Animals Through the Night: Sleepover at the Zoo

We're about to embark on an ambitious project

to explore the fascinating world of animal sleep.

We do know a lot about the animal kingdom,

the evolution of species, their ecology and physiology,

but there are still a few gaps in our knowledge

and one of the greatest mysteries has been the nature and function

of sleep, something that science is only just beginning to understand.

'Tonight we'll be attempting something

'that's never been done before.

'Bristol Zoo will play host to a unique research project.

'For the first time we'll compare and contrast

'different animal sleep patterns across the course of a single night.

'To build up this unprecedented snapshot,

'we've rigged 20 of the enclosures

'with night-vision cameras and sensors,

'and we'll be monitoring just what the animals get up to

'after dark.'

Look at them. Look at them playing together!

'And we won't just be at the zoo,

'we'll be taking an in-depth look at animal sleep research

'around the world.'

It's, I think, been one of the great success stories

of neuroscience over the past 15, 20 years.

'We'll be looking at dolphins.

'These marine mammals spend their entire lives in the water,

'but they must come to the surface to breathe.

'So, how can they get a good night's sleep?

'Can a sleeping meerkat's brain distinguish between different noises

'at night, and tell it when to wake up in the face of a threat?'

A meerkat left the nest box at our level 4 meerkat alarm call.

'And this cat is asleep, but what is it doing?

'Is this evidence that animals can dream?

'We'll see how throughout evolution

'the process of sleep has not only been conserved

'but perfected and advanced.

'And we'll explore how the study of animals

'can begin to unlock the secrets of human sleep.'

I'm fascinated by the underlying mechanisms

that determine how animals sleep,

and I'll be finding out why the latest research

is forcing us to rethink the role of sleep throughout the animal kingdom.

Animal sleep is more varied than we could possibly have imagined,

and Bristol Zoo,

with more than 400 species in 12 acres,

gives us the perfect opportunity to see for the first time

how the animals behave during the night.

Around 600,000 people visit the zoo each year,

but every night the lights go off, the visitors leave

and the animals are left alone.

Our cameras are already in place,

but connecting them to our HQ needs more than 6km of cable,

and we can't finish rigging until all the guests have gone.

OK, it's 5pm, the last of the visitors are leaving,

the zookeepers will be heading home very shortly

and of course normally nobody gets to stay here overnight -

until tonight, that is -

and by morning we should have a good picture of how the whole zoo sleeps.

Right, that's the last of them gone.

Team, that's the zoo closed.

RADIO: 'OK, thanks, Liz.'

'The cameras are starting to come online,

'and we're getting our first glimpse of the animals.

'Animal activity patterns are divided into four

'basic categories -

'diurnal, nocturnal,

'crepuscular and cathemeral -

'and there are animals in all of these categories at the zoo.

'Humans are diurnal animals -

'we're active during the day and we sleep at night.

'The gorillas, flamingos and seals should follow this pattern.

'A nocturnal animal is the opposite,

'active at night and asleep during the day.

'So the red pandas, sloths and crocodiles

'should be awake all night.

'Crepuscular animals are mainly active at dawn and dusk,

'using the most temperate part of the day to feed.

'Watch out for the tapirs, capybaras and fruit bats

'following this pattern tonight.

'And finally cathemeral animals -

'only the lions fit into this category at the zoo.

'These animals can be active at any time of the day or night.

'But these four categories only dictate the timing of sleep -

'we're watching for a huge variety of other behaviours as well.

'The cameras feed into the theatre at the centre of the zoo.

'This is our HQ for the night.'

I've just grabbed John Partridge, Senior Curator here at the zoo,

for a quick chat by the lion enclosure.

John, what do you make of all of this tonight?

-This is all very exciting.

-Is it?

We're really looking forward to seeing what's going to be found,

really, with our animals.

We leave them quite a lot to their own devices at night,

let them get on with what they want to do.

The lions behind us, particularly,

we know will be active during the night,

-but we don't know exactly what they get up to.

-So you don't mind us

laying all these cables down and invading the space back there?

Not at all. Let's see what happens,

-I'm sure there'll be a lot of interesting things.

-Good stuff.

Well, come in and have a look at the monitors with us later on.

-I'd love to do that, thank you.

-Excellent. Thanks, John.

What sleep IS seems such an obvious question.

But it's much more complex than we think.

Across the animal kingdom, sleep is as varied as life itself.

From the amount of sleep animals get -

giraffes sleep as little as four hours a day,

and little brown bats sleep for nearly 20 hours -

to sleep behaviours.

Different animals choose different places to sleep.

They adopt different body positions.

Like the leaf-tailed gecko, for example,

which always sleeps vertically, using tree bark as camouflage.

Some have adaptations so unusual

that scientists are still not sure what they're for.

Parrot fish sleep in a cocoon made from mucus

secreted from their mouths.

This could be to protect themselves from parasites

or to prevent their scent drifting, attracting predators.

Other animals have sleep rituals.

They circle, yawn, stretch or make a nest like this chimpanzee.

Sleeping in groups is common across the animal kingdom.

There's safety in numbers.

Often, big groups will have sentry animals on alert for predators.

And other animals remain vigilant during sleep.

This kangaroo rat wakes quickly

to the sound of a snake moving above.

No-one has ever watched a zoo in this detail at night.

As the animals fall asleep,

we'll be looking out for never-before-seen behaviours

that darkness usually conceals.

Now, this is our control room,

with video feeds from each of the enclosures that we're monitoring,

and we've also got infrared motion sensors trained on our animals.

This is the motion sensor panel -

green means the animals are still moving,

red means the animals haven't moved for 30 seconds or so,

so it's likely they're asleep.

But our experiment actually started a month ago

when we started rigging the zoo with our cameras.

Observing animal sleep is no easy task.

The risk is that sleep research itself may disturb the animal,

stopping it from sleeping.

So it was essential we gave our subjects the time

to become accustomed to the intrusion.

And not all of them did.

We rigged the pygmy hippo enclosure with four infrared cameras.

Sirana, the young female, took one look at the cameras,

hid in the water and refused to come out for two days,

even after we removed the equipment.

Which is why the hippos won't be part of our study tonight.

Many of the other zoo residents

seemed relatively unfazed by the process

and after some initial curiosity,

they've accepted our cameras and should sleep normally tonight.

And then there was Jock, the male silverback gorilla.

He recognizes cameras, and they upset him.

So we'll be using camouflaged cameras already installed by the zoo

in the newly-built gorilla house.

Now, what happened there with Jock our gorilla

is just one example of how difficult it can be

to carry out animal sleep research.

Just by studying sleep

we can often interfere with the animals' natural behaviour.

