Why do we sleep?

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This text is part of: "Science of sleep" by Piotr Wozniak (2017)

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[1])(important: do not confuse this two-stage model with the two-component model of memory (Wozniak et al 1995[2]) or with the two-component model of sleep regulation (Borbely 1982[3]) 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.

We sleep so that the brain can integrate new knowledge and form new associations. As we must sleep for our brain to continue its function, our body attached dozens of important processes to run in sleep as well. In simplest terms, in waking we use and burn, while in sleep we restore and synthetize. Sleep affects the function and health of the entire body.

Sleep control system.jpg

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.

We can compare the brain and its NREM-REM sleep cycles to an ordinary PC. During the day, while learning and experiencing new things, you store your new data in RAM memory. During the night, while first in NREM, you write the data down to the hard disk. During REM, which follows NREM in the night, you do the disk defragmentation, i.e. you organize data, sort them, build new connections, etc. Overnight, you repeat the write-and-defragment cycle until all RAM data is neatly written to the disk (for long-term use), and your RAM is clear and ready for a new day of learning. Upon waking up, you reboot the computer. If you reboot early with the use of an alarm clock, you often leave your disk fragmented. Your data access is slow, and your thinking is confused. Even worse, some of the data may not even get written to the disk. It is as if you have never stored it in RAM in the first place. In conclusion, if you use an alarm clock, you endanger your data

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.

We need to respect sleep, let kids sleep, design smarter night-shift schedules, and minimize sleep deprivation in jobs that weigh on life and death

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.


  1. Buzsáki G., "Two-stage model of memory trace formation: A role for “noisy” brain states," Neuroscience / Volume 31 / Issue 3 (1989): 551-570
  2. Wozniak P.A., Gorzelanczyk E.J., and Murakowski J.A., "Two components of long term memory," Acta Neurobiologiae Experimentalis / Volume 55 (1995): 301-305
  3. Borbély A.A., "A two process model of sleep regulation," Human Neurobiology / Volume 1 / Issue 3 (1982): 195-204