Do Fish Sleep? Sleep-Like Rest States in Fish

How researchers define sleep in fish, what zebrafish studies show about its regulation, how species rest at night, why some fade in colour, and what constant light does.

How sleep is defined in fish

Fish have no eyelids and no neocortex, so the brain-wave patterns and closed eyes used to recognise sleep in mammals do not apply. Researchers instead use behavioural criteria: a prolonged period of inactivity, a typical resting posture (often in a particular shelter), alternation of rest and activity over a 24-hour cycle, and a raised arousal threshold, meaning that a stronger stimulus is needed to provoke a response. A further hallmark is homeostatic regulation: when rest is prevented, the animal makes up for it later with more or deeper rest.

Evidence from zebrafish

The zebrafish (Danio rerio) is the main vertebrate model for fish sleep research. A 2026 review from the California Institute of Technology summarises that larvae are free-living from about the fifth day of development and already show day-night sleep/wake cycles controlled by both a circadian and a homeostatic process. In larvae, arousal threshold rises sharply after one minute or more of inactivity, so one minute or more of stillness is used as the operational definition of a sleep bout. Depriving larvae of sleep by mechanical, visual or drug-based disturbance is followed by increased sleep, which shows homeostatic control.

Melatonin: zebrafish mutants unable to make melatonin (aanat2 mutants) keep normal circadian rhythms but sleep less at night under a light-dark cycle, indicating that melatonin acts downstream of the clock to time sleep.
Galanin: sleep deprivation raises galanin expression in the preoptic area and hypothalamus, and galanin-deficient fish fail to show rebound sleep afterwards.
Hypocretin (orexin): raising hypocretin signalling promotes wakefulness, ablating hypocretin neurons reduces sleep in larvae, and loss of hypocretin signalling fragments sleep in adult zebrafish, similar to findings in rodents.
Serotonin: loss of the serotonergic raphe nuclei reduces sleep amount, sleep depth and the response to sleep deprivation.

In adult zebrafish, according to Wikipedia's summary of the literature, resting fish at night hang horizontally with mouth and gill movements reduced by almost half and are about twice as hard to arouse as during the day.

How different species rest at night

Sleep-like rest has been described in many species, each with its own posture and site. The brown bullhead rests on the bottom with fins spread and body tilted, with lower heart and breathing rates. The Mozambique tilapia stays motionless at night with slower breathing and reduced responsiveness to food. Some wrasses and parrotfish surround themselves or their shelter entrance with a mucus envelope, and divers have been able to lift sleeping Spanish hogfish and bluehead wrasse to the surface without a reaction. A 2024 review in the Journal of Experimental Biology adds aquarium observations: neon and cardinal tetras (Paracheirodon innesi and P. axelrodi) become quiescent at night, cardinal tetras have been seen resting near the bottom, banded knifefish (Gymnotus carapo) rest head-up at about 30 degrees, several Barbus species tilt at 45-60 degrees, and Nothobranchius korthausae rests near the surface with its head tipped up. The cleaner wrasse Labroides dimidiatus uses a sleeping cave in aquaria much as it does on the reef.

Not all fish show sleep. Continuously swimming pelagic fish such as tunas and Atlantic mackerel show no clear signs of it, and cave populations of the Mexican tetra (Astyanax mexicanus) sleep about 2 hours per 24 hours compared with about 8 hours in surface populations. Some fish skip sleep at particular life stages: spawning and parental care suppress night rest in threespine stickleback and convict cichlids, among others, and juvenile Mozambique tilapia do not show sleep while adults do. Draughtsboard sharks are nocturnal and rest during the day.

Colour fading at night

Some fish look different while resting. In the neon tetra the blue and red areas turn greyish or dark at night and return when the fish becomes active in the morning; the iridescent stripe is produced by light-reflecting guanine crystals and shifts from blue-green in a light-adapted state to indigo in a dark-adapted state. In zebrafish embryos, a laboratory study found that surface melanin decreased under constant light and increased under constant darkness, an effect the authors linked to melanin-concentrating hormone and sleep pressure. Pale colour seen at lights-on is therefore not, by itself, a sign of illness in species known to change colour at night.

Light regime and disturbed rest

Light is the main cue that sets the daily rhythm, alongside temperature, food availability and predation risk. The 2024 review notes that constant light disrupts the circadian system as a whole and that many fish species do not reduce night activity under constant light. In older studies, larvae of the shanny (Lipophrys pholis) kept under constant light died at a higher rate than those on a normal photoperiod, and two wrasses of the species Coris julis became erratic and died under intense constant light. Dim light at night caused heart damage in zebrafish embryos in a 2026 laboratory study. Zebrafish deprived of sleep at night by water flow showed fewer startle responses the next day, showing that broken rest changes daytime behaviour.

These findings describe laboratory and wild fish; the reviewed sources do not give a specific number of dark hours for aquarium species. Related topics are covered in the stress-in-fish-guide and fish-stress-physiology-cortisol articles.