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Do sharks sleep?

Science communicator
Hammerhead sharks schooling at Darwin Island. Photo © Pelayo Salinas.

The first sharks evolved around 450 million years ago, that’s before dinosaurs began roaming the earth a mere 230 million years ago and even before trees started growing around 350 million years ago. Over the course of time, and due to millions of years of evolution, sharks have developed some pretty neat adaptations to ensure that the species survives. There are more than 440 species of shark, and one of their adaptations is that they seemingly don’t sleep.

Some sharks rest deeply for periods, but can this be considered as sleep? Rest is defined by scientists as ‘reduced activity without loss of consciousness or greatly reduced responsiveness’. Sleep on the other hand, or should I say the other fin, is defined as a decreased awareness of surroundings or a ‘reduced state of consciousness and the lack of movement or physical activity that can be quickly reversed’. Lost sleep leads to an increased longing for sleep or the need for more sleep known as a ‘sleep rebound’, while lost rest doesn’t. Sharks have periods of deep rest that do at least resemble sleep. However, shark scientists don’t know enough about them yet to prove whether or not sharks actually sleep because they don’t know if they sleep rebound or not.

A shark in deep rest may or may not look asleep to you. For one, they don’t shut their eyes when they rest, well at least not all the way; there’s at least one species that partially closes its eyes when it rests – keep reading to find out which one of the 440 species of sharks it is. Different sharks breathe in different ways, and this influences how they rest. Some species are thought to ‘sleep swim’, others lie stationary on the seafloor, and some snuggle up in groups or someplace out of sight where it’s safe to rest such as in a cave or in the sand.

Shut-eye

Sharks don’t shut their eyes like you and me when they’re in deep rest periods or, what might at least resemble, sleeping. They have an upper and lower eyelid like us but they can’t close them, or at least not fully. However, the majority of sharks have third eyelids, which are called nictitating eyelids and don’t serve the same purpose as our eyelids. Their nictitating eyelids are transparent and do what safety goggles do for us – they protect their eyes while still allowing them to see – unlike our eyelids which keep our eyes moist and block out light while we sleep.

Sharks have one nictitating eyelid per eye. These move in from the inner corners of their eyes to the outer corners or from the bottom of their eyes to the top. Just before sharks bite, these nictitating eyelids slide up over their eyeballs to protect their eyes from being scratched or poked by their squirming prey. Wouldn’t these nictitating membranes come in handy for protecting a great white’s eyes while hunting seals? Surprisingly, this is not the case – great whites are one of the very few species of shark that don’t have nictitating eyelids at all! Instead, just before sinking their teeth into seals, these highly evolved hunters roll their eyes back into their skulls, essentially biting their prey completely blind. Despite some sharks being blind when they bite, they’re not blind when they’re resting. A resting shark’s eyes are open, or at least partially open, to monitor what’s swimming around them.

A Caribbean reef shark with its nictitating membrane half closed. Photo by Shin Arunrugstichai |  © Save Our Seas.
A Caribbean reef shark with its nictitating membrane half closed. Photo by Shin Arunrugstichai | © Save Our Seas.

Do sharks need to keep swimming to stay alive?

To stay alive sharks must keep oxygen-rich water flowing over their gills. Oxygen in the incoming water passes through their gills and is absorbed into their blood. The blood is then pumped around their body supplying all parts with oxygen. Sharks are far better than humans at absorbing oxygen – they can absorb 80% of the oxygen if there’s just 1% of oxygen in the water around them. We, humans, absorb 25% of the 21% of oxygen that’s around us in the air we breathe. Some sharks can breathe (by which I mean keep water flowing over their gills) while lying still on the seafloor or even buried in the sand. Others need to keep swimming to breathe and stay alive – keep reading to find out how and why different species of sharks breathe the way they do.

Just keep swimming

Sharks that have to keep swimming to stay alive need oxygen-rich water to flow in through their mouths and be continually ‘rammed’ over their gills in a type of breathing known as ram ventilation. Sharks that only use ram ventilation to breathe include those species that most people typically think of when they think of sharks: great white sharks, bull sharks and shortfin mako sharks. Researchers originally thought these sharks had to keep moving to stay alive because they didn’t show obvious periods of inactivity and thus didn’t appear to rest or sleep. Some shark scientists thought that because the open ocean is so empty, these sharks’ brains are not sufficiently taxed to need sleep. However, this idea has been dismissed as unlikely because even in the open ocean sharks use complex visual, navigation, predation, and communication skills. Because ram ventilating sharks would drown if they stopped swimming and ramming water over their gills, it’s thought that part of their brain stays active to keep them swimming while they rest. Could this be ‘sleep swimming’?

Great sleep swimmers?

Some great whites have been tracked and shown to remain for hours in the same spots in channels. Shark scientists presume they’re resting, facing into the currents running through these channels with their mouths open and letting water flow in over their gills. One great white has been filmed doing just that – her slow swimming and open mouth supposedly enabling her to keep oxygen-rich water moving over her gills while she rests.

While we aren’t yet sure if this is sleep swimming, we do know that sharks using only ram ventilation to breathe, such as great whites, evolved to be more active cruisers of the open ocean and lost the ability and structures to actively pump water over their gills while not moving forward.

Some sharks have the cheek to stay stationary.

Sharks that pump water over their gills using their cheek and neck, otherwise known as ‘buccal’ muscles, breathe by means of ‘buccal pumping’. This way of breathing was used by the ancient sharks that first evolved and cruised the oceans 450 million years ago, and many shark species still use buccal pumping today. They include Port Jackson sharks, leopard sharks and zebra sharks. Species that use buccal pumping can spend their time resting, or dare I say snoozing, on the seafloor.

Sharks that sleep all day and swim all night

In the wild and aquariums, some sharks that breathe using buccal pumping are often seen lying still on the sand during the day and actively swimming around at night. Including nursehound sharks and nurse sharks in aquariums, they spend most of their days lying still at the bottom of the tank in a relaxed posture. Nursehound sharks’ eyes are even slightly closed while they appear to be resting. Nurse sharks are less responsive when ‘resting’ and don’t react to scuba divers at all until touched by them, which makes the sharks swim away. Wild nurse sharks are typically docile and not known to be aggressive towards humans but have bitten scuba divers that startled them while they were ‘resting’. None of the bites caused fatalities and were likely done in self-defence following a fright. It seems that while these sharks were ‘resting’ they had decreased awareness of their surroundings and lacked movement that could quickly be reversed, which fits the definition of sleep, right? Well, the definition of sleep has a second part to it: if sleep is lost it leads to more sleep later, known as ‘sleep rebound’. This is the main way in which sleep differs from rest because lost rest doesn’t result in more rest later.

It’s tricky to study shark sleep in the wild because sharks are generally shy and elusive and in aquariums they don’t behave in the same way. For example, at night whitetip reef sharks swim three times more in the wild than they do in aquariums. It’s tricky to pinpoint why sharks in aquariums behave differently, but maybe it has to do with the way they’re fed or how the tank is set up. For these reasons, it’s difficult to ascertain whether sharks sleep rebound.

Snoozing in snuggle piles

Nurse sharks often spend their days resting in groups of up to as many as 40 sharks in the wild. Some appear to be cuddling and snuggling while they rest. Shark scientists think resting together may help protect them from predators, including formidable bull sharks.

A hammerhead shark swims over a group of resting nurse sharks. Photo © Chris Vaughan-Jones.
A hammerhead shark swims over a group of resting nurse sharks. Photo © Chris Vaughan-Jones.

Sharks that snuggle in sand breathe through ‘snorkels’.

Sharks, and some other species, that bury themselves in sand can breathe through spiracles. Spiracles are openings just behind the creatures’ eyes that function like snorkels. Oxygen-rich water is sucked in through these spiracles (sometimes from above the sand) and pumped through the gills and out the gill slits. Sharks that have spiracles include serrated snouted sawsharks, flattened angelsharks and furry-faced wobbegongs. These sharks could be snoozing while they lie there breathing through their snorkels, we just don’t know for sure.

Mixing breathing methods.

Many species of shark take in oxygen-rich water through spiracles and use buccal pumping, while other more active species use buccal pumping and ram ventilation. Ragged-tooth sharks can switch between buccal pumping and ram ventilation depending on how fast they’re swimming – you can see these sharks in the Save Our Seas Foundation Shark Exhibit at the Two Oceans Aquarium in Cape Town South Africa and in many other aquariums around the world. Another species that switches between these two types of breathing is the mighty tiger shark.

Sharks that break the rules

Ram ventilating sharks, which usually need to keep moving to keep water moving over their gills, can even survive staying stationary if conditions are just right. Caribbean reef sharks, which typically only use ram ventilation, have been found motionless ‘resting’ in caves in Mexico, baffling shark scientists. Scientists later discovered that the water in the caves had extremely high levels of oxygen and reduced salinity. These conditions allowed the sharks to keep breathing without moving because the conditions allowed for easier absorption of oxygen from the water around them.

