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Sleep is perhaps the
single most effective thing

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that we can do each and every day

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to reset the health
of our brain and our body.

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

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perhaps we can get the chance to improve
both the quantity and the quality

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of our sleep.

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[Sleeping with Science]

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(Gentle music)

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So, exactly what is sleep?

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Well, sleep, at least in human beings,

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is subdivided into two main types.

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

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or non-REM sleep for short.

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But on the other hand,

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we have rapid eye movement
sleep, or REM sleep.

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And non-REM sleep has been
further subdivided

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into four separate stages,

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unimaginatively called
stages one through four,

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increasing in their depth of sleep.

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

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your heart rate starts to decrease,

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your body temperature starts to drop

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and your electrical brain wave activity
starts to slow down.

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

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stages three and four,

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now all of a sudden the brain erupts

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with these huge, big,
powerful brain waves.

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The body is actually recharged
in terms of its immune system.

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We also get this beautiful overhaul
of our cardiovascular system.

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And, in fact, upstairs in the brain,

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deep non-REM sleep
will help consolidate memories

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and fixate them into the neural
architecture of the brain.

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So that's non-REM sleep.

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But let's come on to REM sleep,

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which is the other main type of sleep.

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And it's during REM sleep
when we principally have the most vivid,

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the most hallucinogenic types of dreams.

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The brain wave activity
actually starts to speed up again.

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It's during REM sleep that we receive
almost a form of emotional first aid.

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And it's also during REM sleep
where we get a boost for creativity,

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that it stitches information together

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so that we wake up with solutions

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to previously difficult problems
that we were facing.

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Coming back to these two types of sleep,

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

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in a battle for brain domination
throughout the night,

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and that cerebral war
is going to be won and lost

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every 90 minutes,

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

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And what this produces is a standard
cycling architecture of human sleep,

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a standard 90-minute cycle.

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But what's different, however,

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is that the ratio of non-REM to REM
within those 90-minute cycles

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changes as we move across the night,

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such that in the first half the night,

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the majority of those 90-minute cycles

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are comprised of lots
of deep non-REM sleep,

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particularly stages three and four
of non-REM sleep.

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

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now that seesaw balance
actually shifts over,

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and instead, most of those
90-minute cycles

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are comprised of a lot more
rapid eye movement sleep, or dream sleep,

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as well as stage-two non-REM sleep,

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that lighter form of non-REM sleep.

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And it turns out
that there are implications

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for understanding how sleep
is structured in this way.

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

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and they wake up at 6am,

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so they have an eight-hour sleep window.

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But this morning,
they have to wake up early

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for an early morning meeting,

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or they want to get
a jump start on the day

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to get to the gym.

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

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rather than 6am in the morning.

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How much sleep have they actually lost?

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Two hours out of
an eight-hour night of sleep

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

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Well, yes and no.

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They have lost 25 percent
of all of their sleep,

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but because REM sleep comes
mostly in the second half of the night

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and particularly in those last few hours,

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

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of all of their REM sleep.

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So there are real consequences
to understanding what sleep is

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and how sleep is structured.

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And we'll learn all about the benefits
of these different stages of sleep

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and the detriments that happen
when we don't get enough of them

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in subsequent episodes.