Fish And Rem Sleep: A Deep Dive

Fish don't have eyelids, so it's hard to tell when they're sleeping. But do they sleep? And if so, do they experience REM sleep? While fish don't sleep in the same way that land mammals do, most fish do rest. They remain still, their breathing slows down, and their metabolic functions slow. Some even stop moving completely and can be picked up in your hand without waking up.

Fish do not have eyelids, so they can't close their eyes

Zebrafish, for example, have been found to exhibit brain activity similar to that of humans during slow-wave sleep and REM sleep. During these periods, their metabolic functions slow down, and they become less active and responsive to their surroundings. Some fish even become completely still and can be picked up in your hand without waking up.

The need for "sleep" in fish is likely related to the essential role it plays in consolidating memories of experiences accumulated during their waking hours. While they may not have eyelids to physically close, fish do exhibit signs of sleep through their reduced activity, metabolism, and responsiveness.

The resting habits of fish can vary depending on the species and their environment. Some fish, like reef sharks, lay still at the bottom of the ocean or inside caves when they sleep. Other species, such as parrotfish and wrasses, surround themselves with a mucus cocoon, which may protect them from predators or parasites. Still, others may wedge themselves into mud or coral, float in place, or locate a suitable nest.

While the specific mechanisms may differ, the need for rest and recovery is universal across species, including fish.

Fish do have sleep-like brain activity

The resting habits of fish vary across different species and environments. Some fish lay still at the bottom of the ocean or inside caves, while others float motionlessly near the surface or bury themselves in the sand. Certain species of marine fish, such as parrotfish and wrasses, surround themselves with a mucus cocoon when they sleep, which scientists believe may protect them from predators or parasites.

While most fish remain motionless when sleeping, some species of sharks must keep moving, even while at rest, to ventilate their gills. These sharks have a unique anatomical feature called a spiracle that forces water out of their gills, allowing them to continue breathing. Other fish, like some sharks and rays, need to make larger movements to move sufficient water over their gills, which they can do through unihemispheric sleep.

Unihemispheric sleep allows fish to put one half of their brain to sleep while the other half stays active, enabling them to keep swimming slowly. This type of sleep is also seen in birds and whales, where the eye connected to the sleeping side of the brain would close while the other eye remained open.

Research on zebrafish has provided insights into the sleep patterns of fish. Studies have shown that zebrafish experience two stages of sleep similar to slow-wave sleep and REM sleep, the latter being associated with dreaming in humans. However, it is important to note that during REM sleep, zebrafish do not exhibit rapid eye movement like humans and other animals.

In summary, while fish do exhibit sleep-like brain activity, their sleep patterns differ from those of humans and land mammals. They rest by reducing their activity and metabolism, and some species have unique strategies for sleeping, such as creating mucus cocoons or using unihemispheric sleep to keep swimming. Research on zebrafish has helped scientists understand the sleep patterns and brain activity of fish, providing valuable insights into the ancient origin of vertebrate sleep.

Unihemispheric sleep allows some fish to keep swimming while they rest

Fish do not sleep in the same way that land mammals do. They do not have eyelids, so it can be hard to tell when they are sleeping. However, they do enter a sleep-like state, where their metabolic functions slow down, and they are less active and alert. This state of rest is essential for fish, and they will find ways to obtain a minimum amount of sleep, such as through microsleeps.

Unihemispheric sleep is a unique ability that some fish possess. It allows them to put one half of their brain to sleep while the other half remains active, enabling them to continue swimming and staying alert to their surroundings. This is particularly important for larger fish, such as some sharks and rays, which need to keep moving to ventilate their gills and obtain oxygen. By sleeping with only one hemisphere of their brain, these fish can maintain sufficient movement to keep water flowing over their gills while still getting the rest they need.

Fish sleep, or rest, looks different from the sleep of land mammals. Most fish do not experience REM (rapid eye movement) sleep, a stage characterised by rapid eye movements, muscle twitching, and dreaming. However, research on zebrafish has revealed that they exhibit brain activity similar to REM sleep. They also show signs of slow-wave sleep, a deep sleep stage characterised by low brain activity.

The resting habits of fish vary depending on their species and environment. Some fish, like reef sharks, lay still at the bottom of the ocean or inside caves when they sleep. Other species, such as parrotfish and wrasses, surround themselves with a mucus cocoon, which may protect them from predators and parasites. Still, others, like loaches, float motionless at the top of the water.

While the specific sleep patterns differ, fish, like humans, have biological clocks that regulate their sleep-wake cycles. They follow a circadian rhythm, influenced by factors such as light and food availability. Understanding fish sleep is an active area of research, and it provides insights into the ancient origins of vertebrate sleep and its essential role in the animal kingdom.

Some fish enter estivation, a state similar to hibernation

Sleep is essential for the overall functioning, development, and longevity of all animals, including fish. However, the way fish sleep is quite different from the way humans sleep. Fish do not have eyelids, so they cannot close their eyes during rest. They also lack a neocortex, a part of the brain that shuts down when mammals sleep.

While fish do not engage in deep sleep, they do enter a state of rest, during which their metabolic functions slow down and they become less active. Some fish even become completely motionless and can be picked up by hand without waking up. This resting state is so crucial to fish that they will find ways to obtain at least 5-10% of their normal amount of sleep, even under experimental conditions that interrupt their sleep.

Some fish occasionally enter estivation, a state of dormancy similar to hibernation. Estivation occurs in dry, hot conditions, whereas hibernation occurs in cold climates. During estivation, fish, reptiles, and amphibians experience a drastic slowdown in metabolism, enabling them to survive for a dry season or even years without food or water.

One example of a fish that undergoes estivation is the African lungfish. During the dry season, the African lungfish buries itself in mud and becomes encased in a mucous sheath that gradually hardens. It remains in this state, known as "dry sleep," until the waters return. This estivation process allows the lungfish to survive without food or water for months or even up to five years.

In summary, while fish do not sleep in the conventional way humans do, they do have a form of rest that serves a similar restorative function. Additionally, some fish enter estivation, a state of dormancy similar to hibernation, as an adaptation to survive harsh environmental conditions.

Fish have biological clocks, too

Fish have biological clocks that tell them when to be alert and awake and when to rest, so not all fish sleep at night. Some fish are nocturnal, and some are diurnal (awake during the day, like many humans). These tendencies primarily evolved based on when they are more or less likely to be attacked by predators or what the best times are to find a meal.

Like all animals, fish need to sleep on a regular basis to stay healthy and live long, happy lives. They enter a state of rest where their metabolic functions slow and they are less active. This is a period of restoration when their metabolic functions slow and they are less active. Fish sleep is different from that of land mammals. Most fish don't experience REM cycles, though research on zebrafish suggests they have similar brain activity to REM sleep.

Zebrafish appear to have distinct stages of sleep like humans, and they follow a circadian rhythm. They also experience the 'dreaming' stage of sleep, or REM sleep. When zebrafish are deprived of sleep, they make up for it at the next opportunity. This is called a sleep rebound, in which they catch up on sleep.

The sleep habits across and within different fish species vary. For example, surface cavefish sleep significantly more than those of the same species that live in deep-sea caves. Fish species can also be diurnal, meaning they generally sleep at night, or nocturnal, meaning they are generally active at night.

Frequently asked questions