Helping us to collate and analyse our data this evening

is one of the world's leading experts on animal sleep,

Dr Bryson Voirin.

Bryson, we've been trying very hard

to habituate our animals to the cameras,

give them time to get back into their natural behaviours,

but one of the questions that kind of has to be raised

is how valid will this data be? Because the animals are captive.

Well, clearly there's going to be some major differences

between a captive and a wild animal,

but the data we have right now for most of these species

is based on animals in a box or in a cage inside of a laboratory,

so by setting it here in the zoo

it's a much more naturalish environment

than what's known right now.

Bryson has been working with us

to determine which animals to select for our observations tonight.

He's been studying animal sleep for ten years,

and he'll be able to interpret the data

as our night at the zoo unfolds.

Now, one of the methods used for studying sleep

is direct behavioural observation.

We've got two researchers here on our monitors

who will be collecting all the data from our motion sensors,

and by the end of the night,

hopefully we'll be able to see

what patterns of sleep are emerging.

Our researchers

are examining a range of behaviours.

How active the animals are, how often they move,

if their eyes are open,

if their breathing has slowed,

and, if they're sleeping, how deeply and for how long.

By morning, we'll have seen what all the animals got up to

during the night, and we'll be able to compare the data between species.

Now, Bryson, how useful is observational research

in sleep analysis?

Well, you know, historically speaking a lot of the sleep research

has been actually focused on behaviour observations

so here we're using cameras to record their every move,

and that's a key way to actually see what an animal's getting up to

and to ascertain what the behaviour of sleep is.

And there are so many things we need to learn about sleep.

It's not just about quantity, right?

There's a lot of other factors involved

that, you know, we may gain insight to here.

Absolutely. It's not just the amount that they're sleeping,

it's also the sleep-related behaviours.

Are they sleeping together, are they sleeping by themselves,

are they sleeping in a particular way,

are they doing strange behaviours that we wouldn't normally see?

And so just by watching them

sleeping here we can learn some very interesting things.

OK, let's do our first monitor check of the evening.

Could you bring up the gorillas for me?

Lovely female, there, with her young.

-They seem to be asleep, don't they?

-Completely passed out.

The gorillas are among the first of our animals to fall asleep

and we can see the behavioural ritual Salome,

the adult female gorilla, carries out each night before settling down.

She leaves her platform with her baby on her back

to gather fresh woodchip off the floor.

She's very particular about what she chooses.

She's using it to create a bed for her and her baby.

Salome actively gathers bedding material

and builds a new nest every night.

This behaviour is common in great apes -

chimpanzees and orang-utans also do this.

It's a sleep ritual considered by scientists

to be an example of tool use.

Not only that,

but it's a process that's taught by a mother to her young.

By improving their quality of sleep,

it's believed that the higher primates

are able to enhance their waking survival skills.

With a little encouragement from Salome,

it takes mother and baby just half an hour to settle down to sleep.

Now it's time for a little tour of our zoo,

introduce you to some of the animals that we're monitoring -

first up, the Asiatic lions. See you in a bit.

There is definitely something a bit weird

about walking through a deserted zoo at night.

Most primates, including humans, are monophasic,

which means they get all their sleep in one go.

All other animals are polyphasic,

they sleep in a number of short phases.

So the gorillas should now sleep through the night.

For the rest of the animals, this is the beginning of a cycle

which will see them sleeping and waking several times.

So we anticipate plenty of activity at the zoo tonight.

I'm here with Lynsey, assistant curator of mammals

and her two favourite animals in the zoo, yeah?

The two beautiful lions that we just saw there.

Tell me a little bit about them.

These are two brothers, their names are Ketan and Kamran,

they are just turned one year old.

They are actually hand-reared, by me and a small team of keepers here.

-That's no small feat.

-It was certainly no small feat.

They're very small to start with, quite a lot of intensive work.

OK. How much do you know of what they do at night here?

Very little, really.

We come in - obviously we know what goes on in the day very well -

we get clues when we come in in the morning

as to where they've been and where they've possibly slept.

You see the beds that they've made their own.

But where they've spent all night, or whether it's part of the night

-we actually have no idea, so I'm fascinated to see the results.

-Good.

Well, come and have a look at our monitors later on

-and we'll see how they're getting on.

-Absolutely.

Thank you so much for introducing me to them,

-they're beautiful.

-No problem.

We know that the lions are cathemeral,

so they probably won't sleep through the night.

But what we don't know is how often they'll wake

and what they'll get up to when they do.

OK, so on the right up here we should have some tapirs.

Now, they're herbivores

and they tend to be sleeping and active in cycles

all through the day and night so we might get a bit of action from them.

Ooh, I can hear something in the water. Turn the light on them.

Oh, look at that!

Hello.

So these guys, all through the night,

will go through phases of sleep and then they'll wake up,

and in the wild it's just to allow them to feed as much as possible

all through the day and night because they're herbivores

and they need to get a lot of food into them

to get the energy they need.

Now, we don't want to disturb them too much,

and of course we are still using a white light right now

because it hasn't been dark for long,

but later on we might have a little sneak around

and we'll use infrared lights

to really make sure we don't disturb any of the animals

once they've really settled in for the night.

But it is a bit of a treat to see them.

HIGH-PITCHED CALL

And they're vocalising as well.

Very interesting looking animals.

OK, let's leave them in peace.

The crepuscular tapirs will be most active at dusk and dawn,

but they can still wake in the middle of the night to feed.

How often will they wake and how active will they be?

Across the zoo are the flamingos.

They're diurnal, so they should sleep at night,

but right now they're still active and communicating with each other.

We want to monitor their behaviour as a group through the night.

And then there are those animals

that have turned the daily cycle of activity and rest on its head.

All right, so in this enclosure

we have got two red pandas -

there's one - a male and a female.

And they are nocturnal animals, so this is why they're out and about.

They're actually more closely related to the raccoon

than they are to the giant panda.

And hopefully we'll get to see some more of their activity

with our monitors later on.

The red pandas should be up and about tonight.

But with temperatures at the zoo already plummeting,

will they be able to resist the warmth of their nest box?

That sleep is so varied should perhaps not come as a surprise

when you consider its origins.

All life on earth evolved under the day-night cycle of the sun.

So early life had one phase of activity and one phase of inactivity

in line with light and dark.

As animals evolved central nervous systems,

so inactivity became what we would recognize as sleep,

an opportunity for the brain and body to perform specific functions

not carried out when the animal is active.

Over hundreds of millions of years

the functions performed during sleep became more and more complex.

As life spread into every habitat on earth,

so animals were forced to adapt the way they slept

in order to survive and thrive.

But light and dark are still some of the key triggers

for the chemical processes in the brain

that tell us when we should fall asleep.

