Do Fish Sleep? How Fish Rest, From Sharks to Bass

A parrotfish asleep at night in a hole in the coral — Photo: Jérôme Paillet, Ifremer (CC BY 4.0)

Fish do sleep, though only four species have been put through every test science uses for it: the zebrafish, the Mexican cavefish, the banded knifefish and the Mediterranean rainbow wrasse (Coris julis). The rest, from the bluegill under your dock to the walleye off the point, have not been tested in full; the ones watched at night show many of the signs. They keep a daily quiet spell and a fixed resting spot, breathe slower, and take a moment to wake. For an angler, that decides which fish to chase after dark and which to leave until dawn.

A zebrafish spends about 58% of the night asleep and 5% of the day. A draughtsboard shark asleep on the seabed burns less oxygen than one lying awake, and far less than one swimming.

How do scientists know a fish is asleep?

A fish has no eyelids to close and no neocortex to record a sleep brain wave, so the yardsticks used on mammals fail. A 2024 review of 126 papers sets out the behavioral tests instead. The classic four start with a daily quiet period tied to the body clock (the circadian rhythm). Then come a raised arousal threshold (the fish needs a stronger nudge to react), a species-typical resting site or posture, and quick reversal, which rules out coma and torpor. Later work added a fifth: the fish sleeps more after being kept awake. Only the four species above have been tested against all of them, a measure of how few fish have been studied.

The zebrafish shows what passing looks like. At night it floats in the water column, level or with the head slightly up. Its mouth and gill movements drop by almost half, and it is twice as hard to rouse as by day. Kept from that rest, it sleeps longer and deeper afterward. Melatonin, the pineal hormone long known to promote sleep in people, brings on the same sleep-like state in zebrafish, and so do the sleeping drugs diazepam and pentobarbital.

Why do fish need sleep, and do they dream?

Sleep looks like a risky habit for an animal that other animals eat, so it has to pay. Two payoffs have been measured in fish. In draughtsboard sharks, a lowered metabolic rate marks sleep, which the researchers read as support for sleep saving energy. In zebrafish, breaks in the DNA of brain cells pile up while the fish is awake. Sleep makes the chromosomes in those neurons move more, and that movement is needed to bring the breaks back down.

An older view, Ray Meddis’s, sees sleep as an immobilizer. It keeps an animal quiet and out of sight through the hours it cannot use, and saves energy while it waits.

The zebrafish also shows what that sleep looks like inside. In 2019 researchers recorded two sleep states in its brain. One is slow-bursting, like our slow-wave sleep. The other is a propagating wave, like our REM, with relaxed muscles and a slowed heart. Those signatures may have emerged in the vertebrate brain more than 450 million years ago, the researchers suggest. Whether a fish dreams nobody can say: the REM-like state is there, but a dream cannot be measured.

Do fish sleep with their eyes open?

Yes. Most fish have no eyelids, so they cannot close their eyes; the eyes stay open whatever the fish is doing. Sleep shows in the rest of the body. A resting fish stops swimming or slows to a hover and settles on or near a fixed spot. Fin and tail movement drop to a minimum, and it breathes slower.

Some fish still do something with the eye. Oscars watched with infrared goggles rested on the tank bottom with their eyes turned downward. The draughtsboard shark can close its eyes, yet in a 2022 study eye closure was a poorer sign of its sleep than a flat body posture and a lowered metabolism. At night its eyes were open in about 38% of sleep bouts.

One sense stays on. In the Pachón cave population of the Mexican cavefish, the lateral line (the row of pressure sensors along the flank) keeps the fish awake. Cut its input and they sleep as much as their river-dwelling cousins. In four other cave populations, cutting it made no difference. A fish at rest is still listening for vibration.

Where do fish sleep?

Where they can hide. Divers have watched fish settle in for the night in holes and crevices, under ledges, among weed or coral, inside sponges and buried in sand, and the same shelters held fish all night. Some wrasses bury themselves in sand, the rainbow wrasse among them. The Pacific sand lance burrows into sand to rest, save energy and avoid predators. A sheltered fish stays put: most sleeping fish will not leave their shelter when a light hits them, and if anything they dig in.

Parrotfish go further. Every night one spins a mucus cocoon around itself in under an hour, for about 2.5% of its daily energy. Of cocooned fish, 10% were attacked by blood-feeding isopods; of uncocooned ones, nearly all. Earlier work found spotted moray eels taking more sleeping parrotfish without a cocoon than with one. A sleeping fish is a meal, and the cocoon is a mosquito net.

Fresh water is plainer. The perch, bullheads and tilapia that have been watched at night rest on the bottom, the perch each sitting alone.

How can you tell a fish is sleeping?

The handling reports collected in Reebs’s review of fish sleep are the best guide. A brown bullhead at rest lies with its tail flat on the bottom and its body tilted 10 to 30 degrees, heart and breathing slowed. Tilapia rest on the bottom at night and react less readily than by day to food or an electric current. Spanish hogfish, wrasses, the cunner and even requiem sharks have been picked up by hand at night, some lifted to the surface without a response.

One trap: a sudden bright light at night makes some fish sink to the bottom and hold still for several minutes. Called light shock, it was first studied in bluegill, and it looks like sleep. To see the real thing:

Pick clear, shallow water with something to rest against. A dock, a weed edge or a rock pile in 3–6 ft (1–2 m). Fish rest where they hide.
Return an hour after full dark, and approach slowly. Footfalls on a dock carry into the water.
Keep the light dim and steady, never a sudden white beam in the face. For sleep studies, Reebs advises very dim light kept on all night, for exactly this reason.
Read the posture. Tail on the bottom, body tilted, pectoral fins slow, gill covers beating slower than by day. A school broken into single fish sitting apart is the perch signature.
Look for the arousal gap without touching the fish. A resting fish ignores a leaf drifting past; by day the same fish chased everything. That gap, not stillness alone, is sleep.
Come back at first gray light. Yellow perch run their dusk sequence backward at sunrise, from a standstill on the bottom to schooling and feeding again.

How long do fish sleep?

Less than we do, and in pieces. Adult zebrafish sleep about 5% of the day and 58% of the night. Kept awake for the last six hours of the night, they lost about 30% of their sleep and made it up when the dark was extended; released into light, they showed no rebound. The Mexican cavefish’s river-dwelling form sleeps more than 800 minutes a day; the blind cave populations get by on 110 to 250 minutes.

Young fish can skip it altogether. Juvenile Mozambique tilapia watched day and night showed no sign of sleep at all, and took 22 weeks to develop the adult pattern.

For a bass, a bluegill or a catfish there is no field number at all. Michael Heithaus of Florida International University puts aquarium fish at roughly 7 to 12 hours. That is expert opinion, not a measurement.

Do fish sleep at night or during the day?

Most fish keep one shift, but which one depends on the fish. Many are active by day and sleep at night; for numerous eels, rays and sharks it is the reverse. The rest is seldom total: a few day fish still turn up in traps, or with food in the stomach, at night. Many nocturnal reef fish spend the day in schools that hang in one place yet still react to predators. The California horn shark hides in caves by day but responds quickly when divers disturb it. In fresh water the night shift belongs to channel catfish, which feed by taste and smell, and to flathead catfish, which leave their cover after dark to hunt. Walleye are low-light hunters too: a tapetum, a reflective layer behind the retina, sends light back through it a second time.

The shift is not fixed for life. At a field station at 66° N, above the Arctic Circle, captive burbot, sculpins and brown bullheads that were nocturnal in summer turned diurnal in the short days of the Arctic winter. Tested south of 60° N, they stayed nocturnal all year. Juvenile Atlantic salmon became more nocturnal as their water was chilled. White suckers were diurnal in a shoal and nocturnal alone.

Food moves it too. Goldfish fed only at night, only by day or only at twilight were each active around their old mealtime through a 10-day fast. Food and predators, Reebs concludes, probably decide when a fish rests, and so when it sleeps.

Sleep also bends to life. Spawning fish, yellow perch, minnows, sunfishes and bullheads among them, drop the daily rhythm and stay active day and night.

Do fish sleep while swimming?

Some never seem to stop. In large seawater tanks, two open-water (pelagic) fish, bluefish and Atlantic mackerel, kept swimming day and night. They swam more slowly at night but responded to disturbance and to food around the clock. The same case, that a fish must swim to breathe or to stay up and so cannot settle, has been made for tunas, bonitos and some sharks. The 2024 review lists mackerel, tuna and some sharks as fish with constant activity.

Whether they rest some other way is open. One idea, Kavanau’s, is that a fish in a big school lets its attention drift in short bouts while its shoalmates keep watch. Another is half-brain (unihemispheric) sleep, as in some marine mammals and birds. Popular pages state it as fact for tuna and sharks, but no primary source shows it in any fish; the 2024 review raises it only as a possibility.

Travel costs sleep as well. Lake chubs, day fish in summer, were counted crossing stream fences mostly at night during their spring and fall migrations.

Do sharks sleep?

The ones that can stop swimming do. Port Jackson and draughtsboard sharks, bottom dwellers that pump water over their gills, needed a stronger electric jolt to react while lying still. That 2021 study was the first evidence that sharks sleep, though the sharks did not make up lost rest after forced swimming. A draughtsboard shark asleep on the bottom, body flat, burns less oxygen than one lying awake, and far less than one swimming. Recordings published in 2024 added the first muscle-tone evidence of shark sleep.

