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

in your spinal cord.

They control all of your voluntary skeletal muscles.

So ensuring REM sleep, your brain paralyzes your body so

your mind can dream safely.

It's a bad evolutionary design when you're not perceiving

your outside world to start acting out all

of those dream commands.

And there are plenty of them.

Just as a quick aside on this process note by the way,

sometimes this persists despite you waking up.

Some of you may have experienced this, this

persistence of sleep paralysis on awakening.

It's quite unusual--

well, it's not unusual proportional wise.

About 25% of the population will experience this.

It's about as common as hiccups.

And what seems to happen is that your brain starts to wake

up, but the paralysis isn't released from the body.

So you start to become aware.

But you can't lift your eyelids, voluntary muscles.

You can't move.

You can't say anything.

It is often associated with a sense of sort of anxiety, a

sense of someone else being there in the room.

It turns out that this sleep paralysis, this persistence,

accurately explains most, if not all, of

so-called alien adoptions.

I mean when was the last time you ever heard of someone

being abducted during the day, in the middle of a meeting?

You know.

I mean--

whoosh, what was that?

Well I believe Jimmy just got abducted by aliens.

No, it never happens like that.

It's usually at night, when you're in bed.

People describe a sense of a presence in the room, that you

were paralyzed by these other agents.

You couldn't move.

You couldn't fight back.

You couldn't talk.

It's a strange interesting feature.

That's a little bit about what sleep is, together

with some odd sides.

I don't quite know I threw that in there.

But anyway, let's now come onto what sleep is doing.

And it is serving a whole broad array of functions.

Firstly, let me tell you why it's essential to sleep before

learning, to prepare your brain, almost like a dry

sponge, ready to soak up new information the next day.

And to sort of test this question, we're going to run

an experiment.

Essentially, is pulling the all-nighter a good idea?

Here's how you do this.

You take two groups of participants.

You assign them to a sleep group or a

sleep deprivation group.

Both are awake across the first day.

But then across the following night, those in the

deprivation group, we keep them awake in the laboratory

under full supervision.

They can't fall asleep.

The sleep group they get a full eight hours.

Both of them are awake across the second day.

And then we have them try and cram a whole bunch of facts

into their brain.

And then we're going to test them to see how efficient that

learning has been.

But instead of testing them immediately after learning, we

actually wait until two full recovery nights of sleep

before we test them.

So that any measure of memory that we get is not confounded