Ever had a bad night’s sleep? Imagine trying  to catch some zzzz’s while floating in the middle of the ocean! Or even worse, sinking  to the bottom of the sea. What a nightmare!

But it does make you wonder,  how do animals sleep underwater!

Some oxygen breathers spookily float close  to the surface. Some others only sleep half a brain at a time. And some genius invertebrates  have nightmares that you can see in their skin.

Let’s get into the amazing adaptations  of the animals who sleep underwater.

Hi, I’m Danielle Dufault and you’re watching  Animalogic. Today we’re investigating how marine animals sleep. How do they rest in an  environment that requires constant movement?

But before we get into the specific, let’s take a step back to understand  the basics of sleep in the first place.

Even though sleep may feel like a simple process,  there’s a lot more to it than shutting your eyes and drifting off into the darkness. There are several stages of sleep that make up a ‘sleep cycle’ someone  goes through during the night.

Your brain behaves differently  during each of these stages.

Scientists study them by attaching  electrodes to the head and watching brain activity while a person sleeps. This is called an electroencephalogram, or EEG.

We’ll get back to EEGs in a bit. Without getting too far into the weeds of neuroscience, there are two  kinds of sleep: “REM” and non-REM sleep.

When you start drifting off,  you begin in non-REM sleep, moving into deeper and deeper stages of rest. After your brain has had enough rest time, it shifts into REM sleep, the phase with  Rapid Eye Movement. This is when we dream.

Our muscles usually freeze  during this part of rest, which prevents us from acting out our dreams. Throughout the night, your brain will cycle through several REM and non-REM stages. Both are  essential for the brain and body, and without them, humans, and many other animals, would die! Many aspects of sleep still remain a mystery to scientists, but one thing is for  certain: sleep is critical for life.

Knowing this, it becomes even more important to  ask: how the heck do marine animals sleep at all?

Let’s start with our closest aquatic  relatives: cetaceans like whales and dolphins.

These animals have a unique sleeping adaptation.  They put half of their brain to sleep, while the other half is still awake. Cetaceans have little to no REM sleep, which is weird because without it we have issues  with memory, mood, learning, and even increased risk for certain diseases like dementia. Some scientists think that sleeping with one side of the brain at a time may  help to compensate for this lack of REM.

Cetacean sleep may also simply be more efficient,  doing the work of REM without having to be in it!

Dolphins will spend about 8 hours per day in this  partially awake, partially asleep state, spending half the time resting each hemisphere in 2-hour  long sleep ‘shifts’. Talk about multitasking!

But, why? Obviously, being an air-breather but sleeping underwater  isn’t the best combination, and so these animals have had to figure out how not to inhale water  and how to surface when needed while sleeping.

Hanging out right at the surface of the  water can make you a prime target for large ocean predators, so they have to stay  a little deeper where it’s darker and safer.

So, this kind of sleep gives them the opportunity  to get some rest, while also being able to balance these other, very important, factors. You might be wondering what these animals look like while in this state. Marine mammals don’t  actually need to be totally still while sleeping.

They usually just swim slowly or use basic  movements of their flippers and fins to keep themselves warm and their bodies stabilized. They also sleep with one eye open, to remain vigilant while also  giving half of their brain a break.

While the thought of a human sleeping with one eye  open sounds like something out of a horror movie, for these creatures, it’s nothing but a day’s  work…or rest… in the marine animal kingdom.

Even though most research in marine mammals has  been focused on dolphins, researchers have also discovered unique patterns of sleep behaviour  across other Cetacean species. For example, sperm whales show an unusual and wildly cool behaviour  while sleeping: floating vertically in the water!

These whales will float in a vertical position,  nose-up, near the surface of water, coming up to breathe for air when needed before going down  again to drift in their vertical positions.

Why do some Cetaceans remain moving,  while others just ‘chill out?’ Larger whales don’t have to worry so  much about being dragged by currents, being attacked by predators, or losing  warmth. So, they can park themselves.