At Oxford University, some of the world's leading experts

in the sleep-wake cycles of all life on earth

are breaking new ground with their research

by studying the neurochemistry at work.

Russell Foster is professor of circadian neuroscience

at the university.

Russell, clearly sleep is hugely important in all animals,

otherwise they wouldn't have retained it -

but how much do we know is going on at a physiological level,

inside the brain?

It's been incredible progress over the past 10, 15 years,

and I think the first point to make

is that sleep doesn't arise from a single structure within the brain -

in fact, what's turned out to be remarkable

is that there are multiple brain structures

and multiple brain chemicals, neurotransmitters, involved in sleep.

But I think we can think of three essential elements

as regulating sleep.

First is the light-dark cycle, the detection of light.

Then there's the master body clock,

telling the brain when it's an appropriate time to be awake

and when it's an appropriate time to be asleep.

And then, finally, there's the intuitive part about sleep,

which is the longer you've been awake the greater the sleep pressure,

the greater the need for sleep.

-So, there's three essential elements.

-OK, so let's break them down -

the first one, if it's about reacting to environmental cues,

to light and dark, how does that work?

So for us, if you increase the amount of light,

then we feel more alert - decrease the amount of light,

we're more likely to drop to sleep.

Of course, if it's a nocturnal animal it's the reverse.

Now, the assumption has been for years, 150 years,

that all light detection is by the classical visual system,

the rods and the cones.

The rods providing us with our sense of dim light vision

and the cones our sense of colour and of contrast.

-All channelling to the optic nerve.

-All channelling to the optic nerve.

But what we discovered is that there's a third class of light sensor

within the eye, and it's based upon the ganglion cells.

Now, the ganglion cells are actually those cells in the eye

that form the optic nerve,

and about one or two out of every hundred of those ganglion cells

are directly light-sensitive,

and are not projecting to the visual structures in the brain,

but going to the structures in the brain that are regulating sleep.

So it's nothing to do with actually being able to see or not see,

it's just about the brain registering

-there's light or there's no light.

-That's right.

In fact, that's this new understanding of the eye.

In a sense we've known forever that it's given us our sense of space,

but we've never really fully appreciated

that it also helps us give our sense of time

and has a profound influence upon our sleep and wake systems.

If we look at this model of the brain here,

we see that the optic nerves here and the eyes will be sort of here...

They're coming through this way...

They form a little bridge called the optic chiasm,

but also fairly close to it is the VLPO,

or the ventrolateral preoptic nuclei.

-We'll call it the VLPO.

-We'll call it the VLPO.

It's also known as the sleep switch,

and it's called the sleep switch because it sends projections

all the way down to the hindbrain, here,

and the hindbrain contains the structures which then feed forward

and bathe our cortex,

this, in neurotransmitters that keep us awake.

So the light-dark message goes straight to the sleep switch

or VLPO, and in diurnal animals tells the brain to sleep

when it's dark.

But if this were the only system at play,

we would all fall asleep as soon the sun sets.

Another system regulates this.

Russell has carried out an experiment

to show how it works in a mouse.

What we see here, first of all, is a mouse in the dark.

It's a nocturnal animal, and so it's wandering around.

What I'm going to do in a moment is turn the lights on,

and you'll see that within a short

time it'll actually induce sleep. The mouse will go to sleep.

So if we advance the footage, we see the lights have gone on...

-And the mouse is asleep.

-And the mouse is asleep.

As you'd expect from this light regulation system.

-This acute effect of light on sleep.

-OK.

But of course we've also got the body clock.

What we're going to do next is just keep the lights on

and just see what happens.

For how long do you keep the lights on?

Oh, we could keep the lights on for days,

-so it'll be constant light.

-OK.

So, now this mouse has been under constant light for several days.

So normally, of course, light would acutely put the mouse to sleep -

and do you see here?

The body clock is triggering activity.

So even though the lights remain on and it should go to sleep,

it will actually have periods of activity and sleep,

just as if it was under a light-dark cycle.

'The internal body clock tells each animal

'when it should be asleep, and when it should be waking up,

'fine-tuning the acute response to light and dark.'

Now, of course, because that isn't complex enough,

there is another mechanism at play - homeostatic regulation.

Talk me through that.

Well, I suppose it's the intuitive part about sleep.

The longer you've been awake,

the greater the sleep pressure that builds up.

And when you're asleep, the sleep pressure is reversed.

So what are the drivers of this system?

Well, until fairly recently, we had very little idea.

But the build-up of adenosine in the brain

seems to be part of that sleep pressure.

Now, it's a really interesting substance,

because adenosine is the breakdown product from ATP,

and ATP is the energy currency of cells.

-So it's a great marker of how long they've been active.

-A-ha.

So you're making energy, adenosine is a by-product, it builds up...

and then what happens?

Well, we know that there are adenosine receptors

in the ventrolateral preoptic nuclei, the VLPO, the sleep switch!

And so what's happening is that pressure is building

stronger and stronger on the VLPO until it gets to a critical level.

And then the VLPO is turned on and you go into sleep.

It's just got this incredible elegance and beauty,

because they're all complementing and interacting

to generate what is, after all,

the single most important behaviour we experience.

Around the zoo these three systems are driving sleep.

The gorillas went to sleep almost as soon as the sun set.

Before it went dark,

the seals all began positioning themselves on the rocks

in readiness for sleep.

And all of the animals around the zoo

will experience the adenosine system at work.

The longer they're awake, the greater the pressure to sleep.

But animals have also evolved to ensure they get sleep

when it's most suitable -

when they don't need to feed, and when it's safe.

OK, it's 10:45. Bryson, anything of interest to look at so far?

So, right here the tapirs are having a snack.

OK, that's quite typical of tapirs - they graze,

they sleep on and off all day and night.

They were super-active when we went out to see them as well, in fact.

-What about the...?

-There's the red panda, actually,

-he just came back in his nest box.

-Ahh! Excellent.

-So, they are nocturnal animals...

-Mm-hm.

Let's go over here and have a look at these other monitors.

Could you bring up the seals for us?

See, they were quite active not so long ago,

and now they do look like they've gone to sleep -

-but it's a little bit too early to tell, isn't it?

-Absolutely.

The seals are diurnal so they should be sleeping,

but they seem restless.

On previous nights, our cameras have shown

that they usually sleep by now.

Could something be keeping them awake tonight?

Since we saw them earlier, Kamran and Ketan the lion cubs

have settled, huddled together for warmth.

This is the first phase of sleep, but it won't be their last.

Animals which sleep in groups behave very differently

from solitary animals.

Around the zoo there are several large animal groups.

In the wild, flamingos are particularly vulnerable

to predators, especially at night.