Many sharks, the great white among them, must swim constantly to breathe; how fish breathe explains why. Heithaus describes them slowing their swimming or swimming into a current instead of stopping. Whether that is sleep is unknown, and the half-brain claim has no primary source.

Do aquarium fish and goldfish sleep?

Yes, and they are the easiest fish to watch doing it. Most tank fish rest after dark in a calm, sheltered spot: a corner, a cave, near plants or ornaments. They hover or sink toward the bottom, breathe slowly and sometimes look duller in color. Jena Questen, an aquatic veterinarian in Colorado quoted by Chewy, says “It is important that fish get some periods of darkness each day” and warns against bright light 24 hours a day. The 2024 review adds that extra light at night disrupts the daily clock in many fish.

Goldfish learn when to be up. Through a 10-day fast, goldfish kept to their old mealtime, so a goldfish fed at the same hour every day can learn to be active at that hour.

A still fish is usually a sleeping one. A fish that was active a few hours ago and now sits still is most likely asleep, Questen says, while a sick fish may sit still all day long. A dead fish floats toward the top once it turns buoyant; a sleeping one settles low.

Which game fish rest at night, and which feed?

None of these game fish has been through a full sleep study; the rest periods come from tracking, tank trials and night observation. The fish facts section answers the other questions about how fish live.

Species	When it rests	Where	For night fishing
Yellow perch	Night; schools break up at dusk in a fixed order	On the bottom, single fish sitting apart	Fish the last hour of light and the first; skip the middle of the night
Bluegill	Probably night (no sleep study)	Tight to docks, weeds and the bottom; a bright light makes it sink and freeze	Dusk and dawn; casts in full dark rarely pay
Smallmouth bass	Probably night (no sleep study)	Rock and hard bottom	Dawn and dusk; leave the spring nest guarder alone
Largemouth bass	By day, offshore near wood	Deeper water near large wood	The swing species: it moves inshore to feed in low light, dusk through dawn
Walleye	No sleep study; hunts in low light	On the bottom near structure	A low-light feeder; the tapetum boosts its vision at dusk and after dark
Channel catfish	No sleep study; feeds by taste and smell	Bottom, holes, current edges	Night is the main shift
Brown trout	No sleep study; stream browns, the big ones above all, feed mostly after dark	Undercut banks, deep pools	Night for the biggest fish
Northern pike	No sleep study; tracked pike were more active by day than at night, late summer through winter	Weed edges	A day fish; treat night as an experiment
Common carp	No sleep study	Not described	Baited spots in a Minnesota lake drew carp most from dusk to dawn

Do bass sleep?

Probably, but nobody has measured it in a bass. The answer comes from tracking and tank trials instead. In a year-long tracking study, largemouth rested offshore near wood by day and moved inshore in low light, where they presumably fed; their ranges were largest in summer. Their eyes set the limit on that night shift. In tank trials, largemouth caught prey more than 95% of the time from dim daylight down to moonlight, 62% by starlight and close to none in total darkness.

So a largemouth hunts by twilight and moonlight, and on a black night it can hardly hunt by sight at all. Two seasons break the pattern. In spring a nest-guarding male, smallmouth or largemouth, stays on duty day and night. In winter a bass slows down without shutting off.

Why do dawn and dusk fish so well?

Dusk is not one event but a sequence. Yellow perch run it in a fixed order: more swimming, a last feed, the school breaking up, slowing down, a final stop. Each species keeps its own timing, so for a spell the water is full of fish changing shifts: day fish hurrying to cover, night fish coming out. Many twilight specialists are predators, and that traffic is what they hunt. The best time of day to fish puts hours on that window by season.

Are winter fish asleep?

No. What looks like sleep in a January bass is metabolic depression. A 2025 study tracked wild largemouth through fall and winter. Heart rate and metabolism fell with the water temperature and stayed low, yet the fish kept slow-swimming and probably fed a little. That is a fish on a low idle, not one that is unconscious; whether fish get cold explains where the savings come from. It will still take a bait dropped in front of it; it will not chase one.

The water temperature chart shows how far the idle drops per species, with trout, walleye and catfish each on their own bands.

Will a resting fish take a lure?

Nobody has put a lure in front of a fish confirmed asleep and published the result, so what follows is inference. A resting fish has a raised arousal threshold, so a crankbait passing 6 ft (2 m) away is background. But its lateral line stays on. A fish tilted against a rock still registers the pressure wave of something landing within a body length. The handling reports add a caution: some resting fish let a hand lift them without reacting at all.

So on a night when the bluegill and perch are down, a lure moving through open water fishes for the hunters. The fish holding tight to cover gets a bait dropped on its nose, or nothing. No field study has measured sleep in bass, bluegill or catfish, so any number you hear for them is a guess.

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