How Do Researchers Know That Fish Sleep? How Do Fish Know When It’s Bedtime? Update 05/2022

Almost all animals take a snooze at some point during the day. The rejuvenation of the mind and body that comes from a good night’s sleep cