Two components of sleep
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.