We can see that our zoo flamingos spend most of their day

on the banks of their lake, but as darkness falls

they all begin to move into the relative safety of the water,

in readiness for sleep.

Now, there are many advantages to sleeping in a group

and one particularly interesting adaption

is that oftentimes you'll see individuals on the outside

of a group exhibiting a lighter form of sleep

so that the ones in the centre can get more rest.

Now, this is true of flocks of flamingos, for example,

but take a look at this - it also seems to happen with insects.

Professor Nigel Franks has been studying ants

for more than 30 years, and his discoveries

have revolutionised our understanding of ant behaviour.

His first challenge was to identify when an ant is sleeping.

You see them being utterly stationary,

to the naked eye they look as if they're dead.

They're that stationary.

But the other thing is they have a little bit of a sort of posture

about this, they pull in their antennae

and pull in their legs as if not to take up too much space,

and they'll have this particular posture.

But ants live in colonies,

and Nigel is an expert in how they work together

to optimise their inactive phases.

Nigel has come to a pine forest in Somerset

to collect ants for our study.

But finding ants in a forest is like -

well, finding ants in a forest.

The first thing we want to do, really,

is find some mature pine trees that are over a fairly open understorey,

and the reason for that is you want the sunlight to get down

to the forest floor.

I mean, it's just a case of rooting around.

Sometimes you find a nice dry patch.

A nice slug, lovely woodlice.

None of our friends, yet.

Oof! Yep, too wet.

Nigel is looking for a colony which, in its nest,

will occupy a space barely bigger than a two pence piece.

We'll try this one.

Well, got them.

Fantastic.

Nigel now needs to collect the complete ant colony,

wood and all, and transport it back to the lab.

Ants are eusocial,

the highest recognized level of animal sociability.

The life of each individual is adapted for the good of the colony.

Workers have no offspring of their own,

their sole purpose is to tend to the offspring of the queen.

And these strict social habits

are also evident in their sleep behaviours.

Once Nigel is back in the lab, he transfers the colony

to a specially prepared nest substitute,

where they can be easily observed.

Nigel then sets up a camera

for us to study the ants over the course of a day.

It's hard to see the patterns at normal speed,

but here's what we see when the footage is sped up.

The short periods of activity begin with just a few ants

and grow like a chain reaction.

At certain moments

almost every member of the colony is stationary,

and at other moments they're very much more active.

And we've found that there's a rhythm to it.

They're rhythmically active together and rhythmically inactive together.

But we don't just see synchronised rest phases,

we can also see another behaviour which the ants have developed

to improve the sleep of the colony.

When the main section of the colony is inactive,

there seems to be a group of ants on sentry duty

at the mouth of the nest.

Professor Franks thinks that these may be a special type of ant

within the group, on duty protecting the resting colony.

Well, you've got a reasonably distinct group of ants

near the entrance, and one of the things you'll notice

is that they're always looking outwards

ready to intercept foragers coming home,

so you've got this wonderful sort of reception committee in the doorway,

and that can prevent the beautiful rhythms

inside the deep heart of the colony being disrupted.

This reception committee seems to decide which ants are allowed in

during the colony's sleep phase, and which aren't.

Foraging ants which were still out of the nest

when the colony went into a sleep phase are turned away at the door,

so their return doesn't wake the main group.

Every indication is that these long periods of synchronised rest

are very important for the ants,

these periods of what we might describe as sleep.

And the reason we think that the ants are giving us ample evidence

of the importance of this

is that they seem to have gone to a great deal of trouble

to organise their societies in such a way that they can maximise

the number of individuals that can be inactive at any one time.

It really strongly implies to me

that these patterns of activity and inactivity

are really important to these societies,

and they're structured to preserve

what seems to be a sleep-like behaviour

for the good of the whole society.

One of the groups we're watching at the zoo is the flamingos.

While they're less sociable than the ants,

they still live in a flock and work together.

At night, each bird exhibits a different level of awareness.

Those on the outside of the flock will remain vigilant

for the safety of the whole group.

All right, we're taking an infrared camera into the flamingos.

SCATTERED CALLS

They are vocalising ever so slightly,

but hopefully we should see the beginnings of their behaviours

and their flock interactions that reflect who's sleeping,

who stays awake, throughout the night.

Now, during the day, the flamingos tend to stay further away

on both of the banks here in this enclosure,

and at night, slowly but surely, they all descend into the water.

The flamingos, unlike the ants,

share responsibility for sentry duty.

The birds will take turns adopting that role

so that no one bird is constantly deprived of deep sleep.

Now, the flamingos, at the moment, are still all awake.

But what's interesting is they are getting into the water

and coming closer to the deeper part.

We're going to leave them to it,

and of course our cameras are trained on them,

so we'll keep a close eye on them and see what happens later on.

The other birds we're monitoring tonight are the penguins.

In the wild they live together in a group called a waddle.

And it's difficult to observe any penguins sleeping at all.

They appear alert almost all of the time.

Very few studies have been carried out on penguin sleep.

We're going to take a closer look

at one of the individuals from this group.

For studying birds in the wild,

Bryson uses what's called an accelerometer,

a device which records the tiniest of movements.

But he's never tried one on a penguin before.

Attaching this accelerometer to an aquatic bird is a real challenge.

I don't think anyone's ever tried it before.

Because they have really small feathers, and they go in the water.

So getting the tape to stick onto that

is going to be almost impossible.

But these guys actually have wing bands,

which I think I can put tape around the wing band

and attach that to the accelerometer,

and the nice thing then

is I can see whenever they're flapping their wings at all.

So I can see if they go for a midnight swim, or jump in the water.

Or if they're just immobile all night.

It'll be nice to see what they're up to.

Bryson needs to attach the accelerometer

securely enough so that it doesn't fall off or get pecked off.

And to get the best data from the penguin,

he needs it to stay in place for 24 hours.

OK.

He is really calm.

-All right.

-All right?

Looks good.

I think that'll stay attached very nicely, it looks good.

I only put one piece of tape, but I think that's enough.

It's... Yeah, I think it should be fine.

Bryson left the penguin to go about its business

and came back the next day to retrieve the accelerometer.

And so, for the first time,

we can see exactly what a penguin gets up to overnight.

Our penguin was active, to varying degrees, for seven hours.

It had two periods of marked inactivity.

One for two hours and the other for almost three.

Penguins sleep sporadically, and it's clear from our cameras

that they don't all sleep at the same time.

As opposed to the flamingos, which adopt a very strict pattern

while sleeping, the penguins seem to adopt a more scattered approach.

With the vast majority of individuals remaining awake,

and very few individuals grabbing sleep in short bouts.

Tapirs are thought to live in small family groups in the wild.

