How Do Sharks Sleep? The Five-Minute Test That Shows It

A shark is asleep when it has been motionless for at least five minutes, has dropped from an upright stance onto a flat, recumbent posture, and is burning measurably less oxygen than the same animal burns while merely resting. Those three signs come from the first physiological study of sleep in any shark, published in the Royal Society journal Biology Letters in March 2022, in which Michael L. Kelly and co-authors at La Trobe University, the University of Western Australia and the University of Auckland put seven draughtsboard sharks into intermittent-flow respirometry chambers and recorded them across 24 hours. Closed eyes, the sign most people watch for, failed.

Why five minutes is the number that matters

The five-minute mark is not a convention borrowed from human sleep science. It is where the shark's own metabolism changes.

Kelly's team measured mass-specific oxygen uptake rate on seven sharks weighing 766 to 2,705 grams, collected from the Hauraki Gulf in north-eastern New Zealand. Sharks that had been motionless for less than five minutes showed no significant difference in oxygen uptake from sharks that were actively swimming. A shark that has simply stopped is still running a swimming shark's metabolism. Only past the five-minute mark did uptake fall significantly, and it stayed low whether the episode happened in daylight or after dark.

Responsiveness moves on the same schedule. Writing in The Conversation, Kelly, now a postdoctoral research fellow in neuroscience at Simon Fraser University, reported that "buccal pumping sharks were less responsive to mild electrical pulses following five minutes of inactivity." A raised arousal threshold is one of the standing criteria sleep researchers apply to any animal, and the draughtsboard shark meets it at the same point its oxygen curve bends.

Muscle tone gives a third independent reading. In a 2024 paper in the Journal of Experimental Zoology Part A, John A. Lesku and colleagues implanted six adult draughtsboard sharks, 850 to 915 millimetres in total length, with electrodes recording muscle activity, brain temperature and movement. Neck muscle tone was strongest during active wakefulness, lower during quiet wakefulness and lowest during sleep. Brain recordings were the one measure that failed: the shark brain floats in cerebrospinal fluid rather than sitting against the skull, and the electroencephalogram signal proved too unstable to analyse.

Closed eyes are the wrong sign to watch for

Eye closure in sharks tracks the light, not the sleep. In the Biology Letters study, sharks kept their eyes open roughly 38 per cent of the time during nighttime sleep, and closed them more often during daylight hours regardless of whether they were asleep or awake. The sharks were held under a 12-hour light, 12-hour dark cycle, which is what allowed the two variables to be separated at all.

Kelly put the finding plainly: "Eye state, however, was found to be unrelated to the sharks' state of consciousness, as the animals were often observed sleeping with eyes open." Sharks have no eyelids in the mammalian sense, and many species instead retract the eye or draw a nictitating membrane across it, a protective movement rather than a sleep posture.

Body posture carries the information that eyes do not. The same team described sharks shifting from an upright position, propped up on their pectoral fins, to a completely recumbent one. A shark sitting up on its fins is holding tension. A shark lying flat has let it go.

Which sharks can stop swimming, and which cannot

Everything above applies only to sharks that can breathe while stationary. Two ventilation modes divide the group.

Buccal pumping uses muscles in the mouth and pharynx to draw water in and force it across the gills, which works at zero speed. Ram ventilation uses forward motion to push water through an open mouth, which costs nothing in muscle work but requires the animal to keep moving. Most sharks can switch between the two. Lamnid sharks, the family containing the shortfin mako and the white shark, cannot: Nicholas C. Wegner and co-authors at Scripps Institution of Oceanography described them in the Journal of Experimental Biology in 2012 as obligate ram ventilators that depend on continuous swimming to force water through the gills.

That paper also quantified what continuous means. Six juvenile shortfin makos, 76.0 to 89.0 centimetres in fork length, held a preferred swimming speed of 38.8 centimetres per second in a water tunnel, extracting 53.4 per cent of the oxygen passing over their gills at a branchial pressure gradient of 96.8 pascals. Roughly 80 per cent of the resistance to that flow sits in the septal channels, structures built into every elasmobranch gill that tunas do not have.

