Evie Summers
Fish sleep very differently from humans. They don't have eyelids, so they can't close their eyes, and they don't have a neocortex, the part of the brain that slows down activity during sleep. Instead, they rest by reducing their activity and metabolism while remaining alert to danger. Some fish float in place, some wedge themselves into a secure spot in the mud or coral, and some even locate a suitable nest. They may also hover or drift along motionlessly near the surface, lay on the bottom or an object, or hide in caves or under grottos. Fish sleep habits vary depending on the species and environment.
| Characteristics | Values |
|---|---|
| Fish sleep with their eyes open | They don't have eyelids |
| Fish remain partially alert | To avoid predators |
| Fish don't display rapid eye movement (REM) | N/A |
| Fish sleep by slowing down their movements, brain activity, and breathing | N/A |
| Fish remain motionless for long periods | N/A |
| Fish sleep at night | In caves or between plants, coral, and rocks |
| Fish sleep during the day | If they are nocturnal |
| Fish sleep in a suspended-animation-like state | N/A |
| Fish drift through the water | N/A |
| Fish change their usual swimming elevation | N/A |
| Fish hide in shelters | N/A |
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What You'll Learn
Fish don't sleep like land mammals
Fish sleep in their own unique way. They slow down their movements, brain activity, and breathing, and remain motionless for long periods. Some fish hover or float in the water, while others lay on the bottom or hide in caves, plants, or coral. Some fish even bury themselves in the sand or mud. Fish remain partially alert to avoid predators and don't exhibit rapid eye movement (REM) sleep.
The sleep habits of fish vary depending on the species and environment. For example, some fish that need to swim to "breathe" rely on ram ventilation, which means they cannot sleep without motion. These include certain types of sharks, tunas, and mackerels. Fish that are caring for young also don't sleep, and some species, like Tilapia, don't start sleeping until they are 5 to 6 months old.
While fish don't sleep like land mammals, they do exhibit brain activity that suggests sleep. Researchers at Stanford University discovered that Zebra Danios experience slow-wave sleep and paradoxical sleep (deep sleep), just like mammals, birds, and reptiles. However, they did not exhibit REM sleep, and they didn't close their eyes since they don't have eyelids.
Overall, while fish don't sleep like land mammals, they do have their own unique sleep habits that help them rest and rejuvenate.
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Fish remain alert to avoid predators
Fish do sleep, but not in the same way land mammals do. They remain alert to danger and don't display rapid eye movement (REM). They sleep with their eyes open, as they don't have eyelids, and they don't have a neocortex, which is the part of the brain that slows down activity during sleep.
Fish exhibit a variety of sleep behaviours, influenced by their species, habitat, and biological needs. They may slow down their movements, brain activity, and breathing while staying motionless for long periods. Some fish keep moving while resting to ventilate their gills, while others occasionally flick their fins to stay balanced.
The sleep habits of fish may differ depending on their species and environment. For example, surface fish with the most exposure to sunlight are usually active during the day and sleep at night. Fish living near coral reefs are generally active during the day and hide in the reef at night to stay safe from predators.
Some fish sleep in caves or between plants, coral, and rocks, while others hover or drift along motionlessly near the surface. Certain species of catfish and plecostomus, some knife fish, loaches, and others are nocturnal and spend their days sleeping in caves or crevices.
Fish are more likely to rest well when feeling safe and hidden away. Nocturnal predatory fish may feed on smaller sleeping fish, so it's important to keep them in separate tanks.
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Fish sleep with their eyes open
Fish do sleep, but not in the same way land mammals do. Fish sleep with their eyes open because they don't have eyelids. This makes it hard to know when they're sleeping, but there are other signs to look out for. For example, fish are likely sleeping when they remain motionless for periods of time, appear to be laying on the bottom of the tank or hovering right beneath the water's surface, or hiding in a cave or crevice. They also exhibit slower reaction times to food or other stimuli.
Fish sleep by slowing down their movements, brain activity, and breathing while staying motionless. Different species of fish portray distinct behaviours while sleeping, but generally, fish drift through the water, change their usual swimming elevation, or hide in shelters. Some lay on the bottom or even bury themselves in the sand, some hide in caves or under grottos, some hover or drift along motionlessly near the surface, and some nestle into plants, driftwood, corals or other objects and just hang there, occasionally flicking a fin to stay balanced.
Some fish keep moving while resting to ventilate their gills, such as certain types of sharks, tunas, and mackerels. These species rely on ram ventilation, which is where their forward movement pushes water into their gills and allows them to extract oxygen. Therefore, they cannot sleep or rest without motion.
The sleep habits of fish vary depending on the species and environment. For example, fish living near coral reefs are generally active during the day and hide in the reef at night to stay safe from predators. Surface fish with high exposure to sunlight are usually active during the day and sleep during the night. Most aquarium fish are diurnal, meaning they move about during the day and rest at night, while some species are nocturnal and prowl at night, sleeping during the day in a cave or crevice.
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Fish sleep in different locations
Fish sleep is an area of active research, and while fish do not sleep in the same way that land mammals do, most fish do rest. Research shows that fish may reduce their activity and metabolism while remaining alert to danger. Some fish float in place, and certain species of fish, like Tilapia, don't start sleeping until they are 5 to 6 months old.
Some aquarium fish species are diurnal, meaning they move about during the day and rest at night. However, some species are nocturnal and prowl at night, spending the daylight hours sleeping in a cave or crevice. Certain species of catfish and plecostomus, some knife fish, loaches, and others fall into this category.
In the wild, fish do not sleep when they are migrating or caring for their young. Research also suggests that blind cave fish may not sleep due to the constant darkness of their environment, which disrupts their ability to distinguish between day and night.
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Fish sleep in varying positions
Fish that need to swim to "breathe" rely on ram ventilation, which means that any species that rely on this method, such as certain types of sharks, tunas, and mackerels, will not sleep or rest without motion. However, most fish do not need to keep moving while sleeping. Instead, they remain motionless for extended periods, occasionally flicking a fin to stay balanced. They may also exhibit slower breathing, reduced brain activity, and slower response times to stimuli.
The sleep habits of fish differ from those of humans and other land mammals. Fish do not have eyelids, so they sleep with their eyes open, and they do not have a neocortex, which is the part of the brain that slows down activity during sleep. This makes it challenging to determine if a fish is sleeping, but certain behaviours, such as hovering or floating in place, reduced activity, and slow response times to stimuli, can indicate that a fish is in a resting state.
Overall, while fish sleep positions vary, most fish find a spot to hover or rest in a secure location, remaining motionless with slower breathing and brain activity.
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Frequently asked questions
Yes, fish sleep but it's not the same as human sleep. They don't have eyelids so they sleep with their eyes open. They also don't have a neocortex in the brain, which is the part that slows down activity during sleep.
Fish sleep by reducing their activity and metabolism while remaining alert to danger. Some fish float in place, some wedge themselves into a secure spot in the mud or coral, and some even locate a suitable nest. Certain species of sharks and other sea creatures must keep moving, even while resting, to ventilate their gills.
It can be hard to tell if a fish is sleeping because they don't have eyelids or exhibit rapid eye movement (REM). However, some signs that a fish may be sleeping include hovering or floating in place, reduced activity, and a slow response time to stimulus.
Fish sleep locations vary depending on the species. Some fish sleep on the bottom of the ocean or tank, while others sleep in caves, plants, or coral. Some fish even dig a trench or bury themselves in the sand to sleep.