Whitetip reef sharks resting in a cave. Photo © James Lea.
Whitetip reef sharks resting in a cave. Photo © James Lea.

Yo-yo'ing sharks

Other ram ventilators that travel long distances through the open ocean such as great whites, blue sharks, shortfin mako sharks and even whale sharks probably rest while ‘yoyo swimming’. What’s yoyo swimming? It’s when sharks actively swim towards the surface and then rest as they make a gliding descent back to the depths. This technique of zig-zagging up and down enables ram ventilating species to have water flowing over their gills as they rest on the way down, and to cover extremely long distances efficiently.

To wrap up:

  • Some sharks have rest periods, but whether they’re really asleep while they’re resting is still a mystery.

  • Shark scientists define rest as ‘reduced activity without loss of consciousness or greatly reduced responsiveness’ while sleep is ‘reduced activity and a reduced state of consciousness that can be quickly reversed’. Lost sleep leads to an increased need for sleep or a ‘sleep rebound’, while lost rest doesn’t.

  • Some sharks definitely rest and this can at least superficially resemble sleep. However, to solve the mystery of whether sharks sleep scientists will have to work out if sharks make up for lost sleep. This is tricky because in the wild sharks are elusive and in captivity they behave differently from how they do in the wild.

  • Sharks don’t shut their eyes while resting, as we do, and how they rest depends on which type of breathing they use.

  • For sharks, breathing entails keeping oxygen-rich water moving over their gills.

  • Some sharks must keep swimming to keep breathing to stay alive, but others can breathe while stationary or even buried in the sand.

  • Despite decades of research on sharks, scientists still haven’t been able to prove if sharks sleep or not.

Frequently Asked Questions:

Do sharks sleep?

Shark scientists aren’t sure if sharks really sleep, but some have periods of deep rest that at least superficially look like sleep. To find out if sharks sleep or not shark scientists will need to work out if sharks that lose out on rest need to rest for longer the next time they rest. If they do it will mean that sharks sleep.

Can sharks shut their eyes?

Sharks have eyelids like you and me but they can’t close them all the way, if at all. Most sharks also have transparent nictitating eyelids called third eyelids that slide over their eyes just before they bite. These eyelids protect their eyes while still allowing them to see. During rest, sharks’ eyes are open to monitor things moving in their environment.

Must sharks keep swimming to stay alive?

Not all species of shark need to swim constantly to stay alive. Sharks that only use ram ventilation to breathe do need to keep swimming to keep oxygen-rich water flowing over their gills and stay alive, but species that use buccal pumping and breathe through spiracles don’t need to keep swimming to stay alive.

How do sharks breathe?

Sharks breathe by keeping oxygen-rich water moving over their gills. They extract oxygen from the water moving over their gills into their blood. There are three ways they keep water moving over their gills:

  1. Ram ventilation Sharks swim constantly so that oxygen-rich water is continually rammed over their gills. Under most ocean conditions if they stop swimming they’ll drown.

  2. Buccal pumping Sharks use their cheeks to pump water over their gills. This allows them to stop swimming without drowning.

  3. Spiracles Sharks have openings called spiracles behind their eyes which they use to pump water over their gills. Their spiracles essentially act as snorkels.

References

Lesku, John A., et al. [‘Evolution of sleep and adaptive sleeplessness.’] (https://www.sciencedirect.com/science/article/pii/B9780128137437000207) Handbook of Behavioral Neuroscience 30 (2019): 299-316.

Nelson, Donald R., and Richard H. Johnson. ‘Diel activity rhythms in the nocturnal, bottom-dwelling sharks, Heterodontus francisci and Cephaloscyllium ventriosum.’ Copeia (1970): 732-739.

Kelly, Michael L., et al. ‘Evidence for sleep in sharks and rays: behavioural, physiological, and evolutionary considerations.’ Brain, behavior and evolution 94.1-4 (2019): 37-50.

Siegel, Jerome M. ‘Do all animals sleep?’ Trends in neurosciences 31.4 (2008): 208-213.

Gleiss, Adrian C., Brad Norman, and Rory P. Wilson. ‘Moved by that sinking feeling: variable diving geometry underlies movement strategies in whale sharks.’ Functional Ecology 25.3 (2011): 595-607.

Kelly, Michael L., et al. ‘Diverse activity rhythms in sharks (Elasmobranchii).’ Journal of Biological Rhythms 35.5 (2020): 476-488.

Klimley, A. Peter, et al. ‘Movements and swimming behavior of three species of sharks in La Jolla Canyon, California.’ Environmental biology of fishes 63.2 (2002): 117-135.

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