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  • MATTHEW WALKER: It's a pleasure to be here.

  • And I want to start with a standard disclaimer, which is

  • that when most speakers look to their audience and they see

  • people who are falling asleep or nodding off, it can be

  • profoundly disheartening.

  • However, based on the topic of today's presentation, I'm

  • almost going to actively encourage that kind of

  • behavior from you.

  • In fact, knowing what I know particularly about the

  • relationship between sleep and memory, it's actually the

  • greatest form of flattery for me to see people like you not

  • being able to resist the urge to strengthen what I'm telling

  • you by falling asleep.

  • So feel free just to sort of ebb and flow in and out of

  • consciousness throughout the entire talk.

  • I'll take absolutely no offense.

  • And the talk itself is really going to come in at four main

  • acts, so to speak.

  • Firstly, I want to spend some time telling you about what

  • sleep actually is, the different types, it's

  • characteristics, its structure.

  • And then after that, I'll tell you about the variety of

  • different functions, plural, that we're starting to

  • understand that sleep serves.

  • So I'll tell you about the role of sleep in promoting

  • learning and also memory.

  • But I'll also then tell you how sleep can go beyond simply

  • manipulating individual memories.

  • Sleep seems to be intelligent in that it can cross-link new

  • pieces of information together so you can come up with

  • creative, novel insights the next day.

  • And then finally, I'll describe a role of sleep

  • beyond information processing into your mental health and

  • how sleep seems to be critical for emotional regulation,

  • preparing specific brain circuits for next day social

  • and emotional interactions.

  • So that's the basic overview.

  • Coming on to what sleep is, and I do love this picture.

  • You can just kind of get a sense of the quality and the

  • depth of sleep that's happening there.

  • If we're going on that whole savanna grasslands kind of

  • side street by the way, I just want to come

  • onto this, the giraffe.

  • Firstly, what a strange morphology for a creature.

  • Have you ever wondered how something that

  • looks like that sleeps?

  • Would you like to know how a giraffe sleeps?

  • That's how a giraffe sleeps.

  • Isn't that remarkable.

  • And it tells us at least two things.

  • Firstly, despite such bizarre anatomy, sleep will still find

  • a way to be obtained by the brain.

  • Second, and more generally, in every species that we've

  • studied to date, sleep, or something that looks very much

  • like it, has been observed.

  • What that means is that sleep has fought its way through

  • vehemently every step along the evolutionary pathway.

  • If that's true, sleep must be essential at some of the most

  • basic of biological levels.

  • And that's exactly what we're starting to discover.

  • And sleep in terms of mammalian species at least has

  • been broadly separated into two main types, as

  • some of you may know.

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

  • or non-REM sleep for short.

  • And non-REM sleep has been further subdivided into four

  • separate stages, unimaginatively called stages

  • 1 through 4--

  • increasing in their depth of sleep--

  • or a creative bunch of sleep researchers.

  • So increasing in the depth of sleep, stages 3 and 4 are

  • those really deep stages of dreamless sleep.

  • And they're often grouped together under the term

  • slow-wave sleep.

  • Why?

  • Because of these slow, lazy brain waves that happen during

  • the stage of sleep that we measure with

  • electrodes on the head.

  • But don't be fooled.

  • That's not that your brain is dormant by any stretch of the

  • imagination.

  • What it means is that vast portions of your brain,

  • hundreds of thousands of neurons, have all decided to

  • synchronize together and sing together in time.

  • It's a phenomena like no other brain state that we know of.

  • It doesn't happen whilst you're awake.

  • It's a strange phenomena and we still don't truly

  • understand why.

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

  • REM sleep named, not after the popular Michael Stipe pop

  • band, but because of these bizarre, horizontal, shuttling

  • eye movements that occur during this stage of sleep.

  • And again, we don't truly understand why your eyes move

  • during that stage of sleep.

  • And it turns out that these two types of sleep, REM and

  • non-REM, will play out in a battle for brain domination

  • throughout the night.

  • And that sort of cerebral war is going to be won and lost

  • every 90 minutes and replayed every 90 minutes.

  • And what that creates is a standard architecture of

  • sleep, what we call a sleep cycle.

  • So I'll just unpack this for you here.

  • We've got the different stages of sleep on the vertical axis.

  • And then time of night along the horizontal axis.

  • And I'll speed this up for you.

  • But what you can see is that upon falling asleep, your

  • brain goes on this delightful roller coaster ride in and out

  • of these different stages of sleep.

  • So you'll quickly descend down into the deep stages of

  • non-REM sleep, 3 and 4.

  • And you'll stay there for a while.

  • And then after about 70 or 80 minutes, you'll start to rise

  • back up and you'll pop up and have a short REM sleep period,

  • here in red.

  • And then back down you go again, down into non-REM sleep

  • and then up into REM.

  • As I said, this cycle is 90 minutes, non-REM through REM.

  • And that's stable across the night.

  • However, what changes is the ratio of non-REM to REM within

  • that 90-minute window as you move across the night, such

  • that in the first half of the night the majority of those

  • 90-minute cycles are comprised of deep non-REM sleep,

  • slow-wave sleep.

  • Whereas as you push through to the second half of the night,

  • now that ratio balance shifts across.

  • And instead, they're dominated much more by rapid eye

  • movement sleep, as well as that lighter form of

  • non-dreaming sleep, stage 2 non-REM sleep.

  • And just to come back to REM sleep, REM sleep is the

  • principal stage during which your brain dreams.

  • And REM sleep is a case of essentially how your brain

  • goes completely out of its mind.

  • Because every one here, as long as you slept last night,

  • you became flagrantly psychotic.

  • Now before you reject my diagnosis of a nightly

  • psychosis, let me give you five good reasons.

  • Because last night when you were in REM sleep and you were

  • dreaming, you started to see things which were not there.

  • So you were hallucinating.

  • Secondly, you believed things that

  • couldn't possibly be true.

  • So you were delusional.

  • Third, you became confused about time, place, and person.

  • So you're suffering from disorientation.

  • Fourth, you had wildly fluctuating emotions.

  • Something that psychiatrists call being affectively labile.

  • And then how wonderful, you woke up this morning and you

  • forgot most, if not all, of that dream experience.

  • So you're suffering from amnesia.

  • If you were to experience any one of those five symptoms

  • whilst you're awake, you would be

  • seeking psychiatric treatment.

  • Yet for reasons again that we don't fully understand, it

  • seems to be both a normal biological and

  • psychological process.

  • One of the other fascinating features of REM

  • sleep is this, paralysis.

  • All of you, when you went into REM sleep

  • last night, were paralyzed.

  • It turns out that there's mechanism deep down here in

  • your brain stem-- so here we have the brain, which as Woody

  • Allen suggested, was his second most favorite

  • organ of the body.

  • And so here's the front of the brain, back of the brain,

  • brain stem down here.

  • Now, this war of REM and non-REM sleep essentially

  • plays out down here.

  • And then is beamed up to the top of the wrinkled mass, atop

  • of the brain, called the cortex.

  • But there's also another signal that goes south, down

  • into the spinal cord.

  • And this signal during REM sleep that goes south

  • essentially inhibits what we call the alpha motor neurons