Do sharks sleep?

Sleep. Without it, it’s bleary eyes and cavernous yawns in front of the computer in the morning. It’s a quick temper washed down with desperate gulps of coffee in the rush-hour traffic. “A lack of sleep will age a man by a decade,” declares sleep scientist Matt Walker in his TEDTalk. It’s enough to send you scuttling for your covers. His assertions are backed by another researcher, Jeff Iliff, who says in his own TEDTalk that sleep is an elegant solution to meeting our brain’s basic needs. Yes, laboratory-held rats do die when deprived in experiments from some forms of sleep. Certainly, any parent of a newborn infant has wondered in the desperate hours of the night if death by sleep-deprivation is also to be their fate. But scientists contend that sleep is very variable across the animal kingdom. Bears hunker down to hibernate for up to 7 months, and African lions languish for hours in the savanna daytime heat before reviving themselves for the night. But what is sleep, and what is rest? What’s the difference? And does every animal on this planet experience both?

In 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.

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. Scientists from the Save Our Seas Foundation D’Arros Research Centre (SOSF-DRC) published the first records of resting grey reef sharks that appear to switch from ram ventilation to buccal pumping to ventilate their gills (Sleeping beauties? – Save Our Seas Foundation).

How Do Sharks Sleep?

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:  

1. 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.
2. 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.
3. 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.
4. 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.
5. 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. Grey reef sharks in Seychelles also appeared unaware of divers.

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.

1. 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, as do grey reef sharks. The bull and blue shark are obligate ram ventilators. The grey reef shark was thought to be an obligate ram ventilator but new evidence suggests that they switch to ventilating their gills (like buccal pumpers) during periods of inactivity.
2. 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.
3. 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. Equally, grey reef sharks appeared unaware of and unresponsive to divers in their vicinity in a study at the SOSF-DRC in Seychelles.

Before the publication of the paper Just keep swimming? Observations of resting behavior in gray reef sharks Carcharhinus amblyrhynchos (Bleeker, 1856) in the Journal of Fish Biology in 2023, there were a few suggested hypotheses as to how obligate ram ventilators might sleep:

1. 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.

2. 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).

3. 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.

The publication from the Seychelles provides first evidence of grey reef sharks, which were previously thought to be obligate ram ventilators, resting. The researchers believe that they were switching their mode of breathing to buccal pumping, because the sharks were facing in all different directions (they were not using currents for ventilation) and they were unresponsive to divers in their presence (potentially not sleeping unihemispherically, as hypothesized previously). This paper is a landmark one for furthering shark sleep science, because it provides the first evidence to suggest that actually, all the points above may not always be true for all ram ventilating species.