses of the school system: tutoring one-on-one under the supervision of the most loving people in existence: own parents or other family members. Homeschooling makes it easy to employ the most efficient of the learning methods: self-paced self-directed exploration based on passion and curiosity. This ideal solution solves the problem of matching learning hours with the circadian cycle.
Sleep deprivation in kids
Most kids wake up earlier than they would prefer to. This results in sleep deprivation and a set of negative consequences:
- bad learning: cutting down on sleep dramatically impairs learning. Some groups of schoolchildren show sleep latency of just 1-2 minutes at the time when they sit in the class. This is catastrophic! This is a latency that many would be envious of at evening bedtime! Kids should learn at a time when sleep initiation is near-to impossible! In free running sleep, that would be the first 3 hours of the waking day! After that, learning is impaired and PE classes and lunch could come in. Lots of kids sleep in the class, or are solely preoccupied with "surviving".
- bad emotions: sleep deprivation results in irritability. Without the neural network cleanup executed in sleep, the brain quickly gets overloaded and overwhelmed with the stimuli. This could be learning stimuli, or minor annoyances, such as a colleague's jokes. Bad temper follows even in otherwise well-mannered kids. Comraderie and social interaction are replaced by bullying, fighting, aggression, and sheer meanness. Over many years this can lead to psychological problems, depression, aggression, suicide, and the loss of ability to harmoniously integrate with the rest of society.
- hate of school: by the age of 10, most kids universally hate school! One of the first questions I ask any kid I meet is about his or her fondness for school and learning. If they claim to like school, it is often because of the chance to interact with friends or simply to break free from parental supervision or leave the home environment. Vacation countdown becomes a daily preoccupation. This has disastrous effects on the efficiency of school education, and long-term choices such as going to college. Universal hate of school shapes a generation and the way society copes with challenges of the modern world. And it all begins with the malignant device: an alarm clock!
Poor recall on schooldays
To illustrate the impact of school hours on learning, see the following exemplary graph. A 16-year old high school student logged his sleep patterns in SleepChart and his learning results in SuperMemo. By combining the two we can see the relationship between the waking time and the average grade obtained in learning with SuperMemo. The waking time for school was always ahead of the natural waking time and the teen compensated by sleeping longer on weekends:
Despite a decline in the learning performance on schooldays, the teen would do great at school, do his best learning during weekends, and would later get admitted to an Ivy League school. The dramatic impact of sleep deprivation on learning can be seen when grades are converted to the forgetting index. In this case, the students would forget 53% more on schooldays when he needed to get up early. Clearly, sleep deprivation is not likely to deprive someone of a chance to get to the Ivy League. However, it does affect the performance and undermines a young man's potential. At younger ages it may also have a significant impact on the brain development. Interestingly, in other data sets, I have also found that later waking up (after 11 am) often correlates with lower grades too. Perhaps that is a result of weekend late "partying" that results in poorer sleep and later waking?
Example #1: Long weekend sleep
A typical sleep pattern with short weekday sleep and long weekends sleep is shown in the following sleep log and the corresponding circadian graph.
Exemplary sleep log with weekday sleep deficits and longer sleep on weekends. Typically, Saturday morning sleep is longer than the Sunday morning sleep.
Circadian graph for sleep with weekday sleep deficits and longer sleep on weekends. The graph shows that a day of 16 waking hours and 8 hours of sleep would probably make the desired optimum. Instead, the 7 hour night causes an accumulation of sleep deficit with sleep cut short by one hour per day on weekdays.
Example #2: Phase 12 napping
A more troubling example shows a fragmentation of the sleep schedule caused by short night sleep episodes, and frequent Phase 12 napping. Here a student attempts to sleep in the exactly same brackets, i.e. 23:00 - 6:00:
Circadian graph for short night sleep with irregular napping. Naps are taken ad hoc in various phases. Early naps are short and do not cover for sleep deficits. Late naps cause a delay in night sleep, and possibly a phase delay that compounds the problem of sleep deficits.
Learning in alpha state
There are learning gizmos and contraptions out there, which are marketed as based on learning in a relaxed state. Proper cognitive environment is paramount to learning. However, for clarity, we should rather use the term concentration instead of an all-inclusive relaxation. Concentration in learning should be maximized by taking into account the following factors:
- being cut off from all sources of interference in learning (telephone, e-mail, conversation, radio, and perhaps even one's favorite music)
- finding the optimum circadian timing for learning (e.g. early in the morning in free running sleep cycle, late in the evening in DSPS individuals who cannot afford free running sleep, etc.)
- all aspects of mental and cognitive health (e.g. avoiding stress, substance abuse, etc.)
The concept of relaxation is often associated with alpha wave learning which has attracted lots of companies that are more interested in their bottom line than their customers' actual success in learning. EEG measurements can be used to roughly determine the current state of the brain in the same way as you could detect bustling activity in a major city by scanning the surrounding electromagnetic field. The usefulness of alpha wave scanning in learning can be compared to the usefulness of electromagnetic field scanning for social life of a city. You need to focus on the causes rather than on symptoms. Alpha waves appear primarily in the absence of visual processing and other intense mental processes. This is why they cannot dogmatically be considered a desired learning state. After all, the drowsy alpha state that precedes falling asleep is exactly the worst moment for learning during the day.
In evaluating the "relaxation products" you need to differentiate between the relaxation effect and the actual learning effect. The number of companies making false claims in this field is astounding. It is very easy to fall for a simple solution to a learning problem (e.g. get 10Hz binaural beat difference and your learning problem will go away for life, and perhaps your sex drive will improve at the same time, you will sleep better and you will look younger). The easy learning solution explains why false claims related to "learning in relaxation" are so hard to extinguish.
At the same time, if you need to cope with stress or insomnia, many products in the field may have a legitimate application. Customers of the Polish Sita system jokingly claim that the company would do better if they marketed their product as a napping system. A worthy application on its own. In the 1990s, I appealed to users of SuperMemo to let me know of relaxation products that might be worth mentioning as an effective help in learning. I do not think I have received any credible suggestions until now.
Learning during sleep
When Soviet researchers made a claim of sleep-assisted instruction, they started a powerful meme that could never be reproduced and is now pretty hard to extinguish. You may have heard of sleep tapes that offer effortless learning during sleep. They are a direct follow-up of the Soviet claims and only a part of the whole series of products for learning in sleep. Your investment in tapes for learning in sleep will not be money well spent. Attempts at learning during sleep should be discouraged! It is possible to occasionally recall a fraction of the material presented during sleep. Information may reach and register in memory during short periods of awakening or transition from REM to shallow sleep. There is also ample evidence that some circuits in the brain can be conditioned during REM sleep. However, the connection between the senses and the brain in sleep is rather focused on awakening in danger rather than on processing complex information.
Whatever you might gain from your sleep tapes will by far be offset by damage to the quality of sleep. If the learning stimuli do not reach a certain threshold, they will simply be ignored. However, past a certain value they may prevent the progression of NREM sleep toward stages 3 and 4. They can also shorten REM sleep.
Interestingly, memories acquired minutes before falling asleep do not get consolidated! Even a few minutes of sleep leave a short window of waking time that is totally erased from memory. Luckily, we rarely learn mission-critical information shortly before dozing off.
Counter-recommendation for learning during sleep, does not imply that falling asleep with TV or radio turned on should be discouraged. If you would like to get a dose of education yet before falling asleep, be sure your tapes, TV or radio meet these conditions:
- they turn off automatically no later than in 10-20 minutes
- they have no ability to wake you up from a properly timed sleep. If you wake up in the initial minutes of sleep you may experience a dramatic drop in homeostatic sleepiness that would delay the sleep onset. Awakening may also indicate that you went to sleep too early in reference to your circadian cycle
- they do not include highly emotional content, distressing messages, shrill sounds like doorbells, phones, timers and alarm clocks, as these all have been designed to most effectively interfere with sleep for the purpose of interrupting it
Moreover, if you find it difficult to fall asleep due to the stresses of the day, subtle news channel may actually help you fall asleep by keeping your mind away from the thoughts that might trigger the release of ACTH, cortisol, catecholamines, or other alertness hormones.
TV, radio or tapes in the morning are OK too, on condition you turn them on manually (i.e. they should not work as an alarm clock substitute). If you wake up slightly ahead of your expected waking time, turn on the news and stay in bed. Test your brain for signs of sleepiness. Occasionally, you may still be able to fall asleep and go through one cycle of sleep that will be beneficial to your intellectual performance.
Some self-help personal power gurus keep bringing up the concept of lucid dreaming as a tool for enhancing learning and creativity. Terms such as super-consciousness or hyperreality are tossed around. Polyphasic sleepers often claim that the Uberman sleep schedule helps them achieve lucid dreaming and an enhanced experience of reality. There might be a grain of truth in that claim. Read about the polyphasic rollercoaster to understand why polyphasic sleeper might experience euphoric highs that seem even higher due to the periods of total zombification. Lucid dreaming is as useful for learning and creativity as LSD. Striving at lucid dreaming is rather likely to disrupt the healthy sleep and negatively affect learning. During REM sleep, the prefrontal cortex should normally be de-activated. Hobson's AIM model of 3D sleep-wake space (Hobson et al. 2000) can be used to illustrate the state corresponding to lucid dreaming as a partitioning, in which the cortex and the rest of the brain occupy different points in the AIM space. Such partitioning is likely to interfere with the physiological function of REM sleep. It can be compared to eating your lunch while jogging (i.e. the situation where contradictory targets are fed to the nervous system). Using auto-suggestive tricks to change the AIM state may affect neural processes occurring in sleep with unpredictable consequences that are not likely to be positive. As for creativity, it is conceivable that LSD (and less so lucid dreaming) might boost non-specific creativity or help understand the creative process. However, most of the mankind's creative breakthroughs occur when a healthy refreshed mind focuses on solving a specific problem. Hallucinatory haze is not helpful in directing creativity towards a useful purpose. Creativity is a game of chance. You should look for ways of consciously directing the creative process rather than to increase its randomness indiscriminately (Wozniak 2001). For some tips, see natural creativity cycle and How to solve any problem?.
