Yes, fish sleep just like almost every other animal, but fish remain partially alert to avoid predators and don’t display rapid eye movement (REM). Some fish keep moving while resting to ventilate their gills, while other fish occasionally flick their fins to stay balanced.
Unlike mammals, fish don’t have eyelids and sleep with their eyes open — eyes don’t need the same protection underwater. The lack of eyelids makes it hard to know when a fish is sleeping, but there are other signs that show when a fish is asleep and resting.
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How Do Fish Sleep?
Fish sleep by slowing down their movements, brain activity, and breathing while staying motionless (in a suspended-animation-like state) for long periods. Different species of fish portray distinct behaviors while sleeping, but generally, fish drift through the water, change their usual swimming elevation, or hide in shelters. Fish may flick their tails occasionally to remain balanced and keep oxygen moving through their gills.
Certain fish species, like wrasses and parrotfish, secrete a mucus sleeping-bag-like cocoon to protect themselves while sleeping. Unlike humans, fish don’t need to follow a strict circadian rhythm at all times. Certain fish species adjust their sleeping patterns based on migration patterns, food availability, water temperature, and while raising young fry.
Where Do Fish Sleep?
In natural environments, certain fish sleep by floating near the bottom of the lake, ocean, or river. Other species of fish burrow themselves into the sand or wedge between reef branches and rocks to evade predators and prevent being swept away by the current.
In tanks or aquariums, sleeping fish lay at the bottom of the tank, burrow into sand, or hide in caves, under rocks, and other shelters. Some species of fish in aquariums may float near the surface of the tank or hang in between plants, driftwood, or other objects in the tank. Aquarium fish hide while asleep to protect themselves from potential predators in the tank.
When Do Fish Sleep?
Like humans, most fish species follow regular sleep schedules, but these patterns don’t always follow the typical day and night cycle. The time fish sleep differs based on whether the fish is nocturnal, diurnal, or crepuscular.
Nocturnal fish sleep during the day and become active at night because their large eyes and excellent eyesight allow the fish to hunt in low light and the dark. Diurnal fish, like humans, sleep at night and remain active during the day because the fish have limited vision. Crepuscular fish sleep at night, remain inactive during the day, and are mainly active during dusk and dawn.
Diurnal Fish Sleep Schedules
Diurnal fish hunt and move around during the day and sleep at night in a natural environment. Diurnal fish have the same schedule in a tank or aquarium but rely on a cyclical lighting system to know when it’s time to sleep at night.
Fish that keep a diurnal sleep schedule include:
Nocturnal Fish Sleep Schedules
Nocturnal fish sleep during the day in caves or other sheltered areas and become active at night to hunt for sleeping prey in the dark. In an aquarium, nocturnal fish sleep when the lights are on, then become active and feed when the lights are off.
Examples of nocturnal fish include:
Crepuscular Fish Sleep Schedules
Crepuscular fish become active to feed during dawn and dusk and sleep at night. During daylight hours, crepuscular fish are relatively inactive. These fish are rarely kept in aquariums but require a day and night lighting schedule that mimics dawn and dusk times.
Examples of crepuscular fish include:
How Do I Make Sure My Fish Sleep Enough?
Ensure your fish get enough sleep by attaching a timer to the aquarium lights to create a natural day and night cycle. Lighting timers are cost-effective and automatically switch the lights on and off at set times. The fish adapts its biorhythm to when the lights turn on and off. Add moonlight lights at night to watch nocturnal fish become active without affecting the day and night cycle.
Include plants, coral, caves, and other shelters based on the needs of the fish species to provide plenty of safe sleeping spaces. Fish are more likely to rest well when feeling safe and hidden away. Don’t keep nocturnal predatory fish in the same tank as small diurnal fish because predators like catfish opportunistically feed on smaller sleeping fish like neon tetras when hungry.
Avoid feeding the fish during its natural sleeping hours to avoid a change in the fish’s sleeping habits. Add food to the tank at the start of daylight hours for diurnal fish and just before switching off the lights for nocturnal fish.