Dr. Matt Walker

Why?

Because they are getting sleep, because it's a somewhat sleep-dependent process.

But they're not going to necessarily release as much, in part because they're not experiencing sleep at nighttime phases.

Right.

So you can elegantly separate those two out.

And that's why it's not quite one or the other, but it seems to be both.

Certainly it's a sleep sensitive.

I would say sleep sensitive is a very good way of describing it.

Remove cortisol from an organism, they will not do well.

Right.

Yes, it is.

And that's one of, I spoke about, and we'll come onto this perhaps when we speak about emotional and mental health, and when we are underslept, we shift over into a more sort of activated, sympathetic, agitated state of our nervous system.

That's one aspect of it.

But there's another aspect of the stress

response, which is, yes, you get elevated heart rate, you're more sympathetic, which is this activated state rather than parasympathetic.

But you also get, when you're sleep deprived, a greater release of the stress hormonal axis, which is called the HPA axis, which if you really want to go into detail, it's the hypothalamopathy.

pituitary adrenal axis, which is a fancy way of saying that it's a signal from your brain going down to release cortisol.

So when you go into deep sleep, not only do you shift over into the nice, quiet, rested, quiescent state of the nervous system, but you also get a dissipation in that stress-related axis and the release of cortisol.

Cortisol, however, seems to be also under the strict control of your circadian rhythm where it drops down at night.

And in fact, you have one of the steepest declines right at the moment when you're starting to get sleepy too, almost as though your brain and your body know we can't have cortisol even at sort of normative levels that you would have during the day.