Dr. Matt Walker

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.

They just measured the temperature of someone's feet and they looked at how quickly they fell asleep and when they fell asleep.

Sure enough, the warmer your feet, the faster you fell asleep.

Why?

Because the warmness reflects the blood dilation and the pumping out of the blood to the periphery.

And then they did it in rats where they started to warm the paws of the rats and the rats fell asleep more quickly.

And I love this notion of, again, we don't do animal warfare.

I love the notion of wrapping a beautiful little rat up in cotton wool and warming its feet with this pad and it's just blissed out and then poof, he's gone.

After all, she, I respect their privacy.

So that was the early evidence.

That then led to a series of manipulation studies.

The most notable is brilliant.

It comes from a colleague in the Netherlands, Oes van Sumeren and his group.