Herbie Murray
Fish sleep is an area of active research, and it is known that fish need sleep to stay healthy. While fish do not sleep in the same way that land mammals do, they do exhibit brain activity that suggests sleep. Some fish remain still when they sleep, while others must keep moving, even while at rest, to ventilate their gills. Certain fish species, like parrotfish, create a protective mucus cocoon when they sleep.
| Characteristics | Values |
|---|---|
| Sleep pattern | Fish don't sleep like land mammals. They rest by reducing their activity and metabolism while remaining alert to danger. |
| Sleep stages | Researchers have identified two stages of sleep in fish, similar to slow-wave sleep and REM sleep. |
| Sleep deprivation | Fish seem to be vulnerable to the effects of sleep deprivation. They experience a sleep rebound, similar to other animals, where they catch up on sleep. |
| Unihemispheric sleep | Some fish can put one half of their brain to sleep while the other half stays active, allowing them to keep swimming slowly. |
| Sleep protection | Some fish protect themselves while sleeping by retreating to reefs, burrowing in sand or mud, or creating a protective mucus cocoon. |
| Sleep indicators | Signs that a fish is sleeping include lack of movement, slow response to stimuli, floating in place, and reduced respiratory rate. |
| Sleep schedule | Fish are believed to have regular sleep schedules, with diurnal fish active during the day and nocturnal fish active at night. |
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What You'll Learn
Fish do sleep, but not like land mammals
Fish do sleep, but they don't sleep in the same way that land mammals do. They don't have eyelids, so they can't close their eyes, and they don't have a neocortex in the brain, which is the part that slows down activity during sleep. However, they do exhibit brain activity that suggests sleep, and their sleep stages are similar to those experienced by humans: slow-wave and rapid eye movement (REM).
Fish sleep, or rest, in a variety of ways, influenced by their species, habitat, and biological needs. Some fish float in place, some wedge themselves into a secure spot in the mud or coral, and some even locate a suitable nest. Some fish can receive oxygen while remaining almost stationary by facing a current and occasionally moving a fin to stay balanced. Other fish, like some sharks and rays, need to make larger movements to move enough water over their gills.
Larger fish, like sharks, swim more slowly when they sleep. Smaller fish might only be able to occasionally move their fins. Some fish are able to remain completely motionless when they sleep. Certain species of coral fish retreat within the reef before falling asleep. Parrotfish may sleep under coral, or they may create a protective mucus cocoon. Other fish, like rainbow wrasse, may burrow into sand or mud for protection. Some sharks rest in groups.
Fish sleep can also take the form of "sleep-swimming". Diurnal damselfish sleep motionless in crevices within coral reefs at night, but three species (the green chromis, the marginate dascyllus, and the whitetail dascyllus) spend the night between coral branches, beating their fins at twice the rate of normal daytime swimming. This creates water currents that keep the inner zone of the coral and the fish themselves well oxygenated. Although the fish are active, they do not respond to light or the presence of predators.
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Some fish can receive oxygen while stationary
Fish require sleep to stay healthy and live long, happy lives. While fish do not sleep in the same way that land mammals sleep, most fish do rest. Research shows that fish may reduce their activity and metabolism while remaining alert to danger. Some fish float in place, while others wedge themselves into a secure spot in the mud or coral, and some even locate a suitable nest.
Some fish can receive oxygen while remaining almost stationary by facing a current, staying afloat, and occasionally moving a fin 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. Unihemispheric sleep allows a fish to put one half of their brain to sleep at a time. The other half stays awake, allowing the fish to keep swimming, albeit more slowly than when they are awake.
Fish have a specialized respiratory system that includes gills, which are adept at extracting oxygen from the water. As water flows over their gills, fish pull in the oxygen they need and expel carbon dioxide. Water movement is essential for distributing oxygen throughout a tank. Stagnant water can lead to areas with low oxygen levels, which can be harmful to fish. Good water circulation ensures all parts of the tank receive oxygen.