How do we fall asleep?
Initiation of sleep
We fall asleep when two signals are generated in the brain:
- (H) "too much waking" signal and
- (C) "it is the usual sleep time" signal.
The "too much waking" signal is called the homeostatic signal. While "time to sleep" signal is called the circadian signal. The homeostatic signal is a reflection of network "tiredness". The more you learn, the more you think, the more you process information, the more tired you get mentally. This generates homeostatic sleep propensity. However, homeostatic sleepiness is not enough to fall asleep. You may be dead tired of too much waking or too much learning, but you may still be unable to get a wink. This is where the circadian sleepiness comes in. Circadian sleepiness is maximum during the subjective night period. There is also a mid-day slump in alertness that also has circadian nature. When you are sleepy in both homeostatic and circadian sense, you can finally fall asleep.
The homeostatic signal is generated in the neural networks of the brain. It is associated with slow-wave activity in the EEG. One of its known expressions is an increase in adenosine levels. The effects of adenosine are blocked by caffeine. This is why coffee can temporarily help overcome the homeostatic component of sleepiness. At the same time, caffeine is entirely ineffective against the circadian component. This is why drinking coffee during the subjective night is imprudent and unhealthy. As the waking hours tick on, brain glycogen and ATP reserves are depleted. ATP is degraded to ADP, then AMP, and finally to adenosine. Adenosine then builds up in the brain. This includes a buildup in the basal forebrain (Porkka-Heiskanen 1999), which is the hypothetical source of the neural homeostatic signal. Depletion of the glycogen reserve is also hypothesized to have its own contribution to the homeostatic sleep propensity (Kong et al. 2002). The basal forebrain, which is a cholinergic structure, when active, contributes to the wakefulness and REM sleep. Deactivation of the basal forebrain helps initiate NREM sleep and sleep in general.
The main source of the circadian signal is the suprachiasmatic nucleus (SCN). A set of genes is expressed in a regulatory loop that keeps a 24 hour rhythm of activity. The SCN rhythm can be reset by light, or activity, or other signals (see: Phase response curve (PRC)). The SCN sends most of its fibers to the subparaventricular zone (SPZ) and the dorsomedial hypothalamic nucleus (DMH). One of the hormonal signals produced by the effects of the SCN oscillation is the release of the melatonin from the pineal gland during the subjective night. This led researchers to the idea that melatonin might be a natural help in initiating sleep (given sufficient homeostatic sleepiness).
Integrating homeostatic and circadian signals
The homeostatic signal needs to be integrated with the circadian input. The precise mechanism of the integration is not known, but there are a couple of solid hypotheses on how this might work. The anterior hypothalamus is the presumed site of the integration. The hypothesized integrating nuclei are: the medial preoptic area (MPA), the anterior paraventricular thalamic nucleus (aPVN), and the dorsomedial hypothalamic nucleus (DMH). DMH and MPA send a big bunch of fibers in the direction of the ventrolateral preoptic nucleus (VLPO), which is one of the main brain nuclei responsible for the initiation of sleep.
Sleep control system. See the text for details. See the legend here
Adenosine agonists are also able to activate the VLPO (Scammell et al. 2001). It has been hypothesized that adenosine inhibits anterior hypothalamic and basal forebrain GABAergic neurons that suppress the activity in the VLPO.
The VLPO is thus able to initiate sleep by receiving both the circadian signal from the anterior hypothalamus and the homeostatic signal from endogenous substances (e.g. adenosine) that accumulate in the course of a waking day. The VLPO and its adjacent nuclei are then able to inhibit the histaminergic wake-promoting TMN and other arousal systems in the pons and the midbrain (e.g. LC, DR, LDT, PPT, PeF, vPAG, etc.). Sleep is a direct consequence of the inhibition of the ascending reticular activating system (RAS) which groups those neural structures that keep the cerebral cortex in the waking state. With the depression in the activity of the RAS, we quickly lose interest in demanding intellectual activities. Soon the only thing we can think of is sleep. Once we rest in an undisturbed place, we drift into the dreamland. People who cannot follow their natural body rhythms will often be unable to follow the above scenario.
Neural inhibition of the arousal is also accompanied by a significant drop in ACTH and cortisol, which are chief alertness hormones. Similarly, the levels of serotonin and catecholamines drop, and so does the body temperature. All those processes proceed on parallel tracks and we sleep best when they are all perfectly synchronized. It is awfully easy to put that symphony out of sync by all forms of intervention: excitement before sleep (dopamine), coffee before sleep (homeostat), exercise (adrenaline), etc. Synergistic manipulation also has side effects: sleeping pills, alcohol or marijuana destroy the sleep structure. Even melatonin has its side effects. Sleep is healthiest when all physiological variables change in pre-designed synchrony. This can best be accomplished by following the commandments of one's own body clock.
Not all scientists agree
Dr James M Krueger has championed, for many years, an idea that all advanced neural networks have an inherent ability to enter a sleep state (in particular, cortical columns have this property). A biochemist by education and spirit, Krueger started his investigations from looking for substances that induce sleep. He was inspired by a century old finding that cerebrospinal fluid of sleepy animals contains substances that are able to induce sleep when transferred to otherwise alert animals. Over the last four decades, Krueger has amassed a great body of evidence for the existence of a huge number of sleep regulating substances (SRS) such as adenosine, nitric oxide, TNF, IL-1, GHRH, prostaglanding D2, etc. (Krueger et al. 1999; Krueger et al. 2001). Some of SRSs, like adenosine, build up with mental activity (e.g. as a result of the release of glutamate (Simasko et al. 2005)) and may play a role in sleep homeostasis, while others (e.g. melatonin) are circadian. In his recent publications, Krueger asserts that sleep is a network-emergent phenomenon, and that sleep control nuclei in the brain play only an accessory synchronizing role. Even though the overarching principle may seem to quarrel with the mainstream science of neural sleep control, the body of undisputed facts is overwhelmingly larger than the areas of disagreement. Even though Krueger theories do not seem to explain the computational aspects of sleep, where a neural control of sleep centers seems indispensable, they all align pretty well with the homeostatic aspect of sleep control. In the place where biochemists meet neural network experts and neurophysiologists, we can find the most fruitful field for further exploration of the mysteries of sleep.
The human body clock runs in a cycle of circa 24 hours. That cycle was therefore named a circadian cycle. Understanding the circadian cycle is vital for healthy sleep. My wild guess is that 95% of sleep problems in industrialized nations are caused by the lack of understanding of the circadian cycle, or lack of respect to its power and importance. The cycle is encoded deep in the human genome and cannot be easily changed or overridden. Playing with the circadian cycle may result in long-term health consequences. All cells in the body express various clock genes, however, there is a master clock in the brain that helps synchronize other clocks in the body to run in harmonious synchrony that is vital for health, well-being, longevity, learning, creativity, etc. The master clock is located in the brain and is called the suprachiasmatic nucleus (SCN). Circadian cycles of the SCN result in periodic release of melatonin from the pineal gland. This led to the use of melatonin as a sleep remedy. The popularity of melatonin comes from its natural origins and the possibility of oral administration. However, as melatonin is located downstream of the SCN in the circadian cascade, it does not have the full magic powers of generating complete nighttime circadian states. Even the natural release cycle of melatonin my get misaligned with the sleep-wake cycle in irregular schedules. This limits melatonin applications. It can be used to produce phase shift (e.g. phase advance if taken 1-2 hours before natural bedtime), but it is not the universal sleeping pill as it is often advertised.
An important alertness hormone, cortisol, can be used to map a well-timed circadian cycle. Its levels drop during the first half of sleep, and raise dramatically on waking giving us a sharp waking mind. On the other hand, growth hormone is less dependent on the clock and is released primarily during deep sleep having its hand in the anabolic power of sleep making it important for both the brain and the brawn.
Circadian alertness is partly hormonal and partly neural. The brainstem contains a collection of nuclei know as the reticular activating system. These nuclei, when activated, keep us awake and alert. Those "vigilance nuclei" include the serotonergic raphe nuclei, adrenergic locus ceruleus, parabrachial nuclei, and more. Various lesions to those areas and their connections may result in insomnia or coma.
In 1982, a Hungarian sleep researcher, Alexander A. Borbély published a seminal paper titled "A two process model of sleep regulation". This model has later been described in pretty precise mathematical terms, and is now the mainstay of our understanding how sleep is initiated and how the sleep-wake flip-flop works in healthy sleep in abstraction from the actual neurophysiological interpretation.
In short, Borbely noticed the distinction between the two components of sleepiness: homeostatic sleepiness and circadian sleepiness. Homeostatic sleepiness increases during the day as a result of mental effort. Circadian sleepiness increases at nighttime. Borbely's model argues that for a good night's sleep, you need to go to bed with both components of sleepiness in a high gear. This means that going to sleep early, before your circadian sleepiness kicks in, is a bad idea. You won't be sleepy enough to fall asleep, or your sleep will be shallow and easily interrupted. On the other hand, the model also implies that a premature awakening may clear the homeostatic sleepiness, and we may find it hard to fall back asleep even though the circadian sleepiness ensures we are pretty tired.
An exemplary interpretation of the two process model of sleep for normal sleep and sleep following a sleepless night. Homeostatic sleepiness is denoted as Process S (throughout this article, I use H for mnemonic reasons). Circadian sleepiness is an inverse of Process C. Sleep occurs when C is low and S is high. Additional sleep pressure accumulates after a night without sleep, and the sleep can occur earlier and last longer (it starts at higher homeostatic sleepiness despite slightly lower circadian sleepiness). SWA - slow-wave activity - is a brain wave activity that represents the deepest sleep. TST - total sleep time - is higher after a sleepless night.
