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.

[Sleeping with Science]

(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 brain wave 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 brain waves.

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 brain wave 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-minute cycles

changes as we move across the night,

such that in the first half the night,

the majority of those 90-minute 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-minute 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 10pm,

and they wake up at 6am,

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 jump start on the day

to get to the gym.

And as a consequence, they have to wake up
at 4am in the morning,

rather than 6am 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 percent of their sleep.

Well, yes and no.

They have lost 25 percent
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 percent

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.