Why we get sleepy

At this very moment, a chemical called adenosine is building up in your brain.

It will continue to increase in concentration with every waking minute that elapses. The longer you are awake, the more adenosine will accumulate.

Think of adenosine as a chemical barometer that continuously registers the amount of elapsed time since you woke up this morning.

One consequence of increasing adenosine in the brain is an increasing desire to sleep. This is known as sleep pressure, and it is the second force that will determine when you feel sleepy, and thus should go to bed.

Using a clever dual-action effect, high concentrations of adenosine simultaneously turn down the “volume” of wake-promoting regions in the brain and turn up the dial on sleep-inducing regions.

As a result of that chemical sleep pressure, when adenosine concentrations peak, an irresistible urge for slumber will take hold. It happens to most people after twelve to sixteen hours of being awake.

Source: Why We Sleep by Matthew Walker