Sleep is most well-known physiological process, and yet most challenging to understand and study. We need to sleep long enough, and well enough for healthy life.
Homeostasis means constant state. It is a term used in the study of human body working, a field called physiology. Thus homeostasis simply means that body tries to balance, maintain levels of various components. Homeostasis is essential for existence. Thus body needs to maintain temperature, pH, weight, and adequate sleep.
Necessity to have balanced sleep is sleep-homeostasis. It means that in our body several mechanisms work to ensure that we have enough of sleep. If for some reason sleep is disturbed or we are not getting enough of it, body tries to balance the things. For example, if you do not sleep enough during the night, you feel sleepy next day. It is body’s reaction to maintain homeostasis.
Propensity is nothing but an inclination. During some times of day like in the evening we have higher sleep propensity. Some individuals may have higher sleep propensity during lunch hours. Sleep propensity is necessary to ensure sleep homeostasis.
So if a person did not sleep long enough, or sleep was disturbed, then he or she tries to sleep more in the coming hours. Similarly, a person overslept, then he may find it difficult to go to sleep next day.
Sleep Regulation: Two Process Model
It’s a sleep model that was first mentioned in 1980’s. According to this model sleep drive or propensity is controlled by two factors:
- Circadian rhythm – that is our body’s internal clock. It affects when we sleep during the period of 24 hours.
- Homeostatic (also called process S)- prolonged waking leads to accumulation of waking fatigue.
According to this model, matching of both the factors is necessary for a sound sleep. Studies have further shown that circadian rhythm affects the timing of sleep. Whereas, homeostatic mechanisms regulate the depth of field.
Figure 1 two process sleep model (source: 1. Owens J, Gruber R, Brown T, et al. Future research directions in sleep and ADHD: report of a consensus working group. J Atten Disord. 2013;17(7):550-564. doi:10.1177/1087054712457992)
Based on this model we can say that person starts feeling sleepy at fixed time of the day, depending on the person’s circadian rhythm. Tiredness/fatigue accumulation would affect the depth of sleep.
Thus people who are involved in the physical tasks, may not necessarily sleep longer than those doing less physical task. But physically active person would have better and longer deep-sleep.
It means that fatigue accumulation whether due to physical activity or less sleep in the past, would lead to more intense non-REM sleep.
Very little is known about the biochemistry of sleep. Though we know that deep sleep means prolonged period of delta waves.
As per researchers of homeostatic model we feel sleepy due to accumulation of certain chemicals in the brain. They often call these chemicals as “sonogens”. At present we have limited knowledge of what these sonogens are, though many chemicals are known to have sleep-inducing effect.
One of the sonogens well proven by research is dopamine. As the day passes, its levels increase. Higher levels of dopamine in the brain means higher sleep propensity. On the other hand, sleep is disturbed in diseases with lower levels of dopamine.
We would say that accumulation of sleep propensity is still very poorly understood. Researchers do not know whether it is accumulated steadily during the day, or its rate of accumulation depends on the activities. We do not know what activities are better in building up sleep propensity, like how does jogging in the morning compare with evening walk in the mall?
Though we do not know much about sleep propensity and physical activity, but it seems that relation is week. It means that physical activity may not necessarily affect sleep propensity, though it may influence quality of sleep.
Delta waves in the EEG are now accepted to show deep sleep (stage 3 of non-REM sleep). Studies show that if you work harder during the day, you may have longer period of delta sleep. However, there is huge variance among the people, it means that genetics also play role.
In conclusion, so sleep well you need maintain your circadian clock and sleep drive. So, fall asleep at fixed time. Get rid of insomnia, get treated for sleep apnea. Avoid consuming sleep disturbing substances like caffeine. Avoid sleeping too much during the day, let the sleepiness slip in naturally.
- Borbély AA, Daan S, Wirz-Justice A, Deboer T. The two-process model of sleep regulation: a reappraisal. J Sleep Res. 2016;25(2):131-143. doi:10.1111/jsr.12371
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