Dr. Matt Walker

Thank you very much.

I actually slept pretty well last night, despite it being a foreign location.

Same time zone.

That helps just astronomically.

What is sleep?

So sleep, I think, in some ways you can define as, at least in humans, and in fact in all mammalian species, is broadly separated into two main types of sleep.

On the one hand, we have something that many people will have heard of called non-rapid eye movement sleep, or non-REM sleep for short.

And non-REM sleep has been further subdivided into four separate stages.

And they are unimaginatively called stages one through four, increasing in their depth of sleep.

So stages three and four.

That's the really deep sleep that we can speak about.

And I should explain a little bit at some point what happens during that state within the brain.

It's stunning.

It's astonishing.

So you've got stages one and two, light non-REM sleep, when you sort of look at your sleep trackers and it has light non-REM, deep non-REM, and then REM.

Stages one and two, that's light non-REM.

Stages three and four, that's deep non-REM.

And that's non-REM encapsulated.

On the other hand, we have rapid eye movement sleep, or REM sleep.

And it's named not after the popular Michael Stipe band of the 1990s, but because of these bizarre horizontal shuttling eye movements that occur during this stage of sleep, hence the rapid eye movements.