Do sharks sleep?
Science communicatorIn our oceans, dolphins “swim-sleep”, keeping one eye open and experiencing slower brainwave function in certain brain hemispheres . Sperm whales hang suspended like inverted tear drops in the water column, sleeping with one side of their brain at a time during so-called shallow “drift dives” . But if there’s one group of animals that has really kept scientists scratching their heads, it’s seemingly sleepless sharks. Predators in perpetual motion through the pelagic, they seem almost to defy biology. We’ve been told that most sharks need to keep moving to stay alive and keep breathing, which means – can they stop to sleep? And have we ever seen sharks close their eyes, which would be another way we generally notice sleep behaviour? Emerging evidence shows that some sharks do indeed sleep; perhaps not in a way that would match the recommendations from a TEDTalk, but in different ways that have allowed the over 500 species to persist for over 450 million years on our planet. For many species, conjecture as to whether they really sleep and hypotheses about how this is achieved are currently all that exist.
What Is Sleep?
Scratch around for a definition, and one distillation that essentially emerges is that sleep is a period of motionlessness that can be reversed. Startle a sleeping dog lying blissfully on a lawn in the sun, and it leaps to its feet. Prod your snoozing colleague slumped momentarily in their chair, and they stammer into wakefulness again. Furthermore, it’s a period where your senses are dulled; your awareness of your environment is reduced and it takes a while to hear etc, as anyone who has fumbled through the dark has discovered after the nagging shriek of an early-morning alarm clock. More importantly, sleep is what scientists term “homeostatically regulated”; that is, lose sleep, and the drive to make up for it increases. Sleep is something we can observe through behaviour, but it’s also a brain-state. Mammals experience non-REM sleep with very reduced brainstem activity and REM-sleep where brain neurons fire at rates equal to, or even exceeding, periods of wakefulness . How we sleep is variable, and complicated. You can stare vacantly at a lecture in progress, and yet not be asleep. You can be roused by the softest mewling of a baby in the dead of night, but the hum of a vacuum cleaner or continuous flow of traffic outside keeps you in a sound sleep. Measuring when, how and where sleep happens in the oceans according to these behavioural and physiological definitions is challenging, and so much of the science of sleep for sharks is continuously emerging.
Do Sharks Sleep?
In short, yes, it appears some do. As scientist Michael Kelly and co-authors write , most animals sleep and evidence for sleep in animals as diverse as jellyfish and flatworms suggests that sleep behaviour emerged early in the evolutionary tree. That makes it likely that sharks sleep too. There is published, preliminary evidence for sleep in sharks that can breathe without swimming continuously (Kelly and his colleagues found 80 scientific papers that detailed sleep behaviour in these sharks and bony fishes ). The question is, do all sharks sleep – and in the same way? The science of sleep is still emerging; of the over 500 known shark species, only 30 have been documented showing sleep behaviour. Of these species, only the nurse shark - and as Kelly and his colleagues showed, draughtsboard and Port Jackson sharks, have been shown to fulfil multiple criteria that would suggest sleep behaviour. Conclusive evidence for sleep in actively swimming sharks remains absent, but several hypotheses exist as to how they might do so.
How Do Sharks Breathe?
Understanding how sharks breathe is important to deciphering how they sleep. All sharks breathe by passing oxygen-rich seawater over their gills. However, there are two different ways of doing this. Some sharks swim perpetually, “ramming” seawater rich in oxygen over their gills (“ram ventilators”). These are active, often pelagic species - think of white sharks, hammerhead species and mako sharks, for example. Other species pump seawater over their gills while stationary (“buccal pumpers”) ; to do this, they change the volume of their buccal cavity (basically, their mouth space). These are often benthic species – think of wobbegongs and Port Jackson sharks. Ragged tooth and tiger sharks do both, switching between ram ventilation and buccal pumping, depending on how fast they’re swimming.