The four tapirs at the zoo tend to sleep at the same time,

but only for short bursts.

I'm taking a look at their enclosure to see what they're up to,

but the only animal I can see is a capybara.

-WHISPERS:

-The tapirs can scare quite easily,

in fact in the wild they always rush to water for safety

whenever they feel threatened by predators.

The last thing we want to do is scare them all if they're all asleep

and feeling nice and safe indoors,

but weirdly enough it's the capybara who happens to sleep with the tapirs

who's come out for a look.

They are the most unusual looking creatures, aren't they, really?

If the tapirs are indeed asleep, which I'm sure they are,

it's definitely a good idea not to disturb them

because this is one of their phases of sleep.

We want to get an idea of how long each phase lasts,

how many there are per night,

and that's part of the results we're looking for

at the end of the evening - well, by dawn tomorrow.

So maybe it's a good idea to just sneak out of here now

before we wake them.

Gently does it.

Had we got too close, the tapirs would undoubtedly have woken.

When asleep, an animal is less responsive

and less aware of its environment,

but still needs to be ready to wake in case of danger.

This is all the more important for prey animals like the tapirs.

So how do animals ensure they are getting enough sleep,

and yet remain alert to danger?

Meerkats are notoriously wary animals.

When they're awake, they're very quick to react

to the slightest danger.

At Paradise Wildlife Park in Broxbourne, north of London,

I want to test the meerkats' reactions to threats

while they're asleep.

Meerkats are known for being an extremely vigilant species,

and in the wild, studies have shown that when they're foraging

they can recognize different predator sounds

and they can discern between different meerkat alarm calls.

But what we're interested in tonight

is whether these fellas can perceive auditory threats

when sleeping.

How deeply do they sleep,

can they maintain a certain level of vigilance,

and can they discriminate between different sounds?

We'll find out, won't we?

Meerkats may be nervous animals, but they're also extremely curious.

Oh, my God,

there's one inside my jacket going up my jumper...

So, Jessie, how many in this mob?

There's 16 in here, it's Mum, Dad and all their children.

OK, dominant female, patriarch, all the siblings -

and despite the fact that they are captive bred, and born,

are they still an extremely vigilant, observant

-type of animal here?

-Yeah, definitely.

I mean, we're in their inside enclosure at the moment,

so they're not too worried,

but when we're outside there's always at least one on sentry duty

-looking out for birds of prey.

-Just like in the wild.

Yeah, I mean, they often think that the aeroplanes that go over

are birds of prey so they'll respond to them just like they would.

So when it comes to sleeping,

this mimics as closely as possible a safe haven,

like their burrows in the wild, and what we want to find out

is whether, even in the realms of this safety,

how they might sleep, how aware they might be of external auditory cues.

So we'll have a little go at that and see what happens tonight.

-Mm, great.

-Excellent.

We've set up cameras and speakers in the meerkats' nests,

and our researcher is ready to play the sounds

while we watch what's going on from a safe distance.

While we've been gone,

the meerkat gang have been settling into their favourite nest box,

the smaller of the two.

Right, well, that is unmistakably

-the behind of a meerkat.

-Yep.

Would you say he's asleep?

Yeah, you can see how sort of slow he's breathing.

Right in front of the camera - which I'm not surprised about,

I mean, there's - what? 16 of them all huddled up in that nest box?

Yeah, 16 - that's the smaller nest box, so it is around that large.

And they do like to sleep piled on top of each other,

even in the wild to keep warm, don't they?

Yep, I mean, they've got a bigger nest box in there,

-but they always choose to sleep in the small one together, so...

-Sweet.

So, now we're going to begin our little experiment

and we're going to play two different sounds,

one is a neutral sound, the sound of wind,

and another is the sound of a meerkat alarm call

and we're going to play them at different volumes,

starting with barely audible to louder and louder.

And we've got a researcher positioned

just inside the enclosure door

and we're going to see whether there is any reaction from the meerkats.

We should be able to see it, obviously, inside the nest box

via this camera

and perhaps even just outside in the indoor enclosure,

maybe even in the outdoor enclosure if a meerkat really reacts to it

quite drastically.

OK, so I am going to speak to our researcher now.

Ross, can you hear me, over?

RADIO: 'Yep, I can hear you.'

OK, we're ready to start with the experiments,

so go ahead with the first neutral sound at the lowest setting.

'OK, wind playing now.'

The first sound level is similar to a light whisper,

but each level will get progressively louder.

WIND WHISTLES FAINTLY

-It's quite exciting, really.

-It is quite intense.

Yeah, I know, it's intense! That's the right word!

Is anyone going to shift?

No reaction whatsoever.

OK.

Let's play the first meerkat alarm call

at the same low level, thanks.

'OK, no worries.'

MEERKAT ALARM CALL

'And finished.'

Not a twitch, all fast asleep, well done.

'The nest box is the safest place the meerkats can be.

'So perhaps it's not so surprising

'that the first three volume levels don't cause them to stir.'

OK, Ross, ready for level four, the wind noise, please, thanks.

WIND NOISES AT LOUDER VOLUME

'And finished.'

No reaction, no repositioning.

OK, thanks, Ross, ready for meerkat alarm call, level four.

MEERKAT ALARM AT LOUDER VOLUME

'And finished.'

Oh, oh, oh, oh!

Ross, we have a meerkat outside the nest box,

as far as we can see from the cameras.

Just hold tight for one second.

-Result!

-This one is still fast asleep.

This one does not care, he was not on duty tonight.

That's really interesting, a meerkat left the nest box at our

level four meerkat alarm call, went to investigate, is now back.

He was only out there for a couple of seconds.

Waking up the sleepyhead!

I think it's definitely worth waiting for him to settle.

The most beautiful close-up of a very, very sleepy meerkat.

Look at that.

OK, time to play our very last set of sounds, at the highest level,

and see what happens.

WIND NOISES AT EVEN LOUDER VOLUME

And will they react to the noise?

'And finished.'

Let's give it a second.

OK, Ross, go ahead and play

the meerkat alarm call level five, thanks.

MEERKAT ALARM AT EVEN LOUDER VOLUME

Ooh, someone has left the box!

A meerkat has left the box.

Was it worth getting out of bed for? That's what they're thinking.

Was it worth getting out of bed for? Exactly! Now, that...

is interesting.

On both of our alarm calls that were loud enough for them to hear it,

a meerkat left the box,

and a meerkat did not leave the box after the wind.

Now, the wind may have disturbed them a wee bit, you know,

we can't separate the two completely, but you know, this is

definitely indicative of a very strong reaction to a foreign noise

that isn't neutral, like something they've heard before.

Coming back in to settle, heads and tails and bums

and legs everywhere, in a beautiful meerkat huddle.

