Fish sleep in a manner quite different from humans and other land animals. They do not have eyelids and typically rest with their eyes open[3]. Fish reduce their activity and metabolic rate during periods of rest, which serve similar restorative functions as sleep in humans[4],[5]. Fish may float in place, wedge themselves in shelters, or burrow into sand or mud to sleep[4],[1].
The sleep patterns of fish vary by species. Some fish are diurnal (active during the day and sleep at night), while others are nocturnal (active at night and sleep during the day) or crepuscular (active during dawn and dusk)[3],[5]. For instance, diurnal fish like the Mozambique tilapia sleep at night by becoming motionless at the bottom, while nocturnal fish like the brown bullhead rest during the day[2],[5].
Fish can also display unique behaviors while sleeping. Some species, such as wrasses and parrotfish, secrete a mucus cocoon for protection during sleep[6]. Certain fish like zebrafish have shown sleep patterns regulated by melatonin and follow cycles similar to humans[1].
Fish may remain partially alert during sleep to avoid predators and can sometimes continue minimal movements to facilitate breathing[3],[6]. Additionally, some fish demonstrate a sleep rebound effect, sleeping longer if deprived previously[2].
To summarize, fish exhibit diverse sleep behaviors depending on their species, environmental needs, and lifestyle, all without closing their eyes[1],[2],[3],[5].
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