Why do we feel sleepy?

Published: February 23, 2022

We all need sleep and our bodies will surely show that to us if we stay awake for too long, but what is it that actually makes us feel sleepy? Well, I recently read an amazing book about sleep and discovered the answer to this question.

The level of a one's sleepiness depends on the time of day in relation to the their internal clock and sleep pressure.

The internal clock controls our physiological circadian rhythm in many ways. For example, it controls how much melatonin and orexin are released into our bodies. Melatonin is a chemical that helps us fall asleep and orexin is a chemical that helps us wake up.

The circadian rhythm is naturally synchronized, based on the amount of ambient light, in such a way that the middle point of sleep usually takes place at around 4 am. Although the rhythm can adapt to changes when we travel to different time zones, it can only adapt about 15 minutes per day which is why we experience jet lag.

Even more than the circadian rhythm, sleepiness is based on the so called sleep pressure. When we're awake, our bodies produce a chemical called adenosine. The longer we stay awake and the more time our bodies have to keep producing more adenosine, the more it slowly builds up in our bodies. The more adenosine we have in our bodies, the sleepier we feel.

The effect of caffeine is actually based on removing or dampening the effect of adenosine. Caffeine prevents adenosine receptors in our body from detecting adenosine and thus alleviates sleep pressure. However, caffeine does not remove adenosine from our body which leads to a strong sleep pressure once caffeine is no longer affecting us.


Walker, Matthew P. Why we sleep. New York: Scribner, 2017. Why We Sleep - Wikipedia.

Walker is the professor of neuroscience and psychology and a sleep researcher in the university of Berkeley in California, USA.

Learn more about sleep

Sleep is your superpower - Matt Walker (TED talk, 20min)

Matthew Walker on the Joe Rogan Experience (Podcast, 2h)

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