The results of our little experiment show that animals have

adapted to maintain a level of vigilance which allows them

to distinguish between external stimuli, filtering sensory inputs

into those which require action and those which don't.

Therefore, at least partially reducing the risks posed by sleep.

It's also clear that not all the meerkats reacted.

During the day, there is always one meerkat

on sentry duty, the job

being shared on a rotation amongst the adults.

It could be that the meerkats maintain this system

even during sleep.

We can see from our monitors that the lion cubs are now

stirring, as we expected them to at some point during the night.

Ketan's up and about and I'm going to take the infrared camera,

to see what he gets up to.

Here he comes. Here he comes.

He's so curious.

Probably wondering what on earth

people are doing here at this time of night.

This is highly unusual.

I mean, although they can be active during the day,

at night, lions sleep for a very long time every day.

The thing is, they're carnivores. They're top predators.

They get a lot of nutrition from their food.

OK, let's go and try and see his brother.

Ooh!

Because I can't see him properly, it makes me slightly nervous.

Wow, did you see that?

'In the wild, adult lions will make use of the night to hunt,

'but here, despite the hour, Kamran and Ketan can still

'keep themselves occupied.'

Look at them, look at them playing together.

They're still big babies.

Two lion cubs playing together in the dead of night.

'Lions have adapted to catch up on sleep at any time,

'so they can maximise their opportunities to hunt.

'And we've already seen that ants guard their sleep closely.

'Flamingos work together to ensure they get their sleep,

'and meerkats seem to maintain a sentry system

'throughout the night to protect their sleep.'

We can see how important sleep is from all the behavioural

adaptations associated with it,

but also from the way animal anatomy has evolved to preserve it.

So prey animals, like zebra for example, they need to be able to run

away from predators, so like others in the equine family they've evolved

a special musculoskeletal adaptation to be able to sleep standing up.

We filmed three zebras at The Wild Place Project

on the outskirts of Bristol.

Members of the horse family, including zebra, can sleep

while standing, by using what's known as the stay apparatus.

We recorded zebras standing motionless for hours on end.

The stay apparatus is a special mechanism

that locks their shoulder and knee joints into place so that

when they sleep, their upper bodies can relax into a sort of hammock

shape, using significantly less energy than when standing normally.

It's another example of the importance of preserving sleep

in the face of danger,

and how evolution has come up with physical adaptations to allow it.

Now, this mechanism is made up of all these ligaments

and tendons that stabilise

all the joints in the leg, so that the leg is rigid

and stable, with minimal muscular activity.

Now, another animal with a great skeletal adaptation for sleep

is the sloth, and Bryson has been studying these animals for 10 years.

Bryson's research includes one of the few studies

of sleep in wild animals,

on an island off the coast of Panama.

Using the latest technology to monitor sleep patterns

in sloths, Bryson's uncovered important information

about how these mysterious creatures sleep.

Bryson, it's safe to say you know everything there is to know

about these animals. Tell me about the skeletal adaption.

So, the sloth is a strictly arboreal mammal,

it can actually stand up on its legs, but as you can see from the

skeleton here, they're perfectly adapted to hang from the trees.

Now, unlike us, if I was to hang from a tree branch,

I'd have to exert quite a bit of energy to hold on

but the sloth actually requires energy to open its claws

and let go, so it can hang here and not require any muscle.

So what is it down to,

ligaments and tendons doing the job of just locking everything to place?

Not just that, but actually the bones in their hands

and feet are designed in a way that actually allows them

to lock in on the branch and just hang there, motionless.

While the adaptations in the skeletons of the sloth

and the zebra are relatively easy to see,

the exact source of other sleep adaptations are more concealed.

So, in a moment we're going to check out the seal enclosure

to observe the zoo's only marine mammals,

but first another marine mammal, the dolphin, has evolved a very specific

and extraordinary adaptation which allows it to sleep while it swims.

Some environments are hardly conducive to sleep.

Marine mammals have their own unique problem to solve -

how to sleep without drowning.

Dolphins do have some physiological similarities to land mammals,

but they spend their entire lives in the ocean.

They have to surface to breathe, and although they can inhale and exhale

in just a third of a second and they need to breathe far less often

than a primate, typically they still need to do so between two and

four times every minute. Without a very

specific adaptation, they'd be unable to enjoy

uninterrupted periods of rest.

We visited Duisburg Zoo in Germany

to see how dolphins tackle the sleep problem.

Ulf Schonfeld has been looking after the dolphins here for 25 years.

He's seen these dolphins sleeping, but not how you might expect.

What we observe is when they are sleeping, they have one eye closed.

If the calves are sleeping,

they are swimming right underneath the mother,

have the one eye open, look at the mother all the time

and the other eye is closed.

They do this for one or two rounds

and then you see the baby is changing

the position - open the other eye and close the other eye.

To study what the dolphins get up to at night,

we set up a sensitive low-light camera to monitor them.

And here are the results.

A dolphin passes the camera with its left eye closed.

And a short while later, the same dolphin passes

in the opposite direction with its right eye open.

Jochen Reiter is Head Curator at the zoo,

he can explain some of the science behind what we're seeing.

The dolphins sleep with one eye open

and the other one is closed.

The eye that is open is opposite to the brain half

that is sleeping, so quite a curious thing.

It is what the scientists call unihemispherical sleep.

In order to remain alert enough to continue breathing

but also to be able to sleep, dolphin brains have evolved

in such a way that one half can sleep

while the other half remains awake.

Unihemispheric sleep in mammals is extremely rare.

It's thought to only occur in cetaceans like dolphins

and whales, some seals, and manatees.

There is a second point to the sleeping of dolphins,

it's like they're resting in a sort of banana posture, you could say,

with their head on the surface, maybe to left, to the right.

It's hard to tell if one eye is open or is closed,

but obviously those animals are resting,

but they're not moving forward.

During the night we filmed some instances of,

as Jochen describes it, the "banana posture".

Then later in the night, we caught something unexpected on camera.

A dolphin appears to float along the surface on its side,

go completely limp and drift down to the bottom of the pool -

only coming back to when he hits the bottom.

Bihemispheric sleep has never been unequivocally shown in dolphins.

Could this be evidence that despite the risks,

dolphins can exhibit bihemispheric sleep,

both brain halves sleeping together?

Dr Peter Evans lectures in marine mammal ecology

at the University of Bangor in Wales.

This here is a bottlenose dolphin with one eye shut,

and it's typical of unihemispheric sleep that we observe

not just in captive cetaceans, but we've observed in them in the wild.

So, what we're seeing here is the bottlenose dolphin,

relatively immobile and then it drops down

below the surface and at that point it does look

as though it may be engaged in bihemispheric sleep.

