Colin Snow
Fish do sleep, but not in the same way land mammals do. They enter a state of rest where their metabolic rate slows, and they remain alert to their surroundings. This is crucial for their survival in the wild. While most fish remain motionless when sleeping, certain species of sharks must keep moving, even while at rest, to ventilate their gills. Some fish, like reef sharks, lie still at the bottom of the ocean or inside caves when they sleep. They have a unique anatomical feature called a spiracle that forces water out of the shark's gills so they can continue to breathe.
| Characteristics | Values |
|---|---|
| Do fish sleep? | Yes, fish do sleep, but it's a different form of sleep than what we're accustomed to in land mammals. |
| How do they sleep? | Fish enter a state of rest where they remain still and experience reduced breathing and metabolic rates, as well as lower brain activity. Some fish float in place, some wedge themselves into a secure spot in the mud or coral, and some even locate a suitable nest. |
| Where do they sleep? | Some fish lay on the bottom or 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. |
| When do they sleep? | Most fish are diurnal, meaning they move about during the day and rest at night. However, some species are nocturnal and prowl at night, sleeping during the day in a cave or crevice. |
| How do they breathe while sleeping? | Some fish, like reef sharks, have a spiracle that forces water out of their gills so they can continue to breathe while resting. Other fish, like tuna and some sharks, must keep swimming or slow their swimming to breathe because they don't have spiracles. Scientists hypothesize that these fish may shut off half of their brains, similar to dolphins, to breathe while sleeping. |
| Why do fish sleep? | Sleep is essential for the overall functioning, development, and longevity of all animals, including fish. It helps consolidate memory and promotes good health and disease resistance. |
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What You'll Learn
Fish don't sleep like land mammals
Land mammals typically require a safe, enclosed space to sleep without interruptions. Their sleep patterns involve cycling through different stages, including light sleep, deep or slow-wave sleep, and REM sleep. They also need to breathe while they sleep, which is not a concern for fish.
Fish sleep looks so different from mammalian sleep that many researchers prefer to call it "rest" instead of "sleep". Fish may reduce their activity and metabolism while remaining alert to danger. Some float in place, some wedge themselves into a secure spot in the mud or coral, and some even locate a suitable nest. They don't close their eyes, but they may stop moving completely and can even be touched or handled without waking up.
Some fish, like reef sharks, lay still at the bottom of the ocean or inside caves when they sleep. These sharks have a spiracle, an anatomical feature that forces water out of their gills so they can continue to breathe. Other species of sharks and tuna must keep swimming to breathe because they don't have spiracles to keep water flowing over their gills. In these cases, scientists hypothesize that they shut off half of their brains, similar to dolphins. They slow their breathing and move more slowly but remain somewhat responsive to their environment.
Like all animals, fish need to sleep regularly to stay healthy and live long, happy lives. Sleep plays an essential role in overall functioning, development, and longevity. Fish are vulnerable to the effects of sleep deprivation, and if they are deprived of sleep, they will experience a rebound effect and sleep longer the next time they can.
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They enter a state of rest with reduced breathing and metabolic rates
Fish do not sleep in the same way that land mammals do. However, most fish do enter a state of rest, during which their breathing and metabolic rates are reduced. This state of rest is essential for the fish to stay healthy and live long lives.
During this resting state, fish are much less responsive to their surroundings. Some fish appear to stop moving completely and can even be touched or handled without waking up. For example, the Spanish hogfish, blue-headed wrasse, and even requiem sharks can be picked up by hand without evoking a response. Similarly, Mozambique tilapia are motionless at the bottom at night, with a lower respiratory rate and no eye movement, and they do not respond as readily as during the day to electrical currents or food delivery.
Some fish float in place, while others wedge themselves into a secure spot in the mud or coral, or locate a suitable nest. For instance, diurnal damselfish normally sleep motionless in crevices within coral reefs at night, while some species of sharks and tuna must keep swimming to breathe because they don't have spiracles to keep water flowing over their gills. In these cases, scientists hypothesize that they shut off half of their brains, similar to dolphins. They slow their breathing and move more slowly but remain somewhat responsive to their environment.
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. In a study of zebrafish, researchers interrupted the fish's sleep, either via light exposure or an electrical or mechanical stimulus. Both types of interruptions woke up the fish, but only the light exposure significantly reduced the amount they slept for days at a time.
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Some fish must keep swimming to breathe
Fish do need to sleep to stay healthy and live long lives. However, they do not sleep in the same way that land mammals do. Instead, they enter a state of reduced activity and metabolism, remaining still and experiencing lower brain activity. This state of sleep in fish is an area of active research, and it is believed that fish sleep differs across species. Some fish remain motionless, while others must keep moving to ventilate their gills.
