Sleeping fish
watching fish sleep
by Dr Floor Anthoni (2009)
in-depth logo Floor Anthoni


Many fish species sleep at night, whereas some become active. In this article we'll investigate how fish sleep.
  • introduction: in the night one cannot see well, so it's time to sleep
  • the night shift: those who hide by day, use the night to feed
  • finding a bunk: a good place to sleep is in short supply
  • pyjamas: many fish change colour for the night
  • burrowing: a safe bed is found in the sand
  • feeling well: shallow warm water is excellent for naps and sleeps
  • evening spawning: after a busy day, it's time to relax and to spawn
  • daytime nap: with a full tummy and nothing to do, it's time to nap


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Most fish species have excellent eyesight, even though many are colour blind. It allows them to do everything they need to survive, in daylight. Thus they can find their way around, often covering vast territories, and they can find one another. Most also find their food by eyesight. Plankton feeders have their eyes right in front of their heads, which gives them, like humans a good appreciation of depth. Many species can also distinguish colours, especially those living in shallow water where colour matters. During darkness there is nothing else for them to do but sleep.

Most fish are covered in transparent scales, which lets the colour of their skins shine through. It allows them to change their appearance. Some use it to distinguish young from adults or males from females. Some use it by way of body language to signal what mood they are in. Yet others use it to camouflage themselves. Fish who can change their colours easily, often also have 'pyjamas', the coloration they assume when sleeping.

Finding a safe place to sleep can be a nightmare, because at night a number of predatory stalkers are active, like conger and moray eels, scorpionfishes and others like octopus, predating crabs and even conchs (snails). So most fish never really sleep, but are alert enough for making a quick escape. Fish do not have eye lids, so they cannot close their eyes. But judging by their sluggishness around midnight, they must enter a form of reduced consciousness. Perhaps they also dream, something we may never find out. Some parrotfish species in the tropics excrete a cocoon of slime which may deter predators, or just cleanse their skins. Many species are so drowsy when sleeping, that they can gently be held and returned to their bed. (We prefer to catch fish for our aquariums at night, and do so with a tiny hand-held scoop net that we gently place over the sleeping fish. This happens in the bright light of a diver's torch. It does matter what time one dives, as fish become more dopey later at night.)

Fishes of the open ocean sleep in schools, but predators able to hunt by night, such as dolphins using sonar, can still locate and attack them. When sleeping, fishes do not like to be thrown around by waves, so they find a tranquil spot inside a cave, behind a sheltering wall, and so on. Pelagic fish that sleep in the open ocean, descend to where waves are no longer annoying.

Reef fishes that depend for their entire lives on the rocky substrate, would die if they were swept away by currents. So they sleep close to the rock in sheltered places like cracks, where waves and currents cannot reach. Some semi-pelagic nocturnal fish are able to find their way home again, even when the reef where they sleep is far out of sight.
the night shift
Many fish species are active by night. They are able to hunt with their eyes or by their sense of smell. The night shift hides during the day in caves and cracks or they burrow into the sand. The vulnerable plankton eaters usually have big sensitive eyes, and they are good at intercepting fast prey, such as fast swimming rag worms that come out at night, rising to the surface where they feed on dead plankton.
In the open ocean large numbers of small fish and squid rise towards the surface by night, only to disappear into dark depths by day. Sardines and anchovy descend all the way to the bottom to sleep in the sand by day, only to emerge and begin their slow ascent at night towards the surface. During this ascent they have to vent compressed air from their float bladders, and they can do this through a canal that leads to their mouths. Watching a school of sardines or pilchards ascend in early evening, swimming circles through their curtain of bubbles, is truly amazing.

Compared to tropical seas, New Zealand has only few nocturnal marine species, and even octopus that are mainly nocturnal in the tropics, are not normally active by night in NZ. In the tropical coral reefs, the overwhelming nocturnal majority is made up of echinoderms, particularly sea urchins. They appear from tiny holes in the coral, with spines seemingly far too long to hide there. Feather stars and other stars appear, and sea cucumbers emerge from the sand in large numbers.

