Dr. Matt Walker

You need to warm up to wake up.

What that refers to technically in sleep science are what we call the thermal trigger zones.

So warming up to cool down to fall asleep is what we call the sleep onset thermal trigger zone.

Cooling down or staying cool to stay asleep is about the deep sleep trigger zone.

And then warming up to wake up is the activating alertness trigger zone.

Studies, if you looked at them to begin with before they manipulated that, found something fascinating.

If I take you, Andrew Huberman, and I bring you into my lab and I remove your phone or your laptop and you say goodbye to your friends and family and I bring you into the center and there are no cues as to what time of day, no windows, no nothing.

And I'm just going to say, look, I'll keep asking you, but

at the moment that you feel most sleepy, just let me know.

It turns out that before that, we'd done the delightful intervention of inserting a rectal probe into you, because that's the best way that we can measure your core body temperature.

So we're measuring your core body temperature.

And sure enough, despite you knowing nothing about what time it is,

The moment that you will tell me I am ready to go to bed and I am sleepy is the moment when you are on the greatest decelerating trajectory of your core body temperature.

It is highly predictive of how sleepy you will feel.

The way that your body does this is by pushing blood out to the surface regions of your skin, notably your hands and your feet, because these are these highly vascular regions.

And you had a great podcast from one of my heroes and good friend, Craig Heller, who's done some amazing work on this at Stanford.

So naturally, as we lie down, blood races to our hands and our feet and also our head, and we start to release that heat trapped in the core of our body.

And by releasing that heat at the surface, our core body temperature drops.

Hence, the outer surfaces of you, hands, feet, and face, have to warm up for your core to cool down for you to fall asleep.

And in fact, there was a great nature paper some years ago.