Updated at: 02-03-2022 - By: Jane Brody

The vast majority of animals take a snooze. A good night’s sleep is essential for reviving the body and mind. When we sleep, our eyes are closed and we remain immobile for an extended period of time. Breathing may become shallower and we may become less conscious of what is happening around us. It takes a LOT to get someone who is a heavy sleeper to get them up in the morning!

Underwater, there is no need for fish to develop eyelids, as dust can’t get into their eyes. Fish, on the other hand, are still dozing off. A small percentage of people sleep during the day and are only awake at night, while another small percentage sleep at night and are up during the day (just like you and I).

Do Fish Sleep?

A wide variety of fish appear to be sleeping, yet fish sleep is very different from what people typically think of as sleep. The recognized brain wave patterns that characterize human and many other animal sleep have yet to be measured in the vast majority of fish, therefore researchers commonly refer to fish sleep as “relaxation.” Because fish lack eyelids, they are unable to close their eyes while they sleep.

While sleeping, fish appear more alert3 than humans, which could offer them a leg up on possible predators. However, fish, like people, are able to slow down their metabolic processes while they are in a resting state. They physically slow down as well, with a few fish merely floating in midair as they do so.

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How Do Researchers Know That Fish Sleep?

To tell if a fish is sleeping or awake might be challenging due to the similarity in appearance between the two states. Fish, on the other hand, are far less responsive when they’re resting. If you touch them, they may not wake up and appear to have gone into a state of deep sleep.

Predators are more likely to target fish at night when they’re resting and less vigilant, which shows that some fish are diurnal. According to research, fish species that prefer to fall asleep in the first two hours after sunset are more likely to die.

Researchers have discovered other sleep signals in fish that they share with humans and other animals, in addition to a lack of attentiveness. Melatonin, for example, has been shown to influence zebrafish sleep-wake cycles in experiments. It is also possible that zebrafish, like humans, have separate stages of sleep and a circadian cycle.

The Stages of Fish Sleep

Our bodies go back and forth between light sleep (two), moderate (one), and deep (one) stages of slumber, as well as one stage of rapid eye movement (REM) sleep. In spite of the fact that fish do not have a neocortex like humans, scientists were able to monitor zebrafish brain activity, heart rate, and eye and muscle movement to document two periods of sleeping that are identical to slow-wave sleep and REM sleep.

Poor sleep appears to have a negative impact on fish’s immune system. Zebrafish go through a sleep rebound after being sleep deprived for a night.

This resting condition is so important that fish, like other creatures, can get by on the barest amount of sleep. In tests in which fish’s sleep is disrupted, for example, the fish eventually go into microsleeps and manage to acquire at least 5-10% of their normal amount of sleep, on average.

Do Fish Have a Circadian Rhythm?

The sleep-wake cycle of many fish is comparable to that of humans and other animals. Similar to the human circadian rhythm, which makes humans more alert in the daylight and tired at night, fish’s circadian rhythms appear to be light-sensitive. Using either light exposure or an electrical or mechanical stimulus, researchers disrupted zebrafish sleep in a previous study. The fish were awakened by both sorts of interruptions, but only exposure to light shortened their sleep duration for days at a time.

In many animals, light-dark cycles serve as a common regulator of their circadian rhythms. Cavefish, for example, can’t sense light because they live deep in the ocean. Many cavefish have developed without eyesight because they live so deep in the ocean. Because of this, experts believe there may be other environmental elements that assist govern their sleep-wake cycles outside their circadian rhythm.

Why Do Fish Sleep?

Our ancestors slept, but scientists don’t know why. The origins of sleep can be traced back to two different theories.

Restorative Sleep Hypotheses

It is claimed that during asleep, the body is able to physically mend itself, strengthen its immune system, and form new memories. Researchers have found evidence that sleeping fish have a reduced metabolism and alterations in their heart rate, both of which support this hypothesis.

Adaptive Sleep Hypotheses

In order to keep animals safe and preserve energy, the adaptive sleep theory suggests that sleep evolved as a result of their environment.

How Do Fish Sleep?

It is possible that the sleeping patterns of different species are influenced by factors such as their social status, physical characteristics, and their environment.