crayfish or spiny lobsters (Jasus edwardsii)
f034531: crayfish or spiny lobsters (Jasus edwardsii) are nocturnal and seldom found in the open by day. Their eyesight is rather poor, but even so they have a good idea of where they are, as they forage far away from their sleeping dens. Crayfish have a very good sense of smell and home in on a dying or dead animal, from far away, which also makes them easy to trap in lobster pots.
yellowfoot paua (Haliotis australis)
f052127: the yellowfoot paua (Haliotis australis) comes out in the late evening. It grazes the leaves of large seaweeds, often straying far from home. Yet every morning it finds its way back to its sleeping spot in a narrow crack. Other abalone species also become active at night, and home back to their sleeping spots in the morning.
needle urchins (Diadema palmeri)
f031117: these needle urchins (Diadema palmeri) hide under overhangs near the sandy bottom by day, but forage at night. Some are found at the Poor Knights islands, but this photo is from the Kermadec Islands.
featherstar at night
f046607: featherstars like this one, are thought to be attached permanently to where they are found. But this one walks fast by flailing its arms, pulling and pushing itself along at dusk and dawn, some 3-5m from where it sleeps. Niue.
bigeye (Pempheris adspersa)
f034522: the bigeye (Pempheris adspersa) with its light-sensitive eyes, is truly nocturnal, hunting zooplankton at night. It is one of the most common fishes in the warmer waters of NZ, but not usually noticed because it hides during the day.
orange cowrie shell at night
f046403: in the tropical coral reefs a large number of beautiful shells like this cowry, are nocturnal. Notice its tufted mantle that normally covers its shiny orange shell completely. Many cowrie shells have mantles resembling the environment where they live. Niue.
northern conger eel (Conger wilsoni)
f019909: a northern conger eel (Conger wilsoni) searching for small prey and carrion. Notice its pointed snout.
common conger eel (Conger verreauxi)
f019220: the common conger eel (Conger verreauxi) has very muscled jaws with which it snaps its prey stone dead. I then takes the prey home to eat inside its den.
young red cod (Pseudophycis bachus)
f040017: a young red cod (Pseudophycis bachus) cautiously hunting by night.
rock cod (Lotella rhacinus)
f017017: the rock cod (Lotella rhacinus) is very shy, even while hunting at night.
grey moray eel (Gymnothorax nubilus)
f029303: some moray eels hunt exclusively at night, others both by night and by day like this grey moray eel (Gymnothorax nubilus).
northern scorpionfish (Scorpaena cardinalis)
f029112: the northern scorpionfish (Scorpaena cardinalis) has excellent eyesight and hunts mainly by day. However, it can also see well during moonlit nights, when it is also alert. It even makes use of a diver's light to suddenly snap a fish that was disturbed by a diver. These fish still ambush prey, even when there's no room left in their tummies.

finding a bunk
A god spot to sleep is always in short supply, but some fish are very good at finding a bunk to sleep on/in, and one can admire their inventiveness.
red moki (Cheilodactylus spectabilis) sleeping in cave
f016915: red moki (Cheilodactylus spectabilis) gather to sleep inside 'moki holes', little caves that provide good shelter. Inside the cave each has a ledge to sleep on.
red moki (Cheilodactylus spectabilis) sleeping
f051102: an old red moki sleeping on a ledge (a grey sponge) by a crack where it fitted inside when it was much smaller. Fish often stay with their bunks, even when they have outgrown them.
morse code leatherjacket (Thamnaconus analis)
f031235: a morse code leatherjacket (Thamnaconus analis) lies flat on its side under a hard coral. It was holding on to a tuft of red seaweed on right but let go of it while the photo was taken. Its name refers to the blue spot by its anus. Kermadec Islands.
Ambon pufferfish
f043604: an Ambon pufferfish  (Ambon toby)  (Canthigaster amboinensis) has found a suitable crack in the concrete wall of a wharf, and locked itself in place by slightly puffing up its body. Niue.
black-spotted pufferfish sleeping
 f045037: a black-spotted pufferfish has wedged itself into a suitable dent in the coral, by slightly inflating its body. (Arothron nigropunctatus). Niue.
black-spotted pufferfish
f045028: some fish are rather dopey when they sleep and can be handled with care, like this black-spotted pufferfish which is very shy by day. Niue.
sleeping star-eye parrotfish (Calotomis carolinus)
f046904: a sleeping star-eye parrotfish (Calotomis carolinus) with black spotted pyjamas has wedged itself in a crack. Some parrotfishes excrete a protective cocoon of slime. Niue.
redlipped parrotfish (Scarus ubroviolaceus)
f047913: a large (70cm) sleeping redlipped parrotfish (Scarus ubroviolaceus) fits precisely in this dent in the rock where it braces itself with its hard back fins. Niue.
convict surgeonfish (Acanthurus triostegus) sleeping
f043612: a convict surgeonfish (Acanthurus triostegus) is bedding down, ready to change colour for the night. Niue.
convict surgeonfish (Acanthurus triostegus) sleeping
f043718: a convict surgeonfish in pyjamas, shows blotches on its sides (Acanthurus triostegus). It has wedged itself securely inside a narrow crevice. Niue.
scarlet wrasse (Pseudolabrus miles) sleeping
f028914: a scarlet wrasse (Pseudolabrus miles) has made this grey sponge its bedroom, keeping it open wide enough to allow for growth.
baby leatherjacket (Parika scaber)
f018616: a baby leatherjacket (Parika scaber) only 4cm long, is holding on to the hairs of a nesting mussel, in order not to drift away.
sea horses (Hippocampus abdominalis) sleeping
f021720: sea horses (Hippocampus abdominalis) often live in places with strong tidal currents. Fortunately they can hold on to objects with their prehensile tails. Left the male and right the female. They hunt using eyesight, thus sleep by night.