During sleep, cortical slow-wave activity (EEG power density range of 0.7 to 4.5 Hz) depends on the duration of prior waking. This is why it is considered a hallmark of homeostatic sleep propensity (Daan et al. 1984). It decreases exponentially after the sleep onset. One of the limitations of the model is that it does not account for NREM-REM exchange, while the homeostatic sleepiness (Process S) might actually increase in the REM phase. Circadian sleepiness correlates with the release of melatonin, but can also be mapped onto core body temperature, or release of other sleep inducing or alertness hormones.
Borbély model in practice
Figure: An exemplary interpretation of the two-process model of sleep regulation taken from an actual sleep log in SuperMemo for Windows. Aqua line represents circadian sleepiness. Green line represents homeostatic alertness (an inverse of the homeostatic sleep propensity). Red line represents overall alertness that is an inverse of overall sleep propensity. Best alertness is achieved when both components of sleepiness are at their lowest. Inspired by Alexander A. Borbély and Peter Achermann
In Borbély model, sleep timing is determined by the points in which the curves representing the two processes cross. SuperMemo uses a more intuitive approach, in which both components of sleepiness are integrated heuristically to match the expected course of overall alertness (red line in the graph). Sleep is initiated when the overall alertness drops below a certain level. Sleep may thus be initiated by both components of sleepiness independently, as it may happen in early life, but the timing and duration of sleep will differ for various values of both variables (and the status of the circadian system).
You can "feel" both components of sleep. Homeostatic sleepiness is more likely to be described as feeling "unrefreshed", while circadian sleepiness is more likely to be named "grogginess". In a healthy cycle, you should never see the difference between the two: you wake up fresh, and you get sleepy in the evening when both components of sleepiness kick in making you just "very sleepy". However, if you are jetlagged and groggy, you can feel the unpleasant circadian sleepiness that does not go away and cannot be helped with a nap if your homeostatic sleepiness is too little to fall asleep. On the other hand, after a sleepless night, you may be dead tired and unrefreshed, however, with the morning sunlight you get a new energy to survive yet a couple of hours. As your circadian sleepiness passes by, you may feel homeostatic sleepiness that seems survivable (until the next circadian low hits)(see more: Sleeping against your natural rhythm).
Borbély model and evolution
The homeostatic component of sleep may simply be an unavoidable cost of the evolving neural networks. To prevent catastrophic forgetting, neural networks need to implement an overload protection. That protection is the homeostatic drive to sleep. We do not know how much of that protection is a natural consequence of the network overload, and how much of it is an added effort by the brain to prevent further overload. For example, we can improve cognitive function with the help of caffeine by blocking the adenosine-based component of the homeostatic sleep drive. This proves that the brain provides a degree of network overload protection. The overload will result in progressive decline in recall and memory consolidation in the waking day.
The circadian component evolved long before the neural function of sleep was established. However, it was convenient for the organism to do its neural housekeeping at opportune times. For example, for human hunters and gatherers, night is a time of inaction. This is why hooking up sleep to the night time period made an evolutionary sense. Other animals may have made different choices, however, the circadian cycle is always a good hint on the optimum timing for neural optimization.
Borbély's two-process model has been extended by an additional process W that represents sleep inertia. The basis for that model was self-rated reports of sleepiness (Akerstedt and Folkard 1990). The model used in SuperMemo does not include the sleep inertia factor as it is primarily targeted at studying free running sleep.
Phase response curve (PRC)
Phase response curve (PRC) represents a function that tells us how much a phase of an oscillator shifts in response to selected stimuli depending on the timing of these stimuli. PRCs can be used to study circadian rhythms as well as other biological, physical or electronic system (e.g. the heartbeat).
For example, PRC for light may tell us that applying a green light pulse of a given intensity 1 hour before sleep pushes the circadian cycle forward by 10 minutes (phase delay), while a blue light of a higher density might push the same cycle by 25 minutes. During the subjective night, there is a dead zone when light does not produce shifts in the circadian cycle.
There are many PRCs for different stimuli such as exercise, stress hormones (e.g. cortisol), melatonin, and other stimuli. The crossover time between the delay side and the advance side of the PRC for light is near the core body temperature minimum. The sleep control system seems most sensitive to shorter wavelengths of visible light in suppressing the release of melatonin (Brainard et al. 2001).
Stimuli that cannot shift the cycle or have a negligible impact (e.g. cup of warm milk), can also have a PRC plotted. However, that PRC will be a straight horizontal line running along the phase shift of zero. SleepChart uses an algorithm for plotting the so-called recursive PRC, in which the degree of phase shift is measured in reference to the actual position of sleep episodes in free running sleep without differentiating between the actual causes of the shift. rPRCs differ between people. They also change in response to lifestyle changes.
Changing the length of the circadian period
The existence of the PRC implies that the length of the clock period is under our control. If we apply zeitgebers early or late enough we can affect larger phase shifts that can lengthen or shorten the period of the cycle. Everyone can prove it all to himself or herself with relatively simple measures (e.g. bright lights in the late evening to shift the phase forward, or early morning exercise to shift the phase back). This is why PRCs are very important when treating phase-shift disorders. This is also why lifestyle determines phase shifts and possible sleep problems. This is why the modern lifestyle based on the use of electricity causes an epidemic of DSPS in the young learning generation. Recently, the fact of the adaptability of the body clock period was demonstrated with investigations into a possibility of astronauts adapting to a Martian day (Scheer et al. 2007). This was also demonstrated earlier in rats by various "lifestyle" changes (e.g. wheel restriction increases the circadian clock period).
Phase-shifting neural inputs
Our master clock, the SCN, is affected by 3 major zeitgeber inputs that allow of a phase shifts:
- retinohypothalamic tract (RHT) carries light resetting stimuli and acts via the NMDA receptors. This input bypasses cognitive vision
- intergeniculate leaflet (IGL) projects to the SCN via the geniculo-hypothalamic tract (GHT) and carries motor resetting stimuli
- the raphe nuclei provide serotonergic input that is hypothesized to modulate some aspects of the circadian cycle and might be involved in changes to the circadian cycle in affective disorders. Lesions in this pathway or decrease in serotonin lengthen the active phase in constant darkness without affecting the circadian period
Recursive phase response curve (rPRC)
To study the phase response, scientists need expensive laboratory setups and time-consuming research procedures. However, a simple computational trick makes it also possible to see the effects of phase shifting stimuli in SleepChart without the use of a sleep lab.
SleepChart implements a concept of the Recursive Phase Response Curve (rPRC). The curve is recursive because it is first obtained by computing the impact of phase shifts in sleep episodes in relation to the circadian acrophase computed using statistical methods. Once the first approximation of rPRC is obtained, it can be used to produce a better approximation of the middle of the subjective night line that is then used to generate a better approximation of the rPRC. A few iterations of such a process are sufficient to produce the best fit of the rPRC that corresponds well with the actual sleep data. SuperMemo uses acrophase estimates by using a fixed rPRC that roughly corresponds with rPRCs obtained with SleepChart. Whereas a typical PRC employed in chronobiology maps the response of the sleep system to a single stimulus (e.g. light, exercise, melatonin, or various chemical agents), rPRC is the resultant of all natural sleep delaying factors (incl. light, activity, stress, etc.). It can also be interpreted as a PRC, in which the waking activity forms the input to the free running sleep system. Unlike a PRC which responds to a shifting factor, rPRC responds to the evening phase shift caused by the same factor. As such, rPRC is not a de facto PRC, and all departures from the free running condition invalidate the computation. The main advantage of rPRC is that it can be derived from sleep data without collecting blood samples, saliva samples, or taking core body temperature measurements. This way, SuperMemo can correlate learning with sleep models that use only plain sleep log data as input.
In the presented graphs, Sleep delay (h) stands for the bedtime delay and equals the difference between the actual bedtime and the bedtime as computed by SleepChart from the prior history of sleep. As the measurements refer to free running sleep, little phase advance data is available due to the natural way of waking. The causes of sleep delay may include light, social interaction, stress, a conscious decision to delay sleep, exercise, ingestion of caffeine, medication, etc.
Phase shift (h) stands for a phase shift and equals the difference between two exponentially weighted waking hour averages on two successive days: the day on which the bedtime delay occurred and the following day. Instead of the bedtime hours, waking hours were compared as these are less affected by the homeostatic shift caused by the actual delay thus representing a truer reflection of the actual phase shift.
The flattening of the curve (as compared with a typical PRC) is caused by the recursive reference to actual sleep data, which results from the fact that plotting the circadian acrophase by SleepChart is an approximation based on the same sleep measurements. As a result, polynomial approximation shows a slight increase in phase shifts with increasing delay, which is not the case in typical PRC plots. The deviation of the bedtime hour from the optimum bedtime may result from either environmental delay factors or from the approximation error resulting from heuristic procedures used to plot the circadian function, while sleep onset usually occurs naturally at optimum physiological time. The inherent asymmetry of the graph comes from the fact that earlier bedtime is nearly always natural, while delayed bedtime may be natural or forced. It is the forced bedtime delay that is the main source of phase shifts in free running sleep.
Recursive PRC in DSPS
The graph presented below implies that, in the case considered, delaying sleep by four hours results in a shift of sleep phase equal to 1.4 hours (which seems to be close the maximum shift possible). Phase advance would require a natural onset of sleep that preceded the optimum retirement time by as much as 6 hours. Going to sleep at the optimum hour results in the natural daily delay, in this particular case 1.0 hour, typical of DSPS disorders or conditions of isolation from zeitgebers (e.g. constant lighting).
Recursive PRC and phase advance
Delaying sleep should always be avoided (except for cases where it is used as a form of chronotherapy). The next graph shows how sleep delays can actually advance the sleep phase. This is a reverse situation to the described earlier phase delay caused by an evening melatonin overdose. Where the wakefulness intrudes past the circadian acrophase, which follows the stationary point of the rPRC, phase delays decrease rapidly up to a point where further delay in sleep will push the phase backwards. Naturally, this "method" of phase manipulation is particularly unhealthy as it implies arousal in the middle of the subjective night (see: Health effects of shift-work and jetlag).
