Ashwin Lewis
Fish sleep is an area of active research. While fish do not sleep in the same way land mammals do, they do exhibit sleep-like behaviours. 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 remain still, their breathing slows down, and some can even be picked up in your hand without waking up. Fish sleep looks so different that many researchers prefer to call it rest instead of sleep.
| Characteristics | Values |
|---|---|
| Fish sleep | Fish do sleep but it is different from how land mammals sleep. |
| Sleep patterns | Sleep patterns vary from species to species. Some fish sleep at night while others are nocturnal. |
| Sleep posture | Some fish sleep at the bottom of a water body, some bury themselves in the sand, some hide in caves, some hover or drift along the surface, and some nestle into plants, driftwood, or other objects. |
| Brain activity | Fish do not have a neocortex like humans, but they do experience reduced brain activity during sleep. |
| Eye movement | Fish do not have eyelids, so they do not close their eyes during sleep. However, some species, like sharks, close their eyes during sleep. |
| Metabolism | Fish experience reduced metabolic rates during sleep. |
| Respiration | Fish experience reduced breathing during sleep. |
| Responsiveness | Fish are less responsive during sleep and may even stop moving completely. |
| Memory formation | Sleep may allow the fish brain to consolidate information and form memories. |
| Circadian rhythm | Fish appear to have a circadian rhythm that is sensitive to light, similar to humans. |
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What You'll Learn
Fish don't sleep like land mammals
So, do fish sleep? Well, yes, but not in the conventional way. Fish enter a state of rest where they remain still, experience reduced breathing and metabolic rates, and exhibit lower brain activity. Some fish float in place, while others wedge themselves into secure spots in the mud or coral, or locate a nest. This sleep-like behavior is essential for their health and cognitive performance, just like in other animals.
The sleep patterns of fish vary across species. Some fish lay on the bottom or bury themselves in the sand, while others hide in caves or hover near the surface. Certain species of sharks must keep moving, even while resting, to ventilate their gills. Fish sleep is also influenced by factors such as migration, spawning, and parental care. For example, wild fish do not sleep when they are migrating, and many fish do not sleep when caring for their young.
The circadian rhythms of fish are sensitive to light, similar to humans, and they exhibit sleep-like behavior in response to light-dark cycles. Interestingly, even blind cavefish, which live in perpetual darkness, display behaviors consistent with a circadian rhythm, suggesting that other factors like food may regulate their sleep-wake cycles. Researchers have identified melatonin as a potential regulator of these cycles in zebrafish, which also show distinct stages of sleep similar to those seen in humans.
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Fish remain alert to danger while sleeping
Fish do sleep, but it's not in the same way land mammals do. Fish sleep is a different form of rest than what we're accustomed to in other animals. While they remain still, their breathing slows down, and their metabolic rate decreases, they are more alert than humans during sleep. This alertness may be due to their need to react to potential threats in their environment.
Fish do not have eyelids, so they cannot close their eyes. They also lack a neocortex, which is the part of the brain that shuts down when mammals sleep. Despite these differences, researchers have found that fish exhibit sleep patterns similar to those of mammals, birds, and reptiles. For example, Stanford University researchers identified slow-wave sleep and paradoxical sleep (deep sleep) in Zebra Danios, similar to that seen in other animals. However, during their paradoxical sleep, they did not exhibit Rapid Eye Movement (REM) sleep, which is typical of humans and other animals.
Fish sleep patterns can be disrupted by various factors, such as migration, spawning, and parental care. Some fish, like Tilapia, don't start sleeping until they are 5 to 6 months old. Wild fish also don't sleep when they're migrating, and certain species of sharks must keep moving, even while resting, to ventilate their gills. Additionally, blind cavefish may not sleep due to the constant darkness of their environment, which disrupts their ability to distinguish between day and night.
The circadian rhythms of fish are sensitive to light, similar to humans, who feel more alert during daylight and sleepy after dark. In a study of zebrafish, researchers found that interrupting their sleep with light exposure significantly reduced the amount of sleep the fish got for days afterward. This suggests that light plays a crucial role in regulating the sleep-wake cycles of fish, similar to many other animals.
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Some fish float, while others find a nest
Fish do not sleep in the same way that land mammals do, but they do rest. Some fish float in place, while others wedge themselves into a secure spot in the mud or coral, and some locate a suitable nest. Fish that create nests include sticklebacks, callochromis cichlid fish, hornyhead chubs, gouramis, catfish, plecos, and mudskippers.
Bubble nests, also called foam nests, are created by some fish and frog species as floating masses of bubbles blown with an oral secretion, saliva bubbles, and occasionally aquatic plants. Fish that build and guard bubble nests are known as aphrophils. Osphronemidae, containing the bettas and gouramis, are the most commonly recognized family of bubble nest makers. The nests are constructed as a place for fertilized eggs to be deposited while incubating and guarded by one or both parents (usually solely the male) until the fry hatch.
Bubble nests are built even when not in the presence of a female or fry, though often a female swimming past will trigger the frantic construction of the nest. The nests are built by the male (sometimes females) and their size, position, and shape depend on the species. They are often built near an object that breaks the surface of the water, which forms a base for the nest. Bubble nests created by male Siamese fighting fish (Betta splendens) are made from air bubbles coated with saliva to increase durability. The creation of the bubbles is audible and often frantic.
After spawning, the eggs either float up into the bubble nest or are carried there and subsequently lodged into the nest by one or both parents. Following this, the male protects the brood by chasing away the female (if not a species in which the female also guards the nest) and any other intruders, concentrating on the eggs in the nest, retrieving any eggs or fry that fall from the nest, and keeping the nest in repair.
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Fish have a half-brain sleeping trait
While the nature of fish "sleep" is an area of active research, it is known that most fish do rest. However, they do not sleep in the same way that land mammals sleep. Fish do not have eyelids or a neocortex, so they cannot close their eyes, and their brain activity is different. Nevertheless, they do exhibit a lack of responsiveness and reduced activity, which are often signs of sleep. Some fish even become motionless and can be picked up by hand without reacting.
Interestingly, some marine mammals like dolphins possess a 'half-brain sleeping' trait, which allows them to sleep and swim simultaneously. This trait ensures that they do not drown while sleeping. Dolphins must retain control of their blowhole, a flap of skin that opens and closes under their voluntary control, to avoid sinking underwater. While they may rest one side of their brain, the other half remains alert to ensure they can breathe and recognise when they have reached the surface.
Similar to dolphins, fish also experience reduced brain activity during rest. Researchers have monitored the brain activity of fish like zebrafish and zebra danios, identifying slow-wave sleep and paradoxical sleep (deep sleep) without the presence of REM sleep. This suggests that fish may indeed possess a form of the 'half-brain sleeping' trait, allowing them to remain somewhat alert to their surroundings while resting.
Additionally, fish have been observed to have regular sleep schedules, with most aquarium fish being diurnal and nocturnal species prowling at night. They seek out typical shelters, such as holes, crevices, vegetation, or sand, to rest. Some fish even wedge themselves into secure spots or locate nests. This behaviour indicates that fish do need to rest and may have evolved the 'half-brain sleeping' trait to remain vigilant even while sleeping, thus avoiding potential threats or predators.
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Fish sleep patterns are easily disrupted
In addition, sleep patterns can be disrupted by light exposure. In one study of zebrafish, researchers found that exposing the fish to light significantly reduced the amount they slept for days at a time. This is because the circadian rhythms of fish appear to be sensitive to light, similar to the human circadian rhythm that makes us feel alert during daylight and sleepy after dark.
Some fish species also experience disrupted sleep patterns due to a lack of eyelids. Gills and eyes that remain open can make fish more vulnerable to predators at night, suggesting that they are diurnal, or most active during the day. However, this also means that they are more alert to potential threats in their environment, even while resting.
Finally, fish sleep patterns can be influenced by their environment. For example, blind cavefish, which live in permanent darkness, do not appear to sleep, possibly because they cannot distinguish between day and night. Similarly, some populations of Mexican tetra that live in dark caves exhibit continuous activity, suggesting that they have evolved beyond the need for sleep in their constant dark environment.
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Frequently asked questions
Yes, fish do sleep, but it doesn't look like the way humans or other mammals sleep. Fish don't have eyelids, so they don't close their eyes, and they don't have a neocortex. Instead, they enter a "low power mode", remaining alert to their surroundings while slowing down physically and becoming less responsive.
Fish sleep in different ways depending on their species. 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 tend to seek out quiet, calm locations where they feel protected, such as caves, plants, or other hidden spots.
It can be tricky to know if a fish is sleeping since they don't have closed eyes or any other obvious signs. However, some common signs of sleeping fish include reduced movement, slower breathing, and lower responsiveness to stimuli. They may also exhibit typical resting postures or behaviour patterns, such as seeking out specific locations or shelter.
No, different species of fish have different sleeping patterns and behaviours. Some fish are diurnal, active during the day and resting at night, while others are nocturnal or crepuscular. Some shark species, for example, must keep moving even while resting to ventilate their gills.
Fish don't have eyelids, so they can't close their eyes in the traditional sense. However, they do experience reduced eye movement and lower responsiveness during their resting state, which is similar to sleep in other animals.