How does the evidence for sleep in sharks fare when stacked up against the definitions of sleep? Well, there appears to be a fair amount of research available to confirm sleep in buccal-pumping shark species that do not have to keep swimming to breathe:
- It might be sleep if … animals are immobile or have relaxed postures and reduced environmental awareness. Lemon sharks come to lie stationary on the seafloor, white tip reef sharks gather in caves where they lie motionless, and nurse sharks documented in captivity stayed immobile for much of the day.
- It might be sleep if … animals have closed eyes. A captive nursehound shark has been documented with eyes half-closed during periods of restful behaviour, and draughtsboard sharks were shown to close their eyes during periods of sleep, whereas their eyes were open during periods of rest.
- It might be sleep if … animals show circadian regulation. A host of sharks have been shown to be active during the night. Swell and horn sharks, small-spotted catsharks, Port Jackson, zebra, Pacific angel, Caribbean reef, and white tip reef sharks are all nocturnally active species. In one study, scientists compared the duration, amount and distance travelled by three ram ventilating sharks and two buccal pumping sharks. The school shark, spotted estuary smooth-hound shark and spiny dogfish (ram ventilators) all swam continuously through day and night, but the smooth-hound swam slower and a shorter distance at night. The spiny dogfish behaved similarly but swam the shortest distance overall. The Port Jackson and draughtsboard sharks (buccal pumpers) were nocturnal.
- It might be sleep if … animals show homeostatic regulation. Whether sharks need to “catch up” lost sleep is still up for debate. Scientists would need to record sleep/wake brain activity to measure this, an invasive procedure that requires some experimentation.
- It might be sleep if … sharks take a little longer to rouse out of restfulness. Much like daydreaming in a tedious lecture – is it sleep, or simply quiet wakefulness? Linked to point one (reduced environmental awareness), the nurse shark has been shown to have delayed and reduced responsiveness to divers in their presence.
Buccal-pumping sharks, the benthic and stationary species that don’t need to swim continuously to breathe, meet some of the criteria for sleep behaviour. Most species only meet one or two, not all, of the measured criteria.
But what about ram ventilators that need to swim continuously? There is evidence that ocean mammals that are perpetually on the move sleep-on-the-go. By modifying somewhat the criteria for sleep, many whales and dolphins fit the description by sleeping using half their brain and by remaining swimming.
- It might be sleep if … animals are immobile or have relaxed postures and reduced environmental awareness. The bull, blue, Caribbean reef, and lemon shark all show recorded periods of inactivity. The bull and blue shark are obligate ram ventilators.
- It might be sleep if … animals show circadian regulation. Basking sharks have been known to glide at the sea surface in the sun, displaying circadian patterns in their dive migrations. Grey nurse sharks have been recorded “milling” slowly during the day, and actively foraging at night. Other ram ventilators that have been recorded as more active at night include grey reef sharks, sevengill sharks and smooth dogfish. Some species are found at greater depths at night than during the day, and although it’s not considered strong evidence for sleep behaviour, this “diel vertical migration” behaviour has been documented for species such as shortfin makos, blue, megamouth and big-eye thresher sharks.
- It might be sleep if … sharks take a little longer to rouse out of restfulness. The bull, blue, Caribbean reef, and lemon shark have been documented responding only after rough and excessive handling.
There are a few suggested hypotheses as to how ram ventilators might sleep:
- Ram ventilators don’t sleep.
Many ram ventilators are cruising the vast pelagic zone, an area considered to be of low sensory input. So, it’s been suggested that these sharks have secondarily lost the need to sleep, to facilitate their continuously swimming lifestyle, and is made possible by the lower visual input they receive cruising through the pelagic and open ocean areas. Some scientists reject this hypothesis, arguing that it is unlikely that – given how widespread sleep is in the animal kingdom, and how early it evolved – one group of sharks would develop the behaviour and another not. - Ram ventilators sleep unihemispherically (like whales and dolphins).
Whales and dolphins sleep with one eye open to monitor their environment, connected to their “wakeful” brain hemisphere, and closing the eye connected to the brain hemisphere experiencing slow waves. It is possible that sharks do something similar, allowing them to continue moving, while buccal pumping sharks sleep bihemispherically (with their whole brain). - Ram ventilators sleep in currents.