So I would say that

there's definitely unihemispheric sleep happening here.

There could be bihemispheric sleep happening for a short

period of time, a very short period of time.

If the dolphins can sleep with one half of their brain

and remain safe, why risk this

seemingly dangerous bihemispheric sleep?

The answer lies in REM, or rapid eye movement sleep,

a very special type of sleep which can only happen bihemispherically.

Until very recently it was thought that dolphins

and whales did not engage in REM sleep, because they had never been

observed carrying out anything that looked like bihemispheric sleep.

But a very recent study has observed apparent bihemispheric sleep in

sperm whales, when a pod was found motionless and unresponsive

in the Pacific Ocean, floating in a vertical position,

only waking up when a research boat accidentally

bumped into one of the whales.

There's only one species of marine mammal at Bristol Zoo.

The South American fur seals.

They sleep in the water, but of course, they can also sleep on land.

Vicky is a senior presenter here at Bristol Zoo.

Vicky, tell me about this pod, how many seals are there?

We've got six South American fur seals altogether.

We're a little bit of an unusual group - we have all males,

and it's a little family unit, so we have Otari who's the dominant

seal, he's the bull, and then we have his five sons with him.

OK. Each with individual characters, I'm sure.

Very, very different. It's brilliant to work with them.

Lovely, and tell me about the dynamics of the pod,

when they're active, when they're sleeping, how it works.

During the day, especially in the summer months,

we get the seals sleeping in the pool.

They can look like a little synchronised swimming team.

It's quite cute to see!

And on land as well,

generally evening times as we lock up the enclosure,

they all start to position up on land, ready for a night's sleep.

Would it be OK if we snuck up

ever so slowly with our infrared cameras?

-Of course, that would be brilliant.

-Thank you.

We're going to switch our cameras to infrared now,

get as close as we possibly can without

disturbing the seals, to get a look at what they're up to right now.

Hopefully, they are asleep.

OK, so we're ready to go into the seals now.

We're going to be ever so quiet.

We've got an infrared camera that's going to take the lead

and I'm going to basically use the camera as my eyes.

We're going to follow it in and see

if we can spot the seals as they sleep.

Here we go.

OK. Now, this is where it gets a little bit tricky.

Mind the step.

Now, it's pitch black and we can't see a thing.

We're going to use the monitor of the infrared camera

to see where the dominant male is.

You can hear them chuffing,

and usually that's what the dominant seals do to the more

subordinate ones, sort of to keep them in their place,

and you can hear them doing it right now.

What's interesting about fur seals' sleep is that not only do

they sleep in a unihemispheric way, like our dolphins, but they

also exhibit bihemispheric sleep when they're on land,

and it's very similar to the sleep that we see in terrestrial mammals.

So, when they're in the water, it's unihemispheric.

When they're on land, it's bihemispheric.

And some very interesting research has shown that when the seals

are sleep-deprived, they will opt for bihemispheric sleep,

and that indicates that bihemispheric sleep must have some

critical role that unihemispheric sleep simply can't achieve.

'On previous nights,

'Otari, the dominant male, has been sleeping on his favoured rock.

'But tonight, another male, Quito,

'appears to have forced him from the top spot.

'It's left the rest of the seals agitated.

'They're in and out of the water.

'It's this jostling for position that may be preventing

'the seals from going to sleep,

'but at the moment, Quito's defiantly staying put.'

The seals are clearly aware of us right now,

but it'll be very interesting to see how this dynamic shifts and changes

as they settle down for a good night's sleep later on.

Since making my way back to HQ,

apparently it's all been kicking off in the enclosure.

There's been a lot of vying for a very coveted spot on this rock

over here, and there was a lot of sparring going on with

some of the seals until Otari, the dominant male, had a proper

growl at everybody and has calmed the situation down, I think.

'But Otari is still not in his regular sleep spot.

'The tussle has clearly disturbed the sleep of the entire pod.

'But we mustn't let their fighting distract us

'from the big questions here.

'What's so important about REM sleep that the dolphins

'and the sperm whales seemingly interrupt their unihemispheric sleep

'to sleep with both halves of their brains engaged at the same time?

'And why do the seals opt for bihemispheric sleep

'when they need to?'

Let's talk about non-REM sleep first. What exactly is it?

So, non-REM sleep is non-rapid eye movement sleep and REM sleep is

rapid eye movement sleep, and those are the two main big types of sleep.

So non-REM sleep is what makes up the overall

majority of the amount of time that we spend asleep,

and humans, about 75% of the time is non-REM sleep.

OK, and what is the function of non-REM sleep?

So, non-REM sleep is thought to be the restorative kind sleep.

So when you go to sleep and you wake up

and feel refreshed, that's from slow-wave sleep or non-REM sleep.

OK, and then REM sleep, what's that like?

So, REM sleep is sort of the paradox, because generally you think

when you're asleep, your mind is at ease

and your brain is firing slower with these slow waves,

but REM sleep, you have lots of low amplitude, high frequency waves.

It's almost like your brain becomes real activated

and it looks almost like your brain's awake,

and this is when the majority of our dreams in humans occur.

And during REM sleep, there's also this muscle atonia, what is that?

So, your body basically paralyses itself during REM sleep

and that's a good thing, because that's when we're dreaming

and if your body didn't paralyse itself,

you can act out your dreams.

Has that been proven, that you can actually do that?

It's actually a disorder called REM behaviour disorder

and it's a very scary thing to have.

People have to sleep in padded rooms and sometimes strapped down

to their beds, because they'll get up in the middle of the night,

dead asleep and act out their dreams and they can really hurt themselves.

OK, interesting and worrying phenomenon.

So, during REM sleep, humans are known to dream.

The body paralyses itself so that humans don't act out

their dreams, which I think is a fascinating phenomenon in itself.

We know that some animals, some mammals and birds have

REM sleep - does that mean that animals dream as well?

Well, we're not really sure, the only way to study dreams systematically is to wake

somebody up and ask them, so with an animal it's difficult to do that.

However, we see this REM behaviour disorder in animals too, actually -

in cats, they've shown this to happen.

This is footage of a sleeping cat

demonstrating REM behaviour disorder.

It appears to be pouncing on prey.

Cats have also been known to hiss as if challenging a threat when asleep.

We're not 100% sure if they're acting out their true dreams.

But it's the same phenomenon that we see in humans as in cats.

Even in animals without REM behaviour disorder,

the paralysis during sleep can be incomplete, which is why

one of the characteristics of REM sleep is twitching.

This is most easily observable at the zoo in the larger mammals -

the lions, gorillas and tapirs.

Bryson, you and your team are looking at the evolution

of sleep. When it comes to REM sleep in particular, where are we so far?