Some fish species, such as bluefish, Atlantic mackerel, tuna, bonito, and some sharks, are known as pelagic fish and swim continuously without exhibiting any signs of sleep, either behavioural or otherwise. These fish require a constant flow of water over their gills to breathe, which means they must keep moving at all times. The constant movement allows them to extract enough oxygen from the water to survive. While they may not show signs of sleep, it is believed that they can still form memories and consolidate their daily learnings while swimming, which is one of the essential functions of sleep.
It is important to note that the sleep patterns of fish can be easily disrupted during periods of migration, spawning, and parental care. Additionally, some fish species, such as the Mexican tetra, have evolved to lose sleep altogether. These fish live in environments devoid of daily time cues, such as permanently dark caves, and display continuous activity without any periods of rest.
While the specific mechanisms vary, the need for sleep in fish is universal. They enter a state of reduced activity and metabolism, which promotes health and disease resistance. This state of sleep allows their brains to form memories and process information. Therefore, while some fish species must keep swimming to breathe, they still experience a form of sleep that fulfils the necessary functions for their well-being.
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Fish sleep patterns are easily disrupted
In the laboratory, periods of inactivity often alternate with periods of activity on a 24-hour basis, or a near 24-hour basis when the lighting conditions are constant. Circadian rhythms of activity have been documented in over 40 different fish species, including hagfish, lamprey, sharks, cyprinids, ictalurids, gymnotids, salmonids, and labrids.
One physiological characteristic of sleep is "homeostatic regulation", which refers to the notion that animals need a more or less constant amount of sleep every day. If an animal is deprived of sleep one day, they will tend to sleep more the next few days to compensate. This has been observed in zebrafish, convict cichlids, and brown trout.
The sleep patterns of some fish species have evolved to accommodate their environments. For example, some populations of Mexican tetra live in permanently dark caves and exhibit continuous activity, suggesting that they have lost the need for sleep during their evolution in an environment devoid of daily time cues.
In summary, fish sleep patterns can be easily disrupted by both natural and human influences. They exhibit similar sleep characteristics to other animals, such as homeostatic regulation, but they may not always display the same sleep behaviours due to their unique environments and evolutionary adaptations.
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Fish sleep behaviour varies across species
Some fish species always live in shoals or swim continuously due to a need for ram ventilation of the gills, for example, and were thus suspected never to sleep. Other fish do sleep, especially when purely behavioural criteria are used to define sleep. For example, zebrafish, tilapia, tench, brown bullhead, and swell shark become motionless and unresponsive at night (or by day, in the case of the swell shark). Some fish species even become motionless and can be lifted by hand all the way to the surface without evoking a response. These include the Spanish hogfish and blue-headed wrasse.
Port Jackson sharks and draughtsboard sharks can lie flat on the bottom with their eyes closed for extended periods, with less sensitivity to electrical stimulation, and their metabolic rate is reduced during these periods. At night, cunner Tautogolabrus adspersus, requiem sharks, the wrasse Halichoeres bivittatus, and many other species can be picked up by hand without eliciting a response. A 1961 observational study of approximately 200 species in European public aquaria reported many cases of apparent sleep.
Some fish species are diurnal, meaning they generally sleep at night, while others are nocturnal, meaning they are generally active at night. For example, nocturnal sharks rest during the day and then swim and look for food at night. Some fish can receive oxygen while in a nearly stationary position by facing a current, staying afloat, and moving a fin now and then as water passes around them. Other fish, like some sharks and rays, need to make larger movements to move a sufficient amount of water over their gills. Due to unihemispheric sleep, these larger fish can keep swimming while they sleep, with one half of their brain asleep and the other half staying active.
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Frequently asked questions
Fish do breathe when they sleep, but they slow down their breathing and metabolic processes. Some fish, like reef sharks, have a spiracle that forces water out of their gills so they can continue to breathe while laying still at the bottom of the ocean or inside caves. Other fish, like tuna and some sharks, must keep swimming to breathe because they don't have spiracles. Scientists hypothesize that these fish slow their breathing and movement, remaining somewhat responsive to their environment.
Fish sleep looks different from the sleep of land mammals. Most fish remain motionless and experience reduced breathing and metabolic rates, as well as lower brain activity. Some fish can even be picked up in your hand without waking up. However, since fish do not look very different when they are asleep, it can be difficult to determine whether they are resting or awake.
No, each species of fish likely has a slightly different sleep ritual. Some fish float in the water column, some wedge themselves into a secure spot in the mud or coral, and some locate a suitable nest. Some species of sharks must keep moving, even while at rest, to ventilate their gills.