It is not known why fish change colour while sleeping. Perhaps they relax their bodies and their skins, and the colours and patterns that then appear, are their basic colours and patterns which they alter during the day. But there are enough surprises.
pink maomao (Caprodon longimanus)
f041807: a school of pink maomao (Caprodon longimanus) and one coming inquisitively closer. By day they are soft pink in colour with white bellies.
pink maomao (Caprodon longimanus) sleeping
f048407: a young but mature pink maomao in pyjamas. Notice how its skin has darkened, become blotchy, and its belly darkened as well.
pink maomao (Caprodon longimanus) sleeping
f048417: an old pink maomao looks rather scruffy when it sleeps with random blotches and an orange belly.
snapper (Sparus auratus) in dark pyjamas
f048432: snapper (Sparus auratus) in dark pyjamas at night.
male two-spot demoiselle (Chromis dispilus) sleeping
f020621: a mature male two-spot demoiselle (Chromis dispilus) about to bed down. It still has its blue colour and white tail, but its two white spots are already disappearing.
male two-spot demoiselle (Chromis dispilus) sleeping
f019316: a male two-spot demoiselle has become evenly black while sleeping in its little hole in the rock, while stinging jewel anemones and cup corals guard the entrance.
butterfly perch (Caesioperca lepidoptera)
f020421: a butterfly perch (Caesioperca lepidoptera) is normally pale pink with a black spot and pepper spots all over, but this one is already discolouring for a nap during the day.
butterfly perch (Caesioperca lepidoptera) resting
f020031: when sleeping, a butterfly perch becomes dark brown with its black spot just showing through.
butterfly perch (Caesioperca lepidoptera) sleeping
f039918: this butterfly perch was found sleeping with red blotchy pyjamas, which we had never seen before. It appears able to mimic the colours around it.
parore (Girella tricuspidata) in pyjamas during daytime nap
f039806: a parore (Girella tricuspidata) in pyjamas, taking a nap during the day. Its dark-olive colour and white blotches are also its night pyjamas.
tarakihi (Nemadactylus macropterus)
f027332: the tarakihi (Nemadactylus macropterus) is silver by day, with a black neck band behind its head.
tarakihi (Nemadactylus macropterus) sleeping
f035327: a tarakihi in pyjamas is almost unrecognisable, as it has dark commando spots, and its tell-tale black neck band almost missing.
scarlet wrasse (Pseudolabrus miles)
f052701: the scarlet wrasse (Pseudolabrus miles) can have several day-time costumes, and females look quite different too. This is a male in normal active clothing.
scarlet wrasse (Pseudolabrus miles) sleeping
f020135: a male scarlet wrasse in one of its pyjamas.
male orange wrasse (Pseudolabrus luculentus)
f043422: an orange wrasse (Pseudolabrus luculentus) by day can have various costumes, also marking the difference between males and females. This is a male in one of its daytime costumes. On left the female.
male orange wrasse (Pseudolabrus luculentus) sleeping
f043323: a male orange wrasse in pyjamas, tucked away in its sleeping den. Because so many fish tuck themselves deep inside cracks, they are impossible to take photos of.