Nocturnal vs. Diurnal Fish

They are all diurnal, which means they sleep at night. Smaller fish like rainbow wrasses or three-spot wrasses are examples of this. Zebrafish sleep for long periods of time before being jolted awake by any sort of external stimulus. The color of their skin can also change, making them more difficult to spot by predators. Sharks, on the other hand, are usually nocturnal or crepuscular, which means they sleep during the day and hunt at night or twilight.

Cave-Dwelling vs. Surface-Dwelling Fish

Dark-adapted fish may require less sleep than their surface-dwelling counterparts. Because they evolved to live in caves rather than rivers, Mexican cavefish that can see have smaller eyes or no eyes at all. They also sleep far less than sighted Mexican cavefish.

How do fish know when it’s bedtime?

If a fish is sleeping, you can tell because they don’t move at all, either at the bottom or just above it. They may not respond at all to what is going on around them (see some sleeping catfish here). You can tell they’re breathing quite slowly by looking at their gills.

Those who own fish tanks at home are well aware that the fish are less active at night. You’ll see how still they are if you turn on a light in the middle of the night.

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When it comes to activities like sleeping and eating, fish have an internal clock that instructs them when to do so. It’s possible that your fish will still drift off to sleep if you forget to turn off the lights.

Fish that live in caverns, where it is always dark, have been researched for sleep patterns. Although these animals appear to be sleeping at times, they are actually resting. There is no sunrise or sunset in the cave, thus the rhythm of fish that live at the surface in broad sunlight may be different from those living in caves.

In order to breathe, many fish such as tuna and sharks must swim constantly. It’s possible that these fish, like dolphins, sleep with only half of their brain active at a time.

When they go to sleep at night, parrotfish form a mucus cocoon around their bodies to protect them from parasites.

Fish may dream like people do!

Whether fish have dreams while they sleep is a question that has intrigued many. There’s been no conclusive evidence, but octopuses are known to change color when they’re awake, so it’s possible the octopus in the video was trying to elude detection by a predator or sneak up on its own prey.

Fish sleep is being examined as a way to better understand human sleep patterns. Zebrafish are commonly used in these investigations, which investigate the consequences of sleep deprivation, insomnia, and circadian rhythms (sleep cycles).

Among the species featured in this fascinating video are fish.

Sleep as Brain Activity

Many creatures, including mammals, birds, and reptiles, have particular patterns of brain activity that define sleep, according to scientific research. Slow-wave sleep and rapid eye movement (REM) sleep are different stages of sleep for these animals, having bodily correlations such as a decreased heart rate or muscular tone.

Most animals’ neocortex is where these sleep-related brain waves are often recorded. Researchers have discovered sleep-like brain waves in the dorsal pallium of fish, despite the absence of a neocortex in these aquatic creatures. It’s not unusual for these brain waves to occur in tandem with changes in eye movement, heart rate, and muscular tone, as they do in other animals.

Sleep as Behavior

According to the behavioral definition of sleep, animals exhibit short intervals of inactivity as well as certain body postures that vary based on species. After a period of sleep deprivation, animals tend to sleep more, which is known as a sleep rebound.

When deprived of sleep, zebrafish go through a sleep recovery stage where they cease moving for regular intervals of time. Blueheads, Spanish hogfish, and wrasse are among the fish species that demonstrate times of diminished reactivity that allow scientists to easily handle and transfer them. Similarly, sleeping goldfish and perch do not respond to contact on the tail, while sleeping nurse sharks only respond to stimuli that are greater than usual.

How Do Fish Breathe During Sleep?

Open-mouth breathing is a common method used by many bony fish, sharks, and ray species. As a result of this technique, they are able to float still for extended periods of time, while still breathing.

The water must flow over their gills if other species of fish are to survive. According to the research, they can either sleep in a current, keep their fins moving, or use only part of their brain to breathe while they sleep, depending on the species.

Sleep in Fish vs. Land Mammals

In mammals, REM sleep and slow-wave sleep are well differentiated. Zebrafish appear to have two distinct sleep phases that switch back and forth, according to preliminary research. It’s similar to slow-wave sleep in mammals in terms of nervous system activity. Both of these are similar to the REM sleep stage.

Researchers are still trying to figure out if fish have a proper REM sleep cycle or not. Mammals’ pulse rates and breathing patterns change dramatically, their closed eyes move rapidly, and their brain activity speeds up during REM sleep. Many of the same traits have been discovered in zebrafish, although without the rapid eye movements.

Zebrafish appear to share some of the same hormones and processes with humans that govern sleep.

Where Do Fish Sleep?

Predators can’t see some fish because they float motionless or sleep while swimming. The following is an example:

  • The Rainbow Wrasse burrows beneath the sand at dusk and remains there until the sun rises.
  • Fish like the parrot fish build their own homes out of mucus cocoons and float beneath the coral to avoid predators.
  • In the event of a shark attack, the animal may seek refuge in a cave or on the ocean floor. Whitetip reef sharks like to sleep in groups, but other shark species prefer to sleep alone.

How Do Fish Sleep?

There is a wide range of sleep patterns among fish species, just like there is among other animal species. According to researchers, fish sleep needs may have evolved depending on the environment in which they live5. The surface cavefish, for example, sleep substantially more than their deep-sea cousins.

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Fishery species can also be diurnal or nocturnal, which indicates that they are active during the day and sleep at night. Nocturnal sharks, for example, sleep during the day and hunt for food at night.

Swimming While They Sleep

Fish rely on the flow of water against their gills to obtain the oxygen they need to survive. In order for fish to absorb oxygen, they must face a current, stay floating and move a fin every now and then while the water goes by. Other fish, such as sharks and rays, require bigger movements in order to move enough water over their gills to metabolize food. These larger fish can sleep while swimming because of unihemispheric slumber.

When a fish goes into unihemispheric sleep, it only sleeps one side of its brain at a time. In the other half, the fish are able to continue swimming, albeit at a slower pace. As with many birds and whales, if they possessed eyelids, one eye would close to correspond with the sleeping side of the brain while the other would remain open.

Estivation vs. Sleep

In dry regions, some fish go into estivation, which is a kind of suspended animation comparable to hibernation. A lack of food or water can lead to estivation in fish, reptiles, and amphibians, among other animals. This decreases their metabolic rate significantly, allowing them to survive for months or even years in the absence of food.

Cavefish don’t go into estivation, but they do alter their sleep requirements7 depending on the availability of food in their environment. They sleep more when they are deprived of food. When they have a lot of stuff to eat, they may not sleep as much as they normally would.

Do fish sleep at night when it’s dark? Do they have sleep cycles?

The fact that many fish appear to sleep at night is true, and they are most active during the day. According to a study from Stanford University School of Medicine, fish create hormones that regulate sleep patterns and set internal body clocks in the same way that people do.

As a result, even fish that have evolved without eyes can sleep at the same time each day in dark tunnels, says Webster.

Even more interestingly, if fish are kept awake for long periods of time, they’ll sleep longer the next day. After a hard night, “like us, they love a lie in.”

This remarkable ability to change sleep patterns is found in a number of fish species. The Canadian Journal of Zoology’s seminal research discovered that white sucker fish may go nocturnal when they are isolated from their school. Why? Even though swimming alone is risky during the day, predators are significantly less likely to target them at night.

If you’ve ever been a new parent, you may have noticed that your sleep cycle begins to suffer. However, species like the stickleback make that sacrifice look like a drop in the bucket. He must constantly fan oxygenated water over the eggs in order to keep them from suffocating, Webster adds. “New fish fathers dying of exhaustion is not uncommon!”

Can fish sleep upside-down? And where do they nap?

According to the main body FishBase, there are an estimated 33,230 fish species in the world.

That being said, many fish have been observed sleeping on their sides at the bottom of the appropriately named seabed — or the bottom of a fish tank or river. There have been reports of fish such as the loach sleeping on their backsides as well (and cause any owner a lot of undeserved panic).

Some even go beyond. Some coral fish, including damselfish, wedge themselves between reef branches to prevent being blown away by the ocean current. In order to breathe, however, they must keep their fins beating at twice the regular speed when snoozing: a flow of oxygenated water must flow across their gills.

Even though it’s fascinating to consider, other fish have been known to construct their own beds — none of which would be ideal for unwinding after a long day at the office. “It may sound disgusting, but species like the parrotfish create a mucus bag around themselves before they go to sleep,” he explains.

In addition to making prey more difficult to detect, the cocoon also protects them from parasites like lice. It’s quite ingenious, but I don’t recommend it for human consumption.

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