Recursive PRC showing phase advanced that can be caused by either (1) bedtime delays of above 5 hours, or (2) bedtime advances of more than 2 hours.
In the presented exemplary graph we can read the following:
- going to sleep at one's natural bedtime causes a 1.3 hour phase shift resulting in a circadian cycle period of 25.3 hours
- phase shifts are eliminated only when going to sleep 2 hours ahead of the natural bedtime
- phase advances of 2 hours are possible, but require an unnatural early bedtime (e.g. as induced with intense exercise such as running a marathon)
- phase delays of more than 2 hours are unlikely
- a delay in bedtime larger than 5 hours can result in an actual phase advance due to the impact of arousal on the morning end of the subjective night. Obviously, this should not be considered a "cure" to phase delays because such serious deviation will seriously affect the quality of sleep and produce major ripples in the control of the circadian cycle
If you run your sleep free and have a sufficiently large set of data (e.g. several months of a sleep log), you can generate your own rPRC data with File : Export : Recursive PRC in SleepChart (you need SuperMemo 15 or later).
Recursive PRC in polyphasic sleep
It is possible to feed SleepChart with data obtained from "Uberman experiments". Obviously, the mere departure from free-running condition makes the outcome hard to interpret. Even the recursive nature of the procedure used to obtain rPRC cannot effectively cope with the lack of the leading circadian crest. With all that in mind, it is still interesting to peek at "Uberman rPRC" as it nicely reflects the chaotic nature of the sleep system subjected to a polyphasic experiment.
A polyphasic sleeper pushes his sleep phase back and forth largely at random. That can only result in a chaos and complete asynchrony of all neural, endocrinal and biochemical processes depending on the circadian component of the sleep cycle. One might expect serious health consequences of such a chaotic input to the system; however, natural defense mechanisms make life quite miserable for those who attempt a struggle against the natural sleep cycle. As a result, those who attempt polyphasic sleep are doomed to drop out sooner or later.
Chaotic phase-shifting input
Chaotic signals sent to the phase-shifting inputs as seen, for example, in polyphasic sleep, may have hard to predict negative consequences for the sleep control system. The risk is not fully known and hard to estimate. It could include in the order of decreasing likelihood:
- desensitization to signals sent by the sleep control system
- long-term instability in the sleep control system
- damage to nerve cells involved in the control of the circadian cycle
The first possibility can actually be observed in shift-workers and people running a constant battle with sleep deprivation. In those individuals, the concept of "refreshed mind" and "refreshing sleep" becomes hazy, and one can observe an increased tolerance to permanent degree of tiredness coming from insufficient sleep or sleep in a wrong circadian phase. In other words, a degree of fatigue becomes a norm.
Instability of the sleep control system is also observed in shift-workers. I am not sure if shift-induced instabilities can become chronic or are fully reversible in a relatively short time. Even in a perfectly tuned sleep control system, minor rhythm perturbations, such as a switch to the DST, can produce regulatory ripples lasting for days. Larger perturbations might, in theory, result in uncoupling of master and slave oscillators with a particularly slow return to a fully stabilized control. Perhaps this kind of uncoupling is the primary factor that underlies a myriad of disorders that plague shift-workers in the long-term.
Two-component model of sleep in SleepChart
SuperMemo uses a two-component sleep model inspired by the publications of Alexander A. Borbély and Peter Achermann. Unlike other models, SuperMemo uses user's sleep data to predict the homeostatic and circadian status of overall sleep propensity. This model is helpful in choosing the optimum time for learning on a given day (given a particular history of sleep). It can also help planning the optimum bedtime in cases where the sleep pattern is highly irregular. The model does not predicate on the timing and duration of NREM and REM sleep episodes.
The model is tuned to fit typical SleepChart data logs. However, there are individual genetic differences that affect the length of the circadian cycle, steepness of the homeostatic decline in alertness, sleep length preferences, sleep architecture, spectral properties of sleep, fragmentation of sleep, etc. This model is limited in accounting for these variables. If you are sleepy against the simulations based on the model, you can probably trust your own instincts better. If you feel alert against the simulations based on the model, you can certainly get down to learning and ignore predictions of the model. Moreover, sleep patterns are a good measure of your sleep control systems only if they are not artificially disturbed (e.g. by forcefully delaying sleep, using alarm clock, using medication, etc.). In other words, if you are not free running your sleep, the presented model may fail to map your circadian rhythms correctly. You can mark blocks as artificially shortened or delayed (Forced awakening and Delayed retirement on the context menu available with a right-click). However, marked blocks will have a limited effect as there is no way of knowing the degree of the cut into the sleeping patterns, and, consequently, knowing the resulting perturbation in the sleep control system produced by artificially modified sleep.
In Borbély model, the timing of sleep is determined by the points in which the curves representing the homeostatic and circadian processes cross. SuperMemo uses a simpler, but more intuitive approach, in which both components of sleepiness are integrated into an overall alertness level (red line in the graph). The advantage of that approach is the possibility of instant feedback from an actual learning process, where the level of memory recall is supposed to correlate directly with the level of alertness determined by the model. The formula for integrating the two components of sleep into overall alertness was chosen heuristically with the help of alertness data gathered in SuperMemo. The purpose of the integration was to achieve the best possible match of the predicted alertness in the model with the average recall level in SuperMemo. As it has been shown earlier, both homeostatic sleepiness and circadian sleepiness affect the grades in SuperMemo, however, only a combined effect of both components provides a good match with the changes of recall for different combinations of homeostatic and circadian sleepiness. In the model used in SuperMemo, sleep is initiated when the overall alertness drops below a certain level. Sleep may thus be initiated by both components of sleepiness independently, but the timing and duration of sleep will differ for various combinations of changes in the homeostatic and circadian sleep propensity. Despite using a different approach to determining the sleep onset, predictions of the model fit the actual sleep log data pretty well in free running condition in cases studied.
To see the predictions of the model for your own sleep data for any given day, make sure you have your sleep log filled out for recent days in SleepChart, and shift-click the day in question in the sleep log.
Two-component sleep model in SuperMemo: The horizontal axis represents time. Blue blocks show the actual sleep episodes. Aqua line shows the 24h circadian sleep drive with a mid-day siesta hump. Green line is an inverse of the homeostatic sleep drive and can be interpreted as homeostatic alertness, which declines exponentially during wakefulness and is quickly restored by slow-wave sleep (for simplicity, as in Borbely model, the entire sleep block is assumed to have a contribution proportional to its length, as the SleepChart model does not account for sleep stages). Yellow vertical lines show the prediction of the circadian acrophase (circadian middle-of-the-night peak). acrophase computations are done with the help of a phase response curve model (as opposed to a statistical model used in earlier versions of SleepChart). Red line shows the resultant alertness (peaks are best for learning, valleys are best for sleep). For example, Alertness on Oct 1, 2008 at 7:43 was predicted to be at 59% of the maximum but would keep increasing fast in the first 2 hours of wakefulness (a typical symptom of a night sleep that is terminated too early). The picture shows two peaks in alertness on Oct 1, 2008, at 9 am and at 7 pm. Those periods would likely be best suited for learning on that day.
To see a more accurate presentation of your own homeostatic and circadian alertness in SuperMemo, see the Alertness tab in SleepChart.
REM rebound hypothesis
Researchers know that Borbely's two-process model is not complete and does not explain all known properties of sleep, nor even all possible sleep patterns (e.g. various napping habits, newborn sleep, irregular sleep patterns, sleep in psychiatric disorders, etc.). There have been numerous attempts to expand the model by new variables that may show up in specific circumstances (e.g. adding noise to simulate a sleep-wake pattern in autistic children, ultradian dynamics to model NREM-REM occurrence, adding the impact of light intensity, etc.). Borbely and Achermann keep investigating various aspects of sleep that would help make the model more complete. One of their investigative targets is a REM sleep rebound following a REM sleep deprivation. It has already been discovered long ago that REM sleep deprivation reduces alpha activity, waking, and NREM sleep. These are clear signs of REM homeostatic compensation (Borbely et al. 1990, Brunner et al. 1993). It has been proposed that increases in muscle atonia episodes in NREM (MAN) be considered as markers of an increase in REM sleep pressure (Achermann et al. 2002).
For many years now, I have observed an unusual phenomenon in SleepChart logs that I could not explain with the two-process model. In people with irregular sleep, late naps are often exceedingly long and unrefreshing. Those long naps clear up the homeostatic component of sleep propensity, and often result in later bedtimes. In some extreme cases, this can lead to confusion about the optimum timing of sleep. The affected person will nap long enough to lose the sense of the timing of his or her own subjective night. A graph below demonstrates such a classic occurrence.
In the exemplary sleep log above, a middle-aged woman working from home and suffering from a delayed sleep phase syndrome shows a clear and pretty regular progression of the sleep phase from a bedtime at 2 am to sleeping past midday. The lady claims to suffer from irregular sleep, daytime tiredness, and never knowing when to go to sleep to get a "good night's rest". On Sep 24, due to feeling tired, she went for a nap at 6:30 am. This nap unexpectedly lasted 3.5 hours and produced the impression that no more sleep was required on that day. Despite some tiredness, the lady did not go to bed that evening even though the chart clearly says that it was the period of her subjective night and she should retire. After a particularly tiring evening and night, the lady went to sleep 3:30 am on the assumption this was her "night sleep". That sleep was 6 hours long and refreshing enough to "impersonate" the night sleep. This completed the role reversal between the night and siesta periods. The two circadian lows have been swapped in the sleeper's mind. This swap is then reflected in retirement rituals, expectations, and other habits that can perpetuate the reversal for a few days despite a potentially highly unrefreshing sleep. A night-vs-siesta reversal is not stable though. On Sep 25, in the period of the subjective night, a short nap was taken which seemed particularly refreshing. Still the refreshing impression dissipated fast and the third "night at siesta time" followed. On Sep 26, sufficient sleep debt accumulated leading to a "nap" that suddenly turned into 8 hours of deep and refreshing sleep. The sleep pattern flipped back to the norm after 3 prolonged napping episodes. This role reversal cannot be explained with the two-process sleep model. Nor can it be explained with the model employed in SleepChart. Those three outlier naps taken past the siesta time should rather be shorter due to the fact they were not enhanced by the circadian siesta dip. Those naps were also occurring too early to capitalize on the nighttime circadian low. In other words, those "kinky" naps, despite missing the circadian component of sleep, lasted unusually long.