Well, scientists have shown that we see REM sleep in both birds

and mammals, but to date, we haven't really found anywhere else

in the tree of life.

However, though, if we try to find out where REM sleep first

originated, if we trace back both mammals

and birds, the earliest ancestors

then will be 300 million years ago

and none of the species in between show REM sleep.

So, we're not really sure at what point REM sleep started.

So we know we've got REM sleep in mammals here,

we know we've got it in birds here.

Do we know for sure it's definitely not in reptiles?

We haven't seen any indication to date of it

being in any of these in-between animals.

OK, so how do we go about finding out how it

made its way into the mammal lineage and into the bird lineage?

Well, what we're trying to do now is look at the most basal forms

of these lineages to see if they also have REM sleep.

OK, and by basal forms you mean the most ancient group of birds

or mammals that are alive today?

Exactly, and in the case of birds

that would be birds like the ostriches,

so we've done some studies and look and see if they have REM sleep

as well, and we found some very interesting results.

OK, so interesting, in fact,

that we decided to recreate it in a farm in Bristol.

As the most primitive living bird of its group,

the ostrich has remained virtually unchanged for

50 million years, which in the study of evolutionary biology

makes it very interesting.

So, we've come to St Swithun's Lodge on a beautiful winter's day

to meet Paul Cook here.

You've been keeping ostriches for over 20 years, is that right?

-At least 20 years.

-And how did you get into it?

I was at the Royal Welsh Show in Malvern

and decided that they looked nice in the back garden, as you do!

So where do they sleep?

-They've got a house just down here.

-In that shed?

-Yeah, in the shed.

-So, is it OK if we set that place up

-full of CCTV cameras...

-Wow.

..so we can have a proper look at the changes in their behaviour

as they're sleeping? You can have a proper look as well. Yeah?

Be the first time I've managed to get that close without waking them up.

'For us to have a real look at what's going on with Paul's ostriches,

'we've brought along an array of infrared cameras

'to set up in his barn.'

-Mind the step.

-Thanks.

-And the second one.

OK, so this is it. The sleeping quarters.

So, we need to figure out where to put our cameras.

-We've got four. Is that right, Bryson?

-Yeah, exactly.

It's easy to see how close a resemblance these ostriches

bear to their dinosaur ancestors, in comparison to the more highly

evolved birds that came after them.

For one thing, they are very big animals.

They can be as tall as 3m in height

and they can weigh up to 150kg,

and if you get a look at their feet...

I mean, they're enormous as well and they look suspiciously

like those of the theropod dinosaurs that they evolved from.

With all the cameras in place and our monitors set up in the kitchen,

Paul can now bring the ostriches into the barn.

So, in ostriches, there's two types of sleep -

there's slow-wave sleep and REM sleep.

And when they're in slow-wave sleep, they're actually

sitting there with their head motionless, but their eyes

are open as if they're awake, but they're actually in slow-wave sleep.

Now, during REM sleep, though, their eyes are going to close

and they're going to lose muscle tone, and so, their head

will start to droop down a little bit and sometimes it will actually hit the ground.

So here, OK... If we watch her for a second, I think she might be getting ready

to transition from slow-wave sleep into REM sleep.

Almost goes to like looking slightly drunk, doesn't it?

Exactly, here she goes, right there. So, now she's starting to go into

REM sleep, you see her eyes were

just starting to close a second ago, her head's starting to bob around.

So, she's most likely in REM sleep right now

and this is what they do. And then up, so the eye opens up

and she goes back probably into slow-wave sleep again, and they can

transition in and out of REM sleep and slow-wave sleep many times.

Ostriches are one of the only species in which

we can see this moment of transition between non-REM and REM sleep.

Many animals can be observed twitching or exhibiting

rapid eye movement while in REM sleep,

but the transition between REM and non-REM

is nowhere clearer than in ostriches.

Scientists have now found that REM sleep in ostriches is in fact

rather different to REM sleep in humans.

When the ostrich eyes are closed, their brain is

oscillating between both REM and non-REM.

In a human, when you go to sleep, you either are in REM sleep or

slow-wave sleep, there's no in between. Your brain switches,

and it's a global phenomenon, it's your whole brain doing it.

But in the ostrich, they actually have a mixture,

and the only other animal this has ever been found in is

the platypus, which is also a very basal, archaic species.

So, what we're seeing here is maybe the earliest form of REM sleep.

It's possible that this combined sleep state where non-REM

and REM are not clearly defined was a precursor to

the sort of distinct sleep phases we see in mammals and birds today.

So, on the evolutionary tree in the species that came before the

ostrich and before the platypus, it's likely there was no REM sleep,

and in the species that evolved from the ostrich

and from the platypus, REM developed into its own separate sleep state.

So it seems that ostriches represent a very important junction point

in the evolution of birds, where the two separate stages

of sleep, non-REM and REM, hadn't quite separated yet.

Now, what's really interesting is this same pattern

of REM development seems to have

occurred in a completely different lineage, this time in the mammals,

with the junction point here being represented

by the basal mammal, the platypus.

Now, if REM development has occurred in two separate lineages like this,

this implies convergent evolution, a trait that has evolved in two

different groups, completely independently of each other,

and that adds to the evidence that REM sleep must be hugely

important for these two groups to have evolved it independently.

Now, the very latest research also shows that REM sleep may have

evolved again in another lineage, this time in the cephalopods,

the groups of octopuses and squids.

Studying sleep in cephalopods is a very new line

of research in animal sleep science.

REM-like behaviours have been observed in cuttlefish,

which include distinct sleep phases,

changes in colour not related to camouflage function

and thought to be synchronised with REM,

rapid eye movements and twitching.

I want to know if other members of the cephalopod family

may show the same behaviour.

Bristol Aquarium is home to a very special cephalopod,

a giant Pacific octopus called Priscilla,

and the staff here are convinced she dreams.

Paul, when it comes to their personalities,

because you've worked with different octopuses in your career,

-they all have their own characters, would you say?

-I would say so, yeah.

Each octopus has its own personality.

Our lovely girl here is a bit of a diva.

She does like to think that she rules the aquarium,

but you get octopus that are a little bit shy

and tentative to start off with and you do get ones that just

want to play, they want to interact, they love human interaction.

OK, and they do have a reputation for being incredibly intelligent

invertebrates, tell me a little bit about her.

Well, we reckon she's just as intelligent

as a three-year-old child. They love to play,

they can be taught to do lots of different things.

She is actually able to open jars without any issues.

OK, wonderful, so, have you seen Priscilla sleep?

We believe so.

She does like to sit in certain places in the tank,

she will colour-change quite happily.

We can just see these tiny movements around the tips of her arms

as well, which would suggest that she's dreaming of some sort.