Spiracles, the accessory openings behind the eye, are the usual anatomical marker for a bottom-rester, and they are not a reliable one. The Florida Museum's species profile records that in the nurse shark, the most-photographed motionless shark in the Caribbean, "the spiracles are minute." The species rests anyway, on buccal pumping.

| | Nurse shark | Draughtsboard shark | Shortfin mako | White shark | |---|---|---|---|---| | Ventilation | Buccal pumping at rest | Buccal pumping at rest | Obligate ram | Obligate ram | | Can lie motionless | Yes, for hours | Yes, episodes of 5 min or more | No | No | | Maximum length | 308 cm (Florida Museum) | 850–915 mm (Lesku et al. 2024 sample) | About 400 cm | About 600 cm | | Active period | Night | Night, by metabolic rate | Day and night | Day | | Sleep evidence | Behavioural only | Metabolic, arousal, muscle tone | None published | None published |

How long a resting shark stays down

Bottom-resting sharks hold position for hours, not minutes, and they do it in the same places repeatedly.

The best-documented case is the cave system off Isla Mujeres, on the Yucatán coast, found by a local fisherman in 1969 and filmed by the Mexican diver Ramón Bravo before any scientist reached it. Eugenie Clark, the University of Maryland zoologist known as the Shark Lady, worked the site in 1972 and 1973 on National Geographic Society funding. Her team identified the animals as Caribbean reef sharks, recorded them holding station inside the caves for hours at a time, and counted them pumping water over their gills 20 to 28 times per minute. The cave water carried unusually high oxygen and reduced salinity, which Clark suspected was the reason the sharks tolerated it.

Diel timing sorts the rest of the picture. The Florida Museum profile describes nurse sharks as nocturnal, resting by day at depths of 3 to 75 metres, moving into water shallower than 20 metres after dark, and occurring in groups of up to 40 individuals lying close together and sometimes piled on one another. John E. Randall's 1977 study in Pacific Science established the same pattern for the whitetip reef shark, which shelters in caves through the day, often communally, and hunts at night. Kelly's respirometry found the mirror image in the numbers: nighttime swimming produced the highest metabolic rates the study recorded.

How to tell whether a still shark is asleep

The published criteria translate into a short observational procedure, and the order matters because the cheap signs are the unreliable ones.

1. Time the stillness. Start a clock. Below five minutes, the animal is a shark that stopped, not a shark asleep, and Kelly's respirometry found its metabolism unchanged from swimming.
2. Read the posture. Upright on the pectoral fins is wakeful rest. Flat and recumbent against the substrate is the posture that tracked reduced oxygen uptake.
3. Ignore the eyes. Open eyes rule nothing out at night, and closed eyes in daylight rule nothing in.
4. Count the gill beats. Slow, regular pumping confirms the animal is ventilating without swimming. Clark's Caribbean reef sharks ran 20 to 28 beats per minute.

Where the published numbers disagree

Two figures in wide circulation do not survive contact with the measurement literature.

The first is the mako's speed. The Florida Museum credits the shortfin mako with burst speeds "up to 43 mph (70 km h-1)." Direct measurement gives something far lower. Waller and co-authors, publishing in the Journal of Fish Biology in 2023, attached bio-loggers to two shortfin makos and recorded a mean cruising speed of 0.90 metres per second at a tail-beat frequency of 0.51 hertz. The fastest burst they captured, from a two-metre female, reached 5.02 metres per second, about 18 kilometres per hour, held for 14 seconds, and raised her white muscle temperature by 0.24 degrees Celsius over the following 12.5 minutes. The authors called it among the highest speeds directly measured in any shark, tuna or billfish. The 70 km/h figure descends from extrapolation, including estimates of the escape velocity needed to explain the leap heights of hooked makos, rather than from a tag.

The second is the nurse shark's size. The Florida Museum lists a maximum recorded 308 centimetres. FishBase, following Compagno, lists 4.3 metres and 110 kilograms. Both are in print, and the gap is not a rounding difference.

What is known about white sharks, makos and tiger sharks

Nothing physiological. That is the honest state of the field, and Kelly stated it directly: "Nothing is known about sleep in ram ventilating sharks. Their need for constant swimming to facilitate gas exchange suggests they have likely evolved interesting adaptations to permit sleep."