Some fishes, particularly wrasses, burrow in the sand. Wrasses are the kinds of small fish with a stout body, that row with their breast fins rather than using their tail fins. They are usually gaudily coloured, changing into pyjamas as well. Wrasses are born female and change into male later in life. They form small social groups of females and juveniles, guarded over by one large male. Such groups are called harems.
At dusk, the male takes a position by a suitable spot for burrowing, usually a patch of coarse sand in a sheltered place under a rock. One after another the juveniles and then the females slide into the sand, heads first. Once they are all safely under ground, the male follows. In some species the male stays above ground, as if guarding the sleeping patch. It is very unusual for divers to witness this behaviour, because the fish do not like being watched while burrowing.
male spotty (Pseudolabrus celidotus) sleeping
f010314: a male spotty (Pseudolabrus celidotus) has carelessly burrowed in the sand.
male spotty (Pseudolabrus celidotus) sleeping
f035802: a male spotty in pyjamas. All its male colorations have disappeared, and it looks like a female now, including the black spot and yellow fins.

female spotty (Pseudolabrus celidotus) sleeping
f010309: a female spotty has nestled herself safely in the tangled branches of the tangle weed (flexible weed, Carpophyllum flexuosum).
male spotty (Pseudolabrus celidotus) by day
f048319: a male spotty by day in its male coloration. Note that the round female spot has disappeared to make room for a black spot on its back. Note also the banded saddle straps and bluish anal fins.

male Sandager's wrasse (Coris sandageri)
f028705: the male Sandager's wrasse (Coris sandageri) has gaudy colours compared with the juveniles and females in the background. They do not change colour while sleeping, and neither do they change colour during the day.

feeling well
We've discovered that some fishes migrate to shallow water to benefit from the warmth above the thermocline, a boundary between warm surface water and cooler water below. It is not certain that fish can feel warmth, but when their bodies warm up, their metabolism increases, and this could bring a sense of wellbeing. Thus fish in warm water grow faster and produce egg mass more quickly and also perhaps more offspring. Such warm water migrations happen on a daily basis in summer when there is a marked difference between the warm surface and the colder deep. Such fish forage in the cold deep, and sleep in the warm shallows.
school of mature snapper (Pagrus auratus)
f041232: a school of mature snapper (Pagrus auratus) (±6 years) milling around before sleeping in the shallow warm water of 2-4m deep, usually hidden among kelp plants. Poor Knights, summer.
pregnant female longtailed stingray (Dasyatis tethidis)
f012728: a pregnant female longtailed stingray (Dasyatis tethidis) basking and snoozing in the warm shallows. Stingrays are active at night. Stingrays enjoy internal fertilisation, and bear their young alive.

evening spawning
Some fish species do not spawn massively in a single event, but do so in smaller batches. While they are busy foraging during the day, their minds are focused on finding food, but when evening falls, the fish relax and before bedding down for the night, it becomes time to spawn.
red pigfish (Bodianus unimaculatus) mating
f034704: just before bedding down in summer, these red pigfish (Bodianus unimaculatus) changed into their spawning colours with a star pattern around the eyes. The male is guiding the female to the surface where they spawn. The ceremony is repeated a few times.
female red pigfish (Bodianus unimaculatus) sleeping
f020129: female pigfish sleeping in pyjamas. Notice that the black stripes and spots have gone and  her white has become orange. She will bed down deep inside a crevice.

daytime nap
Many fish species are taking a nap during the day. Once they have fed, solitary fishes have little else to do than take a nap. Plankton feeders take a break when the tide is turning, and the current stops. Some fish do not expend much energy, and they need to feed only once every few days, so it is common to see fish resting during the day.

demoiselles and young blue maomao resting
f041125: demoiselles and young blue maomao share a peaceful moment underneath a protruding stalked kelp. Food was plentiful today and their bellies are full, besides, the tide is turning and the nutritious currents have ceased for a few hours.
young blue maomao resting
f041521: young blue maomao (Scorpis violaceus) taking a nap underneath a protective rock. They seek the shallows for warmth and protection but fear attack from sea birds.
old pink maomao (Caprodon longimanus)
f012133: an old pink maomao (Caprodon longimanus) taking a nap in a vertical position along a vertical cliff face.
goatfish resting
f048822: a group of goatfish (Upeneichthys lineatus) resting with their snouts into the current. Notice how they can colour their skins. When fully at rest, the red colours prevail.
parore returning to their sleeping den for a nap
f001935: four parore (Girella tricuspidata) returning to their sleeping den during the day. Notice the many ledges, as each is owned by a different fish. This sheltered pool can accommodate some 50 fish, each sleeping on its own bunk.

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