Having seen those kinks in sleep patterns dozens of times, I came to believe that the 2-process model of sleep propensity needs to be extended by a third component. However, it was hard to come up with a sensible hypothesis that would plausibly fit with what we know about the function of sleep and its evolution. A big clue came from interviews with people affected by kinky naps. It appears that those long naps are very often triggered by consumption of alcohol or, in some cases, smoking marijuana. If the timing of alcohol or cannabis administration aligned with the late waking hours, shortly before the subjective night, the kinky nap could follow on the next day. In addition, those naps are preceded by a particularly strong feeling of being unrefreshed in the morning, which is a frequent case in alcohol or cannabis abuse (as much as in the application of sleeping pills or even melatonin). As both substances are known to reduce the proportion of sleep spent in REM, I hypothesized that it is the REM-sleep deficit that might be causing the said sleep perturbations.
I have also documented cases were kinky naps followed a healthy and refreshing night sleep that did not involve alcohol, cannabis nor other substances affecting sleep. Those remaining cases also had another common factor: a substantial one-time delay in optimum bedtime and the resulting sleep phase shift. This would agree with the REM-deficit hypothesis. If sleep is delayed past the circadian REM peak, it is also known to be less REM-rich.
Finally, those kinky naps, unlike the healthy well-timed naps were reported to be dream rich. This could also indicate that they might be involved in REM compensatory function.
If the REM-deficit hypothesis was to be right, we would need to always consider separate homeostatic REM and NREM sleep propensity. In healthy sleep, the REM component might be hard to notice. Some researchers hypothesize that homeostatic REM drive depends on the preceding NREM sleep. If so, homeostatically, healthy night would produce no REM deficit, while waking activity would only produce homeostatic NREM sleepiness.
How could REM deficit produce those prolonged naps? There are some indications that REM sleep can also produce an increase in demand for NREM sleep. Thus those two, functionally vital phases of sleep, could produce a mutually amplifying cycle that would run its course until the demand for both sleep components was fulfilled. Why would REM sleep increase towards the end of normal night sleep? Some of that increase is circadian, some of it might come from the fact that homeostatic NREM sleep demand is satisfied faster. The biological explanation of sleep terminated with REM is difficult, esp. in the light of Buzsaki model of hippocampal "training" in REM. Waking up with a clean slate seems biologically more advantageous. Perhaps that last REM period is responsible for creative breakthroughs of the early morning? Only a detailed mathematical modelling and comparisons with actual sleep cycle measurements could answer the questions about the homeostatic interplay between NREM and REM sleep.
Three components of sleep propensity
SleepChart cannot easily verify the nature of the REM-deficit hypothesis. Not only are sleep stages missing from its logs, detecting REM sleep is not practicable in home conditions amongst users of SuperMemo or SleepChart. However, the third variable needed to explain kinky naps in sleep logs, which I will call the RD variable (for REM deficit), could possibly be included in the two-process model in hope of mathematically explaining the impact of kinks on the estimated sleep phase. As mentioned earlier, those kinky naps do not need alcohol or other REM-suppressing factors, sleep blocks marked as Delayed retirement often cause similar effects due to a wrong phase alignment vs. the circadian REM peak. Once sleep misalignments are explained successfully with the RD variable, it would be up to sleep labs to verify the model using EEG measurements. The interaction between the RD variable and the other two sleep variables (H and C) is not straightforward. For example, high RD would not suffice to initiate sleep, as it is not possible to initiate sleep without an appropriate combination of H and C. High RD and high C might also be insufficient (as it is indicated by sleep logs where sleep is pretty short in the nights that follow kinky naps due to the low H). However, high RD and high H could initiate fully blown sleep and result in kinky naps with possible negative consequences for the subsequent night sleep (low H), and sleep phase. At the moment of writing, I am still now sure how the sleep phase is affected, however, I am pretty sure it is. For example, in the example presented earlier, the sleep phase seems to have been shifted back by a few hours, however, it could as well be caused by the deficiency of the model employed in SleepChart (precisely due to the missing RD variable). The need for both high H and high RD to initiate sleep for low C seems consistent with current research on the mutual interaction between NREM and REM sleep stages where one increases the demand for the other.
The sleep-wake flip-flop is a system of two sets of brain nuclei that produce a rapid switch from sleep to waking, and vice versa. One set of nuclei is responsible for inducing sleep and inhibiting the arousal centers, while the other set acts in the opposite way. Both sets inhibit one another. This means that when there is time to sleep, the sleep centers take an upper hand and turn off the wake centers. Later on, in the morning, the wake centers take control and turn off the sleep centers. This sleep-wake flip-flop is constructed in such a way that the transitions from sleep to wake and back are pretty fast and thorough. In a healthy sleep cycle, we should be half-awake only for a very short time before sleep, and perhaps a little while longer in the morning. Unfortunately this does not mean that we can switch the flip-flop wherever we wish. It also does not imply that we won't feel tired before sleep. Homeostatic increase in sleepiness is a natural process and it proceeds throughout the waking period. It is only the transition from wake to sleep that is fast and the time when homeostatic sleepiness meets a sufficient degree of circadian sleepiness. The sleep-wake flip-flop is stabilized by orexin neurons. As demonstrated by Siegel, the level of orexins (also called hypocretins) is not related to the circadian cycle but to a particular behavior. During a waking activity, e.g. during exercise, the level of orexins may remain high thus preventing the switch in the sleep-wake flip-flop. When the orexin stabilizer is off, narcolepsy enters the picture and the flip-flop becomes unstable causing multiple sleep episodes in a single day in hard to predict circumstances.
The most important components of the sleep-wake flip-flop are:
- the ventrolateral preoptic nucleus (VLPO) on the sleep side, and
- the tuberomammillary nucleus (TMN), locus coeruleus (LC) and dorsal raphe nucleus DR on the arousal side.
Once sleep is initiated, another flip-flop starts operating: the one that is responsible for transitions between NREM and REM sleep.
Suprachiasmatic nucleus (SCN)
Human brain harbors a clock that runs in a cycle that is slightly longer than 24 hours. That clock is called the suprachiasmatic nucleus (SCN) and is located in the anteroventral hypothalamus. The SCN is made of two groups of neurons (10,000 each, 0.25 mm3) situated bilaterally just above the optic chiasm. The SCN is slightly more elongated in women, and there is a marked difference in VIP expressing neurons between sexes (up to twice as many in males)(Swaab et al. 1990). Homosexual men have larger SCNs and twice the number of VP expressing neurons than heterosexual men (Swaab and Hofman 1990). Incidentally, I am pretty sure that this difference is not by choice and it cannot be remedied with self-discipline or by prayer.
In 1972, the SCN has been identified as the body's master clock that can run without environmental cues and receives resetting inputs from the retina. Clock genes in the SCN are responsible for a circadian cycle of gene expression that determines the output from the SCN. The neurons in the SCN express the cycle that finds its reflection in signals that travel from the SCN to other brain nuclei and the rest of the body in various neural and hormonal forms. If we surgically damage the SCN, the circadian cycle wanes or disappears. It can be restored with a transplant of SCN cells.
The SCN signal is most active during the subjective day, esp. in the evening hours. It is the weakest during the subjective night, esp. in the early morning when the body temperature reaches its minimum. If you ever tried to sleep polyphasically, it is the SCN that will bother you and make you crave the core sleep and make you oversleep during the subjective night time. The SCN controls alertness, attention, release of hormones, body temperature, melatonin secretion, feeding, and more. Most of the output from the SCN flows to the subparaventricular zone (SPZ) and the dorsomedial nucleus of the hypothalamus (DMH). Neurons in the dorsal SPZ (dSPZ) affect the circadian rhythm of the body temperature, while those in ventral SPZ (vSPZ) are running the wake and sleep cycle. vSPZ in turn commands the inputs to DMH which is the chief command center for waking behavior, motor activities, cortisol cycles, feeding, etc. DMH affects sleep promoting VLPO and the wake promoting LHA (lateral hypothalamus). Lesions to VLPO and LHA can produce loss of sleep or insurmountable sleepiness respectively.
This central positioning of the SCN and the DMH at the crossroads of the most essential and influential neural pathways controlling behavior is a powerful demonstration of how a tiny group of a few thousand neurons exerts a powerful influence on what we do as active feeding and surviving organisms. This should remind everyone that sleep hygiene is essential for the proper function of this tiny structure in the human brain. Disrupting circadian cycles with alarm clocks, shiftwork and the like can lead to a whole volley of physical and mental disorders. For a thorough review of the interaction between the SCN, the DMH and the rest of the body see Dr Clifford B. Saper "Hypothalamic regulation of sleep and circadian rhythms" (Saper et al. 2005). Interestingly, Dr Saper hypothesizes that it is the DMH that integrates the circadian resetting stimuli such as exercise or social interaction. In rodents, DMH can also be reset by the availability of foods or even the temperature. It is unlikely though that you will be able to combat jetlag or adapt to any shift-work pattern with the help of zeitgebers such as food or temperature.