The most-cited observation is a single episode. During Shark Week 2016, a submersible operated with researchers from the Woods Hole Oceanographic Institution and Pelagios Kakunjá filmed a white shark named Emma near the bottom off Isla Guadalupe at around 8:30 in the evening, swimming slowly into a current of roughly 3 kilometres per hour with her mouth held open. The interpretation offered was that she was letting the current do the ventilating while she idled. No metabolic rate was measured, no arousal test was run, and the episode remains a suggestive video rather than evidence.

Tiger sharks sit in the most confused position of all. Adults are generally treated as obligate or near-obligate ram ventilators. Taketeru Tomita of the Okinawa Churashima Research Center and colleagues, reporting a captive birth at the Okinawa Churaumi Aquarium in Copeia in 2018, found by sonography that tiger shark embryos use buccal pumping in utero and shift toward ram ventilation after birth, with the newborn still dropping back to buccal pumping whenever it settled on the tank bottom. The switch is developmental, and no equivalent observation exists for a healthy adult.

An upside-down shark is not a sleeping shark

Inverting a shark induces tonic immobility, a reflex state that looks like sleep and is not one. Alan D. Henningsen of the National Aquarium in Baltimore tested 12 elasmobranch species and reported the results in Zoo Biology in 1994. Induction took between 16.7 seconds in the shovelnose guitarfish and 45.7 seconds in the clearnose skate. Duration ranged from 34.3 seconds in the clearnose skate to 274.0 seconds, about four and a half minutes, in the Caribbean reef shark. Two species tested, the broadnose sevengill shark and the yellow stingray, did not respond at all, though only one individual of each was available.

Tonic immobility is induced from outside the animal and ends on its own schedule. Sleep as Kelly's group defined it is entered voluntarily, sustained past five minutes, and marked by a metabolic drop that the reflex does not produce. Aquariums use tonic immobility precisely because it is not sleep: it holds an animal still for a veterinary procedure without anaesthesia.

Frequently asked questions

How do sharks sleep if they cannot stop swimming?

No one knows. Species such as the shortfin mako and white shark are obligate ram ventilators that must swim to force water over their gills, and no study has yet measured metabolic rate, arousal threshold or muscle tone in a swimming shark. Michael Kelly has stated that sleep in ram ventilating sharks is entirely unstudied.

How can you tell if a shark is sleeping?

Time the stillness and read the posture. A shark motionless for five minutes or more, lying flat rather than propped upright on its pectoral fins, is showing the two signs that tracked reduced oxygen consumption in Kelly's 2022 respirometry study. Eye state tells you nothing, because sharks frequently sleep with their eyes open.

How long does a shark sleep?

Sleep episodes in draughtsboard sharks were defined as five minutes or longer of inactivity, the point at which metabolic rate drops. Bottom-resting species hold position far longer: Eugenie Clark recorded Caribbean reef sharks staying inside caves off Isla Mujeres for hours, and nurse sharks rest through the daylight hours before hunting at night.

Do sharks have to keep moving to stay alive?

Only some do. Lamnid sharks including the shortfin mako and white shark are obligate ram ventilators and depend on continuous swimming for gas exchange. Nurse sharks, whitetip reef sharks, draughtsboard sharks and many others pump water across their gills with mouth and pharynx muscles, so they can lie still indefinitely.

Do great white sharks sleep?

There is no published physiological evidence. A submersible filmed a white shark off Isla Guadalupe in 2016 swimming slowly into a 3 km/h current with its mouth open, which was interpreted as a sleep-like state, but no oxygen consumption or arousal threshold was measured. As an obligate ram ventilator, a white shark cannot stop moving.

Can sharks rest upside down?

Inverting a shark triggers tonic immobility, a reflex, not rest. Alan Henningsen's 1994 study of 12 elasmobranch species found the state lasted from 34.3 seconds in the clearnose skate to 274.0 seconds in the Caribbean reef shark. It is imposed from outside, produces no metabolic drop, and is used by aquariums to handle animals safely.