Factors such as water temperature, stocking density, water movement, and the presence of plants or algae play significant roles in oxygen levels. Water temperature directly impacts the amount of oxygen that can dissolve in the water. Colder water holds more oxygen, while warmer water holds less. If the aquarium is too warm, proper aeration is necessary to compensate for the reduced oxygen levels. Fish in warmer water also tend to have higher metabolic rates, increasing their need for oxygen.
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Other fish need to keep moving to ventilate their gills
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 in the brain, which is the part that slows down activity during sleep. However, they do need to sleep on a regular basis to stay healthy and live long, happy lives.
Some fish can receive oxygen while remaining almost stationary by facing a current and occasionally moving a fin to stay balanced. However, other fish, such as some sharks, rays, tunas, and mackerels, need to keep moving to ventilate their gills. This is called ram ventilation, where their forward movement pushes water into their gills, allowing them to extract oxygen. Therefore, any species that rely on ram ventilation will not sleep or rest without motion.
Some larger fish, like sharks, swim more slowly when they are sleeping. They can do this because of unihemispheric sleep, which means that they can put one half of their brain to sleep at a time. The other half stays awake, allowing the fish to keep swimming, albeit at a slower pace.
Some fish that rely on ram ventilation have been observed to exhibit "sleep-swimming". For example, three species of damselfish (the green chromis, the marginate dascyllus, and the whitetail dascyllus) spend the night between coral branches, beating their fins at twice the rate of normal daytime swimming. This creates water currents that keep the coral and the fish themselves well-oxygenated. Although the fish are active, they do not respond to light or the presence of predators.
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Fish exhibit brain activity that suggests sleep
In addition to brain activity, other physiological characteristics of sleep have been observed in fish. For instance, during periods of rest, fish exhibit reduced activity and metabolism, slower breathing and heart rates, and decreased responsiveness to stimuli. Some fish even become motionless and can be picked up by hand without evoking a response. These behaviours are consistent with the concept of "homeostatic regulation," which suggests that animals need a constant amount of sleep every day. If they are deprived of sleep, they will experience a "rebound" effect, where they will sleep more the next few days to make up for the loss. This has been observed in zebrafish, which exhibit a sleep rebound after a night of sleep deprivation.
The sleep behaviours and characteristics of fish can vary greatly between species. Some fish, such as sharks, tuna, and mackerels, rely on ram ventilation, which requires constant movement to push water into their gills for oxygen. These species do not show signs of traditional sleep and may remain active during their typical resting periods. However, it has been proposed that the low sensory input associated with their repetitive swimming movements may allow for memory formation, even in the absence of sleep.
On the other hand, many fish species exhibit more apparent sleep behaviours. Diurnal fish, such as damselfish, are typically active during the day and rest at night, often hiding in crevices or coral reefs for protection from predators. Nocturnal fish, like black-tip reef sharks, exhibit the opposite pattern, becoming active at night and resting during the day. Some fish even create protective "sleeping bags" made of mucus, such as the parrot fish, which surrounds itself with a jelly-like substance when it's time to rest.
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Fish may sleep in nests, under coral, or in mucus cocoons
While fish do not sleep in the same way that land mammals do, most fish do rest. Some fish float in place, while others wedge themselves into a secure spot in the mud or coral, and some even locate a suitable nest. Parrotfish (Chlorurus sordidus), for example, spend about an hour spinning cocoons from their own mucus before settling down to sleep for the night. These transparent, gelatinous balls of mucus are large enough to envelop the parrotfish from head to tail.
Some ocean fish never stop moving, but many fishes live nearly motionless lives, some active by day and others by night. Fish may reduce their activity and metabolism while remaining alert to danger.
The nature of fish "sleep" is an area of active research. Sleep is described as a period of rest in which the eyes are closed and there is little thought or movement. However, except for sharks, most fish do not have eyelids.
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Frequently asked questions
Yes, fish do sleep. They don't sleep in the same way that land mammals do, but they do exhibit brain activity that suggests 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. Some fish can receive oxygen while remaining almost stationary by facing a current and occasionally moving a fin to stay balanced.
Fish do not have eyelids, so they sleep with their eyes open.
No, sleep behaviours and characteristics differ between species. Some fish, like sharks, swim at a slower rate when sleeping. Other fish, like Tilapia, don't start sleeping until they are 5 to 6 months old.
