Do Sharks Sleep? How Sharks Rest Without Stopping

Do sharks really sleep?

Scientists define sleep by behavior and body changes, not just by closed eyes. A sleeping animal is still for long periods, takes up a typical posture, is slower to react to things around it and saves energy. In mammals and birds, sleep can also be measured through brain waves, but that is much harder to do in a fish.

For a long time, there was little direct evidence that sharks sleep at all. That changed in 2022, when Australian and New Zealand researchers published a study of draughtsboard sharks, a small catshark from New Zealand, in the journal Biology Letters. They measured how much oxygen seven wild-caught sharks used while they swam, rested and lay still for long periods.

Sharks that had been still for at least five minutes used significantly less oxygen than when resting briefly or swimming, and they lay flat on the bottom. The researchers concluded that this energy-saving state is sleep. Surprisingly, the sharks often kept their eyes open while asleep, so eye closure turned out to be a poor sign of sleep in sharks.

Why some sharks must keep swimming

All sharks need a steady flow of water over their gills to take in oxygen. They do this in two ways:

• Buccal pumping. The shark uses muscles in its mouth and throat to draw water in and push it over the gills. Many bottom-living sharks also take in water through spiracles, small openings behind the eyes. This lets them breathe while lying completely still.
• Ram ventilation. The shark swims with its mouth slightly open, so water rushes over the gills. This works only while the shark is moving.

Most sharks can use buccal pumping when they rest. A small number of species, known as obligate ram ventilators, have lost the ability and must keep swimming to breathe. The great white shark and the mako sharks are the best-known examples. If they stop moving, they cannot get enough oxygen.

Sharks that rest on the seabed

Bottom-dwelling sharks rest in plain sight. Nurse sharks spend the day lying on the sand or hiding under ledges, sometimes in groups piled on top of one another, and hunt at night. Whitetip reef sharks rest in caves and on sandy patches by day, like the shark in the photo above, and come out to hunt after dark. Wobbegongs, angel sharks and horn sharks also spend long periods lying still.

These sharks are heavier than water, which helps them stay on the bottom without effort. Their spiracles are large and well developed, so they can keep breathing even when their mouth is close to the sand.

In the early 1970s, shark biologist Eugenie Clark studied a famous case off Isla Mujeres, Mexico: requiem sharks, a group that usually swims constantly, lying motionless in underwater caves. Theories included unusually oxygen-rich or low-salt water seeping into the caves, but the reason was never fully settled.

How do great whites and other swimmers rest?

Sharks that must keep moving seem to rest while they swim. Experiments on the spiny dogfish showed that its swimming movements are controlled by the spinal cord rather than the brain. In theory, this could let a shark keep swimming while its brain rests, sometimes called sleep swimming.

In 2016, a great white shark was filmed at night off Guadalupe Island, Mexico, swimming slowly into the current with its mouth open, in what researchers believed could be a sleep-like state. Some scientists have suggested that sharks might rest one half of the brain at a time, as dolphins do, but this has not been shown. How dolphins manage it is explained in How do dolphins sleep?

Other active sharks seem to slow down and cruise at a gentle pace for parts of the day, using little energy. The tiger shark, a mostly nocturnal hunter, moves close to shore to feed at night and spends the day in deeper water, so any rest probably comes in daylight.

Sleep in sharks compared with other fish

Fish do not sleep the way mammals do, but many species have regular periods of rest in which they become still and less responsive. Some reef fish wedge themselves into crevices at night, and parrotfish wrap themselves in a cocoon of mucus. The wider picture is covered in Do fish sleep?

Sharks are an important piece of this puzzle. Sharks and rays split from the ancestors of bony fish and land animals more than 400 million years ago. If sharks sleep, sleep is probably very ancient, shared by vertebrates since long before animals walked on land.

Myths and facts

Myth: all sharks die if they stop swimming. Only a few species, mainly open-ocean sharks such as the great white and makos, rely entirely on swimming to breathe.

Myth: sharks never sleep. At least one shark species has now been shown to enter a sleep-like state with lower metabolism, and many others rest for long periods.

Fact: sharks can sleep with their eyes open. In the 2022 study, open eyes did not mean a shark was awake.