Dr. Matt Walker

downward sort of stroke of its awesome downward movement.

But also don't forget that at that moment, your sleep pressure, your adenosine is also now at its peak.

You've been awake for now almost 16 hours.

So the moment when your circadian rhythm is on its nice downward swing and your highest

in your levels of adenosine, in your sleep pressure, that's the moment truly that will determine, okay, now is when I feel nice and sleepy.

So then what happens?

Then you go to sleep.

You come down that curve of your circadian rhythm and you kind of hit its nadir, its lowest point in the middle of your sleep phase.

But also when you go to sleep, that second factor of sleep pressure is

your brain gets the chance to clear away that adenosine.

And it seems to be about a seven to nine hour period of sleep is enough time for your brain to jettison all of that adenosine that has been building up across the 16 hours of prior wakefulness.

And then these two things align beautifully again.

When it comes to your natural wake-up time, you've been asleep for, let's say, seven and a half hours.

You've cleared out all of that adenosine, so you no longer have the weight of that sleepiness pushing you down.

But also, your circadian rhythm is now on its awesome upswing.

And when those two things align, when you dissipated and jettisoned all of that sleep pressure and your circadian rhythm is starting to rise, now that's the time when you would naturally wake up.

So that's things when they are working well and in alignment.

Let's say that I now take you and I'm going to deprive you of sleep for 24 hours.

So now coming into sort of 10 p.m., your circadian rhythm is dropping down and your adenosine is starting to get high.

And by about 2 a.m., you're probably not going to be happy.