SCN oscillates with a period slightly longer than 24 hours. To adapt to the 24h world, the oscillation needs to be reset daily to match the daylight cycle of the Earth. The resetting is done with the help of zeitgebers ("time givers") such as light, exercise, feeding, etc. The most important zeitgeber is light. Light signals are received by glutamatergic melanopsin-expressing retinal ganglion cells in the retina (pRGCs). From there, they are transmitted to the SCN via the retinothypothalamic tract (RHT). The impact of light signals and other zeitgebers on the sleep phase is described by the so-called phase response curve (PRC). Most importantly, morning light signal helps reset the cycle. The circadian period gets shortened to match the 24h daylight cycle. With the help of zeitgebers, the oscillator with a slightly longer period is brought back to synchrony with the daylight by a minor SCN-mediated reset. This provides for a stable oscillation. People who cannot effectively cue their oscillators suffer from phase-shift disorders. People suffering from DSPS could experiment with light dimmers, toning down their schedules in the evening, properly timed exercise and bright light in the morning. People with ASPS should use opposite measures (e.g. 3000 lux light in the evening). In addition to light, the SCN is affected by activity. Locomotor activity affects the SCN by activating NPY-containing neurons in the intergeniculate leaflet (IFL) and serotoninergic neurons in the median raphe nucleus (MRN). This is why exercise and social interaction act as powerful zeitgebers.
The neural symphony commanded by the SCN goes awry when we use artificial lighting or do exciting evening activities such as watching TV, surfing the net, playing computer games, reading, etc. It has been hypothesized that light (as well as other stimuli) may affect the SCN in two different ways during the subjective night. Short light pulses simply change the expression of clock genes and result in phase shifts along the PRC. However, constant lighting may result in uncoupling between the SCN neurons and the downstream nuclei affected by the SCN resulting in dangerous arrhythmicity (Ohta et al. 2005). Continuous disruptions to circadian cycles as seen in shiftwork or jetlag may lead to a gradual mental decline as indicated by research in rodents (Ree et al. 1985). Circadian changes associated with aging and Alzheimer's can be correlated with loss of cells in the SCN or changes in its inputs. Vasopressin-expressing cells are particularly prominent in their decline in Alzheimer's. All forms of artificial control of sleep cycles, including the use of alarm clocks, can affect the health of those few precious neurons.
The SCN sends projections to the dorsal PVH (parvicellular paraventricular nucleus) whose neurons project to sympathetic preganglionic neurons in the spinal cord that in turn affect the pineal gland and the release of melatonin. This tells us that melatonin, which is often advertized as a "natural sleeping pill" is produced by the pineal gland downstream from the SCN control. This is why it cannot be considered a central factor controlling circadian sleepiness. Melatonin does produce phase shifts along its unique melatonin PRC (see picture in the PRC section). It is possible that this effect is caused by a direct impact of the melatonin on the SCN. However, early sleep will also result in earlier waking and this will also have a phase shifting effect.
Dorsomedial Hypothalamic Nucleus (DMH)
Dr Saper and colleagues demonstrated that excitotoxic lesions to the dorsomedial nucleus of the hypothalamus (DMH) in rats cause a major impairment to circadian rhythms (Saper et al. 2003). As lesioned animals sleep more, it was suggested that the impact of DMH is predominantly activating even though other explanations of the available findings are also imaginable. It appears that a great deal of output from the SCN travels via the subparaventricular zone (SPZ) to the DMH and only then, via inhibitory tracts, to the VLPO that is responsible for the initiation of sleep. The DMH also projects to the lateral hypothalamic area (LHA) that contains wake-promoting orexin neurons. It has been hypothesized that the DMH might be in the center of control of various variables that change along the circadian cycle such arousal, feeding, locomotor activity, cortisol levels, body temperature, melatonin, etc. Restricted feeding synchronizes circadian rhythms of the DMH so that the highest c-Fos expression and locomotor activity coincide with mealtimes (Saper et al. 2006). As most of the input arrives to the DMH via SPZ, it is important to note that dorsal and ventral portions of the SPZ seem to play different functions. Lesions to the dSPZ reduce circadian rhythms of body temperature, while it is the vSPZ that seems to control sleep-wake cycles and locomotor activity (Saper at al. 2001). The dSPZ controls body temperature via the medial preoptic area (MPO) that includes the median preoptic and ventromedial preoptic nuclei. The DMH is affected by the hormones controlling the appetite, ghrelin and leptin, via the ventromedial nucleus (VMH) and the arcuate nucleus (ARC). VMH enhances lipolysis in adipose tissue and decreases feeding. Dr Saper hypothesized that the DMH may serve as a secondary circadian control center that would enable entrainment of the rhythms to the availability of food. However, from the standpoint of control systems, it would seem biologically more sensible to phase-shift the SCN rather than to employ a second asynchronous or phase-locked oscillator. In humans, it is very hard to influence the circadian cycle in any way other than via a minor phase-shift with the use of various zeitgebers, of which, food is a very weak one. It therefore seems highly unlikely that shiftworkers or jetlagged travellers could tangibly benefit from changes to the timing of their diet. Differences between rats and humans cannot, naturally, be excluded. Nevertheless, the DMH is definitely a very interesting further research target.
Ventrolateral Preoptic Nucleus (VLPO)
The ventral lateral preoptic nucleus (VLPO) is one of the chief brain centers needed to initiate sleep and to maintain slow-wave sleep. Lesions in this area halve the amount of sleep, and result in insomnia combined with persistent tiredness. Both NREM and REM can be affected depending on the type of the lesion. For its role, the VLPO is often called a "sleep switch". In both nocturnal and diurnal animals, the SCN is active during the period of daylight, while the VLPO is primarily active during sleep. Once the VLPO is on, it is believed to maintain inhibition of the monoaminergic and cholinergic excitatory systems that keep the brain cortex "awake". Those VLPO projections go to the tuberomammillary nucleus (TMN) (histamine), lateral hypothalamus-perifornical region (LHA/PF) (orexin), ventral periaqueductal grey (vPAG) (dopamine), locus ceruleus (LC) (noradrenaline), parabrachial nucleus (PBN) and dorsal raphe (DR) (serotonin), lateral tegmentum (LDT) (acetylcholine) and the pedunculopontine tegmental nucleus (PPT) (acetylcholine). The inhibitions enacted by the VLPO are mediated by GABAergic neurons as well as by galaninergic inputs to the histaminergic tuberomammillary nucleus (TMN). Inhibition of the TMN and other alertness nuclei results in a drop in alertness hormones and a drop in cortical activation causing drowsiness. A hypothesis says that separate populations of the VLPO might be responsible for expressing circadian aspects of NREM and REM sleep. A subset of VLPO cells is able to stimulate cholinergic neurons in the LDT and PPT. This contributes to inducing REM bursts that activate the cortex without wakefulness during REM sleep.
The VLPO receives its circadian signal input from the SCN (the main body clock) via the dorsomedial nucleus of the hypothamalus (DMH), which is the other brain clock that is usually synchronized with the SCN. The VLPO neurons do not build up a homeostatic need for sleep, however, some homeostatic mechanisms, such as the intracellular build-up of adenosine, may inhibit aminergic or cholinergic wake centers and thus activate the VLPO. For example, infusion of adenosine agonists into the basal forebrain increases both NREM and REM sleep (Satoh et al. 1999) and increases c-Fos in the VLPO (Scammell et al. 2001). In sleep deprivation, the activity in the VLPO is not much higher than in ordinary waking. This low level of activity persists until the bedtime. Once the sleep begins, VLPO neuron firing rate may double in conditions of sleep deprivation (Saper et al. 2005). This indicates that even though the VLPO does not build up homeostatic sleep propensity, it is impacted by the homeostatic mechanisms in the end. This also indicates that the VLPO is located downstream the circadian and homeostatic signal integrator. Aminergic arousal nuclei such as the TMN, LC and the raphe form a part of the sleep wake switch. That switch is stabilized by orexin/hypocretin cells from the lateral hypothalamus-perifornical region (LHA/PF), esp. during motor activities or feeding. The arousal can thus be maintained uninterrupted despite competing inhibitory influences. The arousal nuclei inhibit the VLPO in waking as much as the VLPO inhibits them back in sleep. Scientists believe that this mutual inhibition forms a classic unbalanced flip-flop with sharp state transitions. This is what helps us fall asleep fast, and wake up fast, spending minimum time in transition, and maximum time in the desired states: alertness or deep sleep.
For more on the place of the VLPO in the sleep control system see Figure 39 in Functional Anatomy of the Hypothalamus and Pituitary.
For a thorough review of the role of various sleep and wake centers, see Saper's "Hypothalamic regulation of sleep and circadian rhythms" (Saper et al. 2005).
Nucleus of the Solitary Tract (NTS)
This section speaks more of fads and fashions in science than of the actual involvement of the nucleus of the solitary tract (NTS) in sleep. I heard of the importance of the NTS for sleep in the early 1980s during my college years when studying biology. Some time later, I added a couple of items on the NTS to SuperMemo to consolidate that knowledge for years to come. When writing my Good sleep, good learning, good life article in 2000, I still mentioned the NTS and how rocking babies to sleep might work even though I knew that destruction of NTS does not lead to insomnia, which should be a big clue. The NTS seems to be more involved in processing signals received from the gut. These signals play only a minor part in sleep control. In the end, I fell victim to the same old affliction that pesters science since its inception. Sometimes it takes the old generation of scientists to die out for a new idea to take hold. Old knowledge makes us more conservative, because not knowing makes us seek answers while knowing makes us passive even if our answers are wrong. Once you believe you know all the answers, there is less pressure to investigate. In the end, many other brain centers play a role comparable to that of the NTS. After all, the brain is a highly connected structure and few things happening in one corner of the central nervous system have no bearing on events in other corners. Consequently, activation of nearly all major nuclei will have an arousing or inhibiting effect within the reticular activating system, which has also been for years a mainstay of our thinking about arousal. At the same time, back in 2000, I hardly mentioned the VLPO, as it was perhaps not fashionable enough. A similar situation, we may or may not face with the SLD, which has emerged as an alternative to the well-established PPN/LDT REM on system. Even the DMH might not be immune to fashions. Like the NTS, it is also involved in feeding behaviors. Perhaps, in a decade, this article will warrant a complete rewrite with a great deal of old fads gone. Equally well, in the era where all new findings in science are available at our fingertips, and we can easily communicate via e-mail and other means, we will all show a lesser tendency to swim with the crowd. More importantly, new investigative technologies are likely to open new areas that might still be subject to fads, while the subject matter discussed in this article will gel out and solidify.
