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With over 30,000 species of fish inhabiting freshwater, marine, and aquariums worldwide, every fish requires rest to support cognition and overall health. While fish do sleep, their sleep patterns differ markedly from those of mammals and even other fish species.
Unlike humans, who enter a state of unconsciousness, fish can remain partially aware of their surroundings during rest. This allows them to detect predators while their metabolism slows, swimming ceases, and gill pumping reduces significantly.
Many fish use their swim bladder to float at a stable depth, keeping them from drifting upward or sinking while they sleep. Others find refuge among coral, vegetation, rocks, or even in mud. Certain species, such as parrotfish, create a mucus cocoon around themselves to shield against parasites and predators during rest.
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Sharks and whales share some oceanic behaviors—like being drawn to seamounts—but their sleep strategies are distinct. Great white sharks, for instance, must keep swimming to ventilate their gills, so they glide through currents or reduce speed while resting. Other species that can breathe without continuous movement—such as the thresher, Port Jackson, and wobbegong sharks—may rest on the seafloor, in shallow caves, or under reef rocks, often with one or both eyes partially or fully closed.
Marine mammals such as whales and dolphins have a different challenge: they breathe air at the surface. They cannot rest on the seafloor and instead employ unihemispheric sleep—one half of the brain sleeps while the other remains alert, periodically surfacing for air. Dolphins may float motionless near the surface, swim slowly, or rest on a shallow seabed, closing one eye. Sperm whales are known to sleep vertically, resembling logs, allowing them to rise for air efficiently.
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Research published in Nature (2019) revealed that zebrafish exhibit two distinct sleep stages analogous to human slow‑wave and REM sleep. These stages suggest that neural sleep signatures evolved over 450 million years ago. Zebrafish also display a circadian rhythm, sleeping predominantly at night—except for species like eels and rays that have different patterns.
In aquariums, fish typically sleep for 7–12 hours daily, though wild populations may vary. Sharks seem to have circadian patterns but often take naps of 10 minutes or less. Sperm whales and some other marine mammals also sleep in short bursts, usually 10–15 minutes at a time.
These findings highlight that even marine life possesses complex sleep mechanisms essential for survival, cognition, and health.
