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What decides when you get sleepy
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The time you get sleepy is not decided by one thing: it is decided by two systems working at once. One is the circadian clock, the body's internal clock. The other is the sleep homeostat, which keeps track of how long you have been awake.
The homeostat works like a tank. What builds up in it is called sleep pressure, and it rises with every hour you spend awake; while you sleep, that pressure dissipates. That is why sleep does not depend only on the time on the clock, but on how long you have gone without it.
There is a molecule that keeps that count: adenosine. It accumulates in the brain during waking hours, and it accumulates even more when there is sleep deprivation, that is, when you stay up.
The other system sets the time. Its clock sits in the suprachiasmatic nucleus, a small cluster of neurons in the brain, and its job is to push toward wakefulness during the day and toward sleep at night.
In brief
Sleep is set by two systems at once: the circadian clock and the sleep homeostat.
A tank that fills with every waking hour and empties during sleep, next to a clock keeping its own time.
Sleep pressure builds while you are awake and dissipates while you sleep.
Adenosine accumulates in the brain during waking hours, and even more with sleep deprivation.
The circadian system, with its clock in the suprachiasmatic nucleus, promotes wakefulness by day and sleep by night.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingPhysiology of Sleep, StatPearlsPublished
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