Incidentally, the pain of fashions was once the main factor that pushed me away from peer-review writing to blogging. Writing the presented article was the acme of fun. The article was written using incremental writing, and polished collaboratively as a wiki. Being part of a commercial company, I am not subject to publish-or-perish pressures. This is a precious freedom. Back in 1992, with Dr Gorzelanczyk we studied the literature of the spacing effect and came to conclusion that the mountain of data we collected with SuperMemo, as well as a clear computational formulation of the concept of spaced repetition will sweep the world of education and memory science off its feet. A vast majority of the spacing effect literature of that time was focused on short-term studies (e.g. checking the memory effect after just a week from the trial). Bahrick's study of the retention of Spanish vocabulary was a major and stellar exception. However, Bahrick could only study the retention of vocabulary many years after the original training with no specific data on the timing of exposure to individual words during the period of learning, or during the long period preceding the measurement. In that light, we thought we have all we needed to start a new revolution in learning and in the science of memory. To our monumental disappointment, we could not push our paper through to be published in Memory and Cognition. Our failure came partly due to our inexperience, and the lack of credentials. We both have just come out of the university with MSc degrees. However hard we tried to phrase our paper around the fashionable spacing effect, we were not able to mold it to match the mainstream science of memory. From some old obscure journal, we picked the best-sounding scientific name for our repetition scheduling methodology. We called it repetition spacing. This term mutated later into spaced repetition and remains the only tangible legacy of the original paper, even though it has not yet penetrated the scholarly namespace. Perhaps it never will. More general "distributed spacing" or "distributed presentation" still predominate, while users of computerized flashcard now consistently speak of "spaced repetition". The editors of Memory & Cognition congratulated us on our results and mentioned that the computational aspect of the paper made it suitable to journals devoted to computer algorithms. The world of fashion in memory science was so different from our proposition that no top model took our stance. In the end, we published in a lesser known Acta Neurobiologiae Experimentalis. Sadly, the paper has got only 18 citations in the course of 20 years since publishing, and when it is mentioned, it is quoted with caution. After all, the "optimization algorithm" feels like a black box. It was offered free for anyone, and yet it is hard to study it in action. It is not a neat formula. It is an algorithm, and it can best be run on a computer and studied with computer means. Our line of clothing appeared to be highly unfashionable. It is now commonly used by millions, and new designers hop on board monthly. Scientific community remains largely impervious for now though. It awaits a wave of new talent grown on the feed of spaced repetition.
Fashions in science are part of our collective cognitive prejudices. They slow down progress. They are unavoidable. And still, in the long-run, they regress to the mean of the approximate truth. It is important that for each step back, we can make a dozen steps forward.
Adenosine is one of the endogenous markers of the homeostatic sleep drive. During the waking period, as the cortex and other parts of the brain keeps burning their glycogen reserves, ATP is converted to adenosine which accumulates extracellularly. The role of adenosine was first discovered upon the finding that its systemic administration promotes sleep (Radulovacki et al. 1984).
The increased activity of the cholinergic neurons in the basal forebrain neurons causes a buildup of adenosine that in turn inhibits the activity in that region via its A1 receptor (Strecker et al. 2000). This is one of the hypothetical homeostatic triggers of sleep. The accumulation of adenosine in the basal forebrain is particularly important as it is here that its effect is most pronounced (Strecker et al. 2000). The accumulation in the basal forebrain causes the inhibition of some aminergic waking centers and the disinhibition of the VLPO, which promotes sleep. Infusion of A2A receptor agonists into the rostral basal forebrain increases both NREM and REM (Satoh et al. 1999). Moreover, adenosine A2A receptor agonists stimulate the VLPO. The resulting activation of the VLPO may be measured by the increase of c-Fos activity (Scammell et al. 2001). Some of the presented scenario has recently been questioned upon finding that rats with a 95% loss to cholinergic neurons in the basal forebrain show intact sleep homeostasis despite the lack of the hallmark increase in adenosine.
Adenosine is particularly interesting as its well-known antagonist is caffeine. Caffeine binds to adenosine receptors thus blocking the homeostatic sleep propensity. This proves that network overload is not the cause or at least not the sole cause of the homeostatic sleep drive. The brain has evolved sleep protection mechanisms, in this case involving adenosine, to ensure that before a network overload leads to any significant consequences, homeostatic sleep drive pushes an animal to take a sleep break and do the necessary neural housekeeping.
NREM sleep and REM sleep
NREM and REM alterations
In the course of the night, we alternately enter two phases of sleep:
Using EEG measurements, scientists are able to distinguish 4 phases of NREM sleep which correspond to progressively deeper sleep. In newer literature you may read of three stages due to the fact that Stages 3 and 4 of NREM have been bundled together as a single stage of slow-wave sleep.
As we close our eyes, it takes 3-15 minutes to enter Stage 1 NREM sleep (in a healthy and well-regulated individual). In this stage we will often experience little jerks associated with the impression of falling. Minor disturbances will wake us up and often we will even deny being asleep! Once State 1 NREM solidifies, we move towards Stage 2 NREM sleep which is still relatively light. After that we move to Stage 3 (and Stage 4) NREM, which is also called deep sleep or slow-wave sleep (SWS).
Historically, the importance of REM sleep for memory and learning was documented before we became truly aware of the role of slow-wave sleep. Consequently, articles and books on sleep are peppered with an overemphasis on the role of REM sleep in learning as compared with SWS. Over time, REM deprivation studies received lots of criticism. Today, we know that the natural harmonious interplay of uninterrupted NREM and REM sleep is essential for memory, learning and creativity (Salzarulo et al. 2000).
Cruel sleep deprivation studies actually show that sleep deprived rats can live longer if REM deprived than if NREM deprived. Rats deprived of sleep survive for 2-3 weeks. Rats deprived of REM sleep only survive for some five months.
Napping human subjects reported that it is Stage 4 NREM that feels most restorative. The release of norepinephrine, serotonin and histamine is inhibited during REM. During dreaming, the primary visual cortex is not active, while its secondary areas are active. This is similar to the situation in which people are asked to imagine a visual scene as opposed to a situation in which they actually see the scene. Blind people have dreams that are more auditory and more tactile. This seems to confirm the role of REM sleep in the replay of experiences and in optimization of memories. They do not show the typical eye movement pattern in REM sleep either. Those observations led to an idea that REM sleep is vital for creativity (more than NREM sleep). During REM, cholinergic modulation suppresses the flow of information from the hippocampus to the neocortex. This is supposed to help build new associations within the neocortical areas.
Evolution of NREM and REM
REM sleep is phylogenetically younger than NREM sleep. Fish, amphibians or reptiles do not show typical REM sleep. Yet, interestingly, REM sleep is present in both mammals and birds. This made some scientists hypothesize that REM sleep has been invented twice by evolution! Clearly, REM sleep plays a role critical for survival of creatures with bird-mammal IQ levels (see: How much do animals sleep?). However, Dr Siegel who studied REM sleep in echidna concluded that this animal's sleep combines aspects of both REM and NREM sleep. As a result, he suggested that REM and NREM might have evolved from a phylogenetically older unified form of sleep (Siegel et al. 1996). If REM sleep is as disparate from NREM in its function and as complex as implied by the theories on the neural optimization in sleep, the re-invention factor might be used by evolutionists as an argument against neural optimization. However, like aerial flight, re-invention combined with complexity could equally well add weight to emphasizing the vital neural function of sleep.
NREM and REM deficits
Cruel sleep deprivation studies actually show that sleep deprived rats can live longer if REM deprived than if NREM deprived. Rats deprived of sleep survive for 2-3 weeks. Rats deprived of only REM sleep survive for some five months. After sleep deprivation, it is the SWS deficit that is repaid first. SWS deficit is a result of NREM sleep deprivation. REM deficits are paid off later. A frequent scenario is that the SWS deficit is paid fully on the first night of recovery, while REM sleep deficit may persist through to the second recovery night.
Forced desynchrony protocols are sleep protocols in which subject sleep is dissociated from its natural circadian cycle. In condition of forced synchrony, we can observe that slow-wave activity that characterizes NREM sleep is associated with homeostatic sleep propensity, while the proportion of REM sleep in sleep episodes depends on both homeostatic pressure and the circadian cycle.
NREM sleep is primarily controlled via a homeostatic mechanism. During the waking day we build a pressure to initiate sleep and its deeper NREM stages. If sleep is initiated without a contribution of the circadian component, it is likely to be short and NREM-only. One of the signals correlating with homeostatic sleepiness is the buildup of adenosine. It is the adenosine receptors that are affected by caffeine resulting in its short-lived impact on reducing the homeostatic sleep pressure. One of the consequences of the buildup of adenosine is the inhibition of the aminergic wake centers, inhibition of the basal forebrain, and the disinhibition of the VLPO: the chief brain center responsible for the initiation of sleep. REM sleep also has a homeostatic component, however, in times of deficit, it is NREM sleep deficit that is compensated for first. There is also more evidence indicating that REM sleep increases NREM sleep pressure (Beersma et al. 1990). In addition to adenosine, other signals such as interleukin-1, tumor necrosis factor, interferon, prostaglandin D2, NO, GHRH, and others have also been associated with the increase in homeostatic sleep propensity (Krueger et al. 2008).
Sleep deprivation increases both NREM and REM sleep propensity. Short sleepers have less NREM 2, but there is little data on the actual quality and effectiveness of their sleep. Thomas Edison or Nicola Tesla, on one hand, are well-known for sleeping relatively little, while Einstein is a well-known long sleeper, who, supposedly, slept over nine hours per night. Interestingly, all these geniuses also belonged to notable nappers. It is true that by getting less sleep you compress the less critical NREM 2 sleep, but there is no evidence that this can become your regular habit without hurting the quality of your NREM-REM interplay. With the currently available sleep data the conclusion is: do not try to compress NREM 2 by sleeping less. You are likely to hurt the memory optimization process occurring in sleep.
Some scientists believe that during sleep, an ultradian oscillator in the mesopontine junction controls the regular alternation of NREM and REM sleep. However, the term oscillator is rather misleading as the mechanism of NREM-REM mutual interaction is more of a flip-flop nature, and the timing of alternation is pretty irregular indicating significant internal and external homeostatic influences that ultimately culminate in the extinction of the sleep cycle.
Neuromodulation in sleep
In NREM sleep, cholinergic systems in the brainstem and the forebrain are less active than in waking. Serotonergic raphe and noradrenergic LC are also less active. On the other hand, in REM sleep, these aminergic structures are strongly suppressed, while the cholinergic systems flare up. Release of histamine is also down in REM. It has been hypothesizes that cholinergic modulation suppresses the flow of information from the hippocampus to the neocortex. This is supposed to play an important role in the dual network theory model of learning in which the hippocampus plays a role in building up new associations on the basis of old information (see: Neural optimization in sleep).
Dr Siegel, who does not believe in the role for sleep in memory and learning, believes that REM sleep serves recovery as serotonergic, noradrenergic, and histaminergic neurons stop firing. It is as if they were overused and attempted to replenish their resources. This interpretation might pass the shutdown test (see: Sleep theories) as many of these neurons are vital for maintaining arousal. However, it is hard to imagine that evolution would not find a way to re-design the brain in which neurotransmitter replenishment would be possible without the shutdown. Some areas of the brain keep firing in waking and as well as in all stages of sleep. The neurons involved are able to replenish their resources without going offline.
Growth hormone and cortisol
In addition to changes in firing patterns of neurons releasing different types of neurotransmitters, circulation of systemic hormones also changes during sleep. Of these, growth hormone and cortisol are of particular importance as they impact glucose metabolism. Growth hormone increases at sleep onset and peaks in deeper stages of NREM. On the other hand, cortisol levels increase in the later stages of the night dominated by REM sleep. Unlike the release of cortisol, which is largely circadian, the increase in GH is associated with a sleep onset (Van Cauter et al. 1997). sleep deprivation or sleeping in a wrong phase are both involved in major disruption of glucose metabolism for different reasons. This is why healthy sleep is vital for preventing obesity.
In the same way as sleep in general, REM is controlled via homeostatic and circadian processes. Acrophase of the circadian REM cycle comes late in the subjective night. Benington and Heller proposed that it is the presence of NREM sleep rather than the absence of REM sleep that leads to an increase of REM sleep propensity. Slow-wave sleep builds homeostatic REM propensity, and the best REM comes from the combination of slow-wave "exhaustion" and the circadian REM peak which comes in the last hours of sleep. There is also a strong homeostatic link between learning and the demand for REM sleep. The more you learn, the stronger the drive towards REM. There is an increase in both the number of minutes of REM sleep and the density of REM sleep following intensive learning (De Koninck et al. 1989). It is not clear if learning affects REM demand directly or via NREM demand. However, it is more than clear that heavy learners should be heavy sleepers!
Stimulating the basal forebrain causes a release of acetylcholine, which induces wakefulness and is also conducive to REM sleep. This means that the basal forebrain that takes part in the initiation of sleep is also involved in NREM/REM transitions. Similarly, a subset of VLPO cells contribute to generating REM sleep.
The impact of adenosine antagonists, such as caffeine, is also important. Adenosine agonists infused into the basal forebrain increase c-Fos in the VLPO as well as increase the release of acetylcholine by the basal forebrain. Acetylcholine is known to induce the states of wakefulness and REM sleep. As a result of the agonist infusion, both the total amount of NREM and the total amount of REM sleep increase (Satoh et al. 1999, Scammell et al. 2001).
Of other homeostatic hormonal influences, increased levels of VIP and prolactin in sleep promote REM. It is possible that substance abuse, delaying sleep, as well as the use of alarm clocks can all read to REM sleep deficits (see: REM rebound hypothesis).
One of the sleep theories says that REM sleep helps the brain recover from NREM sleep to speed up the responses in waking. This theory fails the shutdown test as the same recovering might simply be taking place in a waking state unless the hard work of the networks in dreaming is a faster recovery method for some unknown reason. However, why would a brain experience a REM rebound in conditions of full "recovery" to waking? The claim that histamine, serotonin and noradrenaline neurons need recovery time sound more plausible, however, it does not explain why we would need different populations of neurons with different neurotransmitters with different restoration and recovery strategies.
Transition to REM
After an hour or so of healthy NREM sleep during the subjective night sleep, there is a gradual increase in the activity of cells in the pontine tegmentum which is responsible for triggering REM sleep. Structures responsible for triggering REM sleep might include pedunculopontine tegmental nucleus (PPN) and sublaterodorsal tegmental nucleus (SLD). GABAergic SLD and cholinergic PPN send their signals in multiple directions. One of the outcomes is muscle atonia. Another is the activation of the thalamus, hippocampus, and the cortex with an appearance of the typical REM EEG. As a result the brain behaves as if it woke up internally! Injections of acetylcholine into the pons during an ongoing NREM episode may trigger REM sleep, which illustrates the importance of this neurotransmitter in sleep cycle regulation. During REM sleep, the cortex behaves as in the state of wakefulness. Dreams experienced at that stage seem to be generated by random impulsation sent from the brainstem to the cortex. The cortex produces best possible and most coherent imagery of that chaotic input. During dreams we experience connected events, real people, realistic scenery, etc. However, all these are put together in most improbable configurations as if the brain was testing "what if" scenarios. Yet we cannot act upon our dreams (except for people who suffer from violent sleeping). Pontine structures responsible for REM control make sure that the cerebral output gets cut off from motor nuclei that move the muscles. It happens often that we want to act in sleep (e.g. to escape a ferocious dog), and yet we remain motionless as if mired in molasses. At that time, only the eyes move rapidly, while the muscles in the middle ear also twitch.
The movements of eyeballs that gave REM its name is controlled by impulsation generated in the pontine nucleus that projects to the superior colliculi. That impulsation is associated with generating of the ponto-geniculo-occipital waves (PGO) that are also used to detect REM sleep.
During dreaming, the primary visual cortex is not active, while its secondary areas are active. This is similar to the situation in which people are asked to imagine a visual scene as opposed to a situation in which they actually see the scene. Blind people have dreams that are more auditory and more tactile. This seems to confirm the role of REM sleep in the replay of experiences and in optimization of memories. They do not show the typical eye movement pattern in REM sleep either. Those observations lead to an idea that REM sleep is vital for creativity (more than NREM sleep).
The interplay between NREM and REM sleep is most likely controlled by a REM flip-flop. Some scientists believe that during sleep, an ultradian oscillator in the mesopontine junction controls the regular alternation of NREM and REM sleep. However, a flip-flop model is a better analogy considering the timing of the alternations.
As it is the case with the sleep-wake flip-flop, the REM flip-flop causes a continuous switches between a relatively stable NREM and REM states, however, the flip-flop is under far greater influence of various homeostatic inputs resulting in a somewhat chaotic succession of NREM/REM states that gradually become dominated by REM circadian peak towards the end of the subjective night.
For many years, an oscillation between cholinergic and monoaminergic states seemed like a final answer to the control of REM sleep. However, some inconsistencies and new research lead to a newer similar model involving GABAergic structures (see the next section).
Models of REM flip-flop
In 1962, Jouvet showed that stimulation of the caudal mesencephalic region or pontine tegmentum in cats produced a state similar to REM sleep. This led to a hypothesis that mesopontine cholinergic structures are responsible for the activation of the thalamus and the cortex in REM sleep. The hypothesis would also be supported by the fact that injections of cholinergic agonists into the pontine reticular formation would enhance REM sleep.
The original reciprocal interaction model in which pontine aminergic and cholinergic neurons have formed a classical REM-on/REM-off flip-flop has been accepted as a fact for a quarter of a century. New research has identified GABAergic populations that might be part of the REM flip-flop on both on and off sides of the switch.
The old REM flip-flop included cholinergic PPT and LDT, which are particularly active in REM (and wakefulness), as well as the BRF (brainstem reticular formation). The REM-off component was composed of DR (serotonin) as well as the LC (NA) (Saper et al. 2001).
Newer research questioned some inconsistencies in the model. For example, selective lesions to cholinergic or monoaminergic nuclei of the brainstem have only limited effect on REM sleep. Low c-Fos expression in REM-on structures during REM was also troubling. Instead, it was suggested that the key REM-on area is the GABA-ergic SLD (sublaterodorsal tegmental nucleus)(Lu et al. 2007). As the SLD does not directly inhibit DR-LC, their direct participation in the flip-flop was questioned as well. GABA-ergic neurons of SLD project to the vlPAG (ventrolateral periaqueductal grey matter) and LPT (lateral pontine tegmentum), which thus became REM-off suspects. Lesions of the SLD cause a loss of REM sleep.
The precise nature of the REM flip-flop must yet be determined. Components of the old and new models show some interaction as well. For example, PPT/LDT do excite SLD neurons, while DR/LC may inhibit the SLD or activate the REM-off components. However, that interaction is not directly mutual. Hence the exclusion of the old components from the core of the new model.
Termination of sleep
The sleep control system would act as an infinite seesaw were it not for the circadian component of the sleep drive. Towards the end of sleep, the circadian sleepiness determined by the suprachiasmatic nucleus (SCN) will produce decline in sleep propensity, and the sleep will be terminated after one of the REM sleep episodes. It is the SCN which provides the link between the strongest zeitgeber, the light, and the circadian cycle. SCN generates the rhythm endogenously, but is able to get reset by light. Light impulses from the retina travel to the hypothalamus and the SCN to produce a stop&reset signal. End of sleep will see the end of melatonin release. Instead, another neurohormone starts building up: serotonin. A hypothesis says that it is the high level of serotonin that we feel as the morning sunshine happiness. High serotonin combines with the alertness hormone cortisol to give us a good alert start into a new day. Unless you suffer from sleep phase advancement, always make sure the sunshine streams into your sleeping room in the morning to wake you up.