Smaller cetaceans worry about all of those  factors, so it’s safer for them to always be on the move, even while sleeping. While animals like whales and dolphins have evolved their approach to sleeping with  half a brain, what about animals that spend some time in the water, and some time on  land? Pinnipeds like seals, sea lions, and walruses have evolved a sort of hybrid sleep.  For example, seal migrations can include up to 10 months at sea before returning to land. When the seals are on land, they sleep pretty much the same as us, with  cycles of rem and non-rem sleep.

Like this little guy, who seems like he’s  about to have the sleep of a lifetime. Lucky!

But when they’re in the water,  they can do unihemispheric sleep, like whales and dolphins, with some distinct  features like short bursts of REM sleep.

You might be wondering how we know  all this cool info about seal sleep, while they’re far out of reach in the open  ocean. Over the past few years, scientists developed a harmless neoprene headcap - complete  with EEG electrodes - that could go over a seal’s head and stay with them while they travelled. When the animals returned to the beach months later, the researchers collected  the caps and analysed the data.

Seals have also adapted their own unique  way of positioning themselves in the water, using flippers to keep their heads above the  surface for breathing, while the majority of their body remains submerged. In walruses,  although most sleep occurs on land or ice, an amazing adaptation has been discovered that  allows them to also be able to sleep at sea.

Kind of similar to the floating position  of sperm whales, walruses can sleep while floating using special air pouches in  the neck area to keep them buoyant.

They can also use their tusks  to anchor themselves on the ice.

And because they’re so big, strong, and thick-skinned predators don’t  seem to bother them too much.

Clearly, marine mammals have some pretty  cool sleep features that differ from humans.

But what about species that are farther  away from us on the evolutionary tree?

Enter in some of the most iconic creatures  of the watery world: fish and sharks.

Fish have to worry about predators at all  times, so their sleep has had to adapt.

Even though they don’t breathe air, many  fish species need to move constantly to get oxygen through their gills. This  makes their sleep a bit complicated.

When thinking about bony fish, it’s hard to  provide a one-size-fits all description of sleep.

These fish species can range from  household goldfish in a tank, to clownfish on a coral reef, to piranhas  in the Amazon, and everything in between.

Fish have adapted several different sleep  strategies, but we're only beginning to understand them because until recently, many scientists  believed that their brains didn't need sleep.

Luckily, more recent research has started to shine  a light on sleep in fish, which takes into account broader ways that ‘sleep’ can exist. One of the most studied species for sleep is the zebrafish. This species shows patterns of activity and rest throughout the day, following  light and darkness in their environment.

Their sleep is similar to ours in that it  seems to have slower-wave and REM-like phases.

One big difference is the fact that even though  their total sleep may be hours during the night, they sleep in short ‘bouts’ of seconds to minutes.  Wondering if scientists have little caps similar to seals to study fish sleep? Nope! But an equally  ingenious set-up has given us the answers we want.

Larval zebrafish are safely  immobilized in a jelly-like material and then are given full-body scans. The fish are genetically modified to have fluorescent cells that glow  when the neurons are active.

Then, eye movement, heart rate,  and brain activity are measured.

These are the kinds of studies that have shown  the human-like sleep patterns in these fish.

Unsurprisingly, nature has taken advantage of  reduced responsiveness in fish during sleep, leading to both helpful and harmful  cross-species relationships. For instance, cleaner shrimp will act as living garbage  collectors during the night, eating dead scales and debris from the surface of a fish’s skin. But fish are hungry and it's risky for the shrimp to do its job while the fish  is awake. So, it hides if the fish is active, and comes out when it’s sleeping. It’s like the tooth fairy, but instead of taking your teeth, it just takes away your dead scales. We don’t have any EEG data from cleaner shrimps, but all the evidence points to them also  having time for sleep-like rest at some point during the day. Coming back to fish, we have talked about how they rest, but how do  they protect themselves when they’re napping?

Parrotfish have developed the ability  to secrete a cocoon of mucus around them - kind of like a slimy sleeping bag. These cocoons take about half an hour to make each night, and help to protect from  larger predators as well as parasites.

No predator wants a mouthful of slime. What does a sleeping fish look like? It really depends on the species,  and the habitat they are in.

For those living in places with nooks and crannies  like a coral reef, these hiding spots can be the perfect place for a napping fish to stay safe. Once asleep, fish will become nearly still, with only slight movements to help them  stay floating. Luckily, swim bladders, which are basically internal floaties,  keep them from sinking to the bottom.

Fish don’t have eyelids, so they can’t actually  ‘close their eyes’ to sleep. But their eyes do move differently. Their pupils have  a slower response and can no longer tracking movements in their environment. In other cases, fish have been found to sleep while floating with their heads pointed  downward; they're like the anti-sperm whale.

Some others sleep upwards at the surface… And others simply rest at the bottom of their environment. It’s not fully clear which species take the different positions,  or if they even do this at all in the wild.

That’s because these sleeping positions have  mostly been seen in the lab. A floating fish would be an easy target for hungry predators! More  work needs to be done to know for certain all the creative positions fish might take while sleeping. For example, how do sharks do it?

Remember what we said about needing water to  flow over a fish’s gills to keep them alive?

Bony fish, and some types of cartilaginous  species do “buccal pumping,” which is simply sucking in oxygenated water and  exhaling it through their gills.

This is useful because it means they  can stay still for long periods of time, and get all the oxygen they need from the water. But then there are sharks who can't suck in water, so they have to move forwards to  oxygenate their gills. How do they sleep?

Shark sleep is actually one of the greatest  mysteries in fish biology. Some species seem to slow down throughout the day, and others don’t  seem to have much change in their activity at all!

Scientists are still uncertain about what  on earth is going on in their soft heads.

Is it possible that they don’t even sleep at  all? This is unlikely, especially for creatures with complex brains like sharks or rays. Or maybe they sleep half brain at a time, like dolphins? There’s no evidence that they  can do this, but it seems that some can put smaller parts of their brain into sleep mode. Some species could also use ocean currents to keep water flowing over their gills while they rest. Scientists continue to try and crack this sleep mystery. This mostly involves observations in  their natural habitat, looking at changes in things like movement patterns to figure  out what sleep looks like in the wild.

If we don’t know this, you might wonder how  much we can know about invertebrate sleep, like octopuses and jellyfish. Surprisingly,  a lot! Another spoiler: their sleep actually closely resembles ours! These animals even have their own versions of REM and non-REM sleep. When in their version of REM sleep, octopuses have beautiful changes in skin patterns and rapid eye  movements - and maybe even dreams… or nightmares!

Some researchers have noticed octopuses  behaving like they are under predatory attack during this stage of sleep. Sort of  like sleep-walking… or… is that sleep-swimming?

Although it may appear that these sleep patterns  have a common evolutionary origin with humans, our most recent common ancestor with these  creatures was hundreds of millions of years ago, and only had a very simple nervous system. That means that sleep patterns actually appeared independently, likely because  of similar pressures in the environment.

This makes sense, given how common  sleep is across the animal kingdom!

It’s hard to say how many times sleep evolved  in different animal groups, but as far as we know all of our mammal cousins need to sleep. And speaking of how broadly sleep is observed, we can’t forget about brainless animals.  Yes, you guessed it, sleep-like states exist in creatures like the jellyfish. Even though jellyfish and their relatives only have nerve nets rather than true  brains, they still have rest states that could be considered ‘sleep.’ When jellyfish are deprived of this rest time, they show less activity and  responsiveness the next day - much like you feel if you stay up too late yourself! The finding of sleep-like states even before central nervous systems evolved is an  amazing discovery, pointing to sleep as something so vital for survival that it arose  extremely early in the development of life on earth. What a great excuse to hit the snooze  button next time your morning alarm goes off!

As you can see from this deep dive into the  sleep of marine animals, rest in aquatic worlds is truly an evolutionary wonder. From our closest  marine relatives to creatures that don’t even have brains, sleep has found a way to prevail. It’s a great reminder not only about how important sleep is, but also that we  have a lot more in common with sea cre atures than might first meet the eye. So what should we talk about next? Let me know in the comments below and be sure to  subscribe for new videos every single week!