If sharks can sleep with their whole brain, but face into currents that help them ventilate by running oxygen-rich water over their gills, they might be able to remain motionless (or reduce swimming activity). However, there is currently no convincing evidence that sharks do this.
Where Do Sharks Sleep?
If sharks do sleep, the places they’d choose to rest will be as varied as the myriad habitats they inhabit in our oceans. Nurse sharks and Port Jackson sharks have been photographed recumbent under rocks on reefs, or in shallow caves. White tip reef sharks huddle together in big groups, piled on top of one another in caves. Lemon sharks have been recorded simply coming to lie still on the open sand of the seafloor. Californian horn sharks rest in shelters in the rocky reefs of the Californian coast’s kelp beds . Ram ventilating sharks that show some potential evidence for sleep-like behaviour might also favour certain resting spots, like scalloped hammerhead sharks that have been shown to gather around seamounts during the day and during periods of low activity. Basking sharks, however, simply glide through the open ocean closer to the sea’s surface without necessarily coming to rest in a defined “habitat”; there is some suggestion that sharks are following particular zones of light or temperature that best allow them to rest.
When Do Sharks Sleep?
Several shark species have been shown to have circadian regulation: their activity and sleep patterns are influenced by circadian (night and day) rhythms. So, when they sleep will differ according to a host of different factors; for instance, species type, breathing mechanism and trophic level (where in the food web they sit). Nocturnal buccal pumping sharks include swell and horn sharks, small-spotted catsharks, Port Jackson, zebra, Pacific angel, Caribbean reef, and white tip reef sharks. Ram ventilators like basking sharks, white sharks, short fin mako sharks and scalloped hammerhead sharks show different patterns of swimming, diving and cruising at different times of day – but it’s difficult to classify these as defined sleep periods.
How Long Do Sharks Sleep?
Studies have shown that draughtsboard sharks were considered to be sleeping after five minutes of inactivity. Scientists think that the lack of evidence for sleep debt in sharks (one of the criteria for sleep being that it is homeostatically regulated, and means that the more that is lost, the greater the drive to ‘catch up’) means that most sharks sleep on short timescales of less than ten minutes.
A Sleep Summary
So far, of the over 1600 species of chondrichthyans (cartilaginous fish – sharks, rays, skates and chimaeras) in our oceans, only 33 sharks and 1 ray are formally recorded showing possible sleep behaviour. There is plenty research needed in the field! The work that remains to be done, say some scientists, is to monitor circadian patterns in some species, test arousal thresholds in others, and examine homeostatic regulation, which hasn’t really been done or published for any sharks (that is, do sharks experience and catch up on sleep debt?) and how do their brains function during rest and possible sleep?
Frequently Asked Questions:
Do sharks sleep?
It appears some do, but none fulfil all the criteria to meet traditional definitions of sleep. There is more evidence for sleep in buccal pumping sharks (that do not need to move to breathe) than in ram ventilators (continuously swimming sharks). There’s a lot more to study, and potentially, some traditional criteria to revisit or amend.
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.
Do sharks need to move to breathe?
Some do. These are called ram ventilators and they breathe as oxygen-rich seawater is forced over their gills as they swim. But not all sharks need to move to breathe. Buccal pumping sharks, and those that can switch between the two mechanisms (like tiger and bull sharks), can manually pump water over their gills using their mouths. These sharks are capable of remaining stationary for long periods of time.
How do sharks sleep?
Some sharks appear to lie stationary (like lemon sharks, white tip reef sharks and nurse sharks). Some, like draughtsboard sharks, close their eyes. Some sharks show circadian regulation, timing their activities based on time of day (night or day); these include Port Jackson sharks, swell sharks, horn sharks and small-spotted catsharks. Scientists hypothesise that some ram ventilating sharks might sleep using only half their brain, or by facing into ocean currents to breathe passively.
Where do sharks sleep?
Sharks have been shown resting in caves and shelters on rocky reefs, around seamounts and on the sandy seafloor.
How long do sharks sleep?
Scientists think that most sharks don’t sleep for periods longer than 10 minutes, but that this might happen frequently over a longer timescale.
References
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