Do Fish Sleep? How Fish Rest Without Closing Their Eyes

How do scientists know a fish is asleep?

In people and other mammals, sleep is easy to recognize: the eyes close and brain waves change in typical ways. Fish make this harder. Most have no eyelids, and their brains lack the outer layer, the neocortex, where mammal sleep waves are usually recorded.

So scientists look for behavioral signs instead. A fish is considered asleep when it shows:

• long periods of stillness;
• a typical resting posture, often in a particular shelter;
• a daily rhythm, with rest alternating with activity over about 24 hours;
• slower reactions, so it takes a stronger touch, light or sound to rouse it.

A further test is the rebound effect. If an animal truly needs sleep, keeping it awake should make it sleep longer or more deeply afterward. Several fish pass these tests, while others show few or none of the signs.

What zebrafish reveal about fish sleep

The small zebrafish, a favorite laboratory animal, has become the main model for fish sleep. At night, zebrafish hang almost motionless near the bottom or the surface, their mouth and gill movements slow by about half, and they are much harder to startle. When researchers keep them awake, they sleep longer and more deeply the next night, a clear rebound effect.

In 2019, a team at Stanford University published a study in the journal Nature that watched the brains of young zebrafish as they slept. The fish showed two distinct sleep states. One resembled the slow-wave sleep of mammals, and the other resembled REM sleep, the stage in which people dream most, with relaxed muscles and changes in heart rate, but without rapid eye movements. The researchers concluded that these sleep states probably evolved more than 450 million years ago, in the common ancestor of fish and land animals.

How fish sleep on a coral reef

Night divers see reef fish settling down for the night in many ways. Some tuck into holes and crevices, rest under ledges or among seaweed, or squeeze inside sponges. Many wrasses bury themselves in sand. Several reef fish switch to paler or blotchier colors after dark. Some sleep so soundly that divers have lifted resting Spanish hogfish and bluehead wrasses in their hands without waking them.

The ocellaris clownfish spends the night nestled deep among the tentacles of its host anemone, protected by the anemone’s stings. Research on clownfish has found that they do not stay completely still all night: they move and fan their fins among the tentacles at intervals, which raises oxygen levels around their host while it is in the dark.

Parrotfish sleeping bags

Some of the strangest sleepers are parrotfish. At dusk, many parrotfish wedge themselves into a crevice and produce a cocoon of clear mucus from glands near their gills. The cocoon wraps around the whole body, and the fish breaks out of it in the morning.

For years, people guessed the cocoon hid the fish’s scent from night predators such as moray eels, but this was never shown. A 2011 study on Australia’s Great Barrier Reef found a different benefit: parrotfish in their cocoons were attacked far less by gnathiid isopods, tiny blood-feeding parasites that swarm over the reef at night. The researchers compared the cocoon to a mosquito net. Some wrasses make similar mucus envelopes.

Fish that never seem to stop

Not every fish appears to sleep. Many fast open-ocean fish, such as tuna, mackerel and bluefish, swim continuously and show no clear signs of sleep. They may rest by slowing down and cruising, perhaps with parts of the brain less active, but this has not been demonstrated. Some sharks must also keep swimming to breathe, as explained in Do sharks sleep? Air-breathing dolphins solve a similar problem by resting one half of the brain at a time, described in How do dolphins sleep?

Living in a group can also make resting safer. The red-bellied piranha of South America lives in shoals mainly for protection from predators such as river dolphins, caimans and birds. In experiments, piranhas in shoals of eight calmed down faster after a simulated predator attack than piranhas kept in pairs. Larger piranhas feed mostly at dawn, in the late afternoon and in the early evening, but how they sleep has not been studied in detail.

Some damselfish in the Red Sea show what researchers call sleep-swimming. They spend the night among coral branches, beating their fins rapidly and repetitively yet not responding to light or predators. Their fanning keeps the water inside the coral well oxygenated.

Cavefish that barely sleep

Blind cavefish offer one of the most striking examples of how flexible fish sleep can be. The Mexican tetra lives both in rivers and in completely dark caves. Fish from several separate cave populations sleep far less than their river-dwelling relatives, and this sleep loss evolved independently in different caves. Scientists think that in a dark, food-poor cave, staying active to search for food matters more than a regular night’s rest.

When fish skip sleep

Fish sleep is easily disrupted. Species that normally rest at night may stay active around the clock while they spawn or migrate. Many parent fish give up sleep to care for their eggs, fanning them day and night for days at a time. This has been seen in sticklebacks, cichlids, damselfish, smallmouth and largemouth bass and brown bullheads. Even within one species, sleep habits can shift: goldfish may become active by day or by night depending on when food is available.