A walk through the stages of sleep

Sleep is perhaps the
single most effective thing

that we can do each and
every day to reset the health

of our brain and our body.

And by understanding a little
bit more about what sleep is,

perhaps we can get the chance to improve

both the quantity and
the quality of our sleep.

(gentle music)

So exactly what is sleep?

Well, sleep, at least in human beings

is subdivided into two main types.

On the one hand, we have
non-rapid eye movement sleep

or non-REM sleep for short.

But on the other hand,

we have rapid eye movement
sleep or REM sleep.

And non-REM sleep has
been further subdivided

into four separate stages,

unimaginatively called
stages one through four,

increasing in their depth of sleep.

And as we go into those light
stages of non-REM sleep,

your heart rate starts to decrease,

Your body temperature starts to drop,

and your electrical brainwave
activity starts to slow down.

But as we move into deeper
non-rapid eye movement sleep,

stages three and four,

now all of a sudden the brain erupts

with these huge, big, powerful brainwaves.

The body is actually recharged

in terms of its immune system.

We also get this beautiful overhaul

of our cardiovascular system.

And in fact, upstairs in the brain

deep non-REM sleep will
help consolidate memories

and fixate them into the neural
architecture of the brain.

So that's non-REM sleep, but
let's come on to REM sleep,

which is the other main type of sleep.

And it's during REM sleep

when we principally have the most vivid,

the most hallucinogenic types of dreams.

The brainwave activity actually
starts to speed up again.

It's during REM sleep that
we receive almost a form

of emotional first aid.

And it's also during REM sleep

where we get a boost for creativity,

that it stitches information together

so that we wake up with solutions

to previously difficult
problems that we were facing.

Coming back to these two types of sleep,

it turns out that non-REM
and REM will play out

in a battle for brain
domination throughout the night

and that cerebral war is
going to be won and lost

every 90 minutes.

And then it's going to be
replayed every 90 minutes.

And what this produces is a
standard cycling architecture

of human sleep, a
standard 90 minute cycle.

But what's different, however,

is that the ratio of non-REM to REM

within those 90 minutes cycles

changes as we move across the night,

such that in the first half the night,

the majority of those 90 minutes cycles

are comprised of lots
of deep non-REM sleep,

particularly stages three
and four of non-REM sleep.

But as we push through to
the second half of the night,

now that seesaw balance
actually shifts over.

And instead, most of
those 90 minutes cycles

are comprised of a lot more
rapid eye movement sleep

or dream sleep, as well as
stage two non-REM sleep,

that lighter form of non-REM sleep.

And it turns out that
there are implications

for understanding how sleep
is structured in this way.

Let's take someone who typically
goes to bed at 10:00 pm

and they wake up at 6:00 am.

So they have an eight hour sleep window,

but this morning they
have to wake up early

for an early morning meeting,

or they want to get a jumpstart on the day

to get to the gym.

And as a consequence, they have to wake up

at 4:00 am in the morning,

rather than 6:00 am in the morning.

How much sleep have they actually lost?

Two hours out of an
eight hour night of sleep

means that they've lost
25% of their sleep.

Well, yes and no.

They have lost 25% of all of their sleep,

but because REM sleep comes
mostly in the second half

of the night and particularly
in those last few hours,

they may have lost perhaps
50, 60, maybe even 70%

of all of their REM sleep.

So there are real
consequences to understanding

what sleep is and how sleep is structured.

And we'll learn all about the benefits

of these different stages of sleep

and the detriments that happen

when we don't get enough of
them in subsequent episodes.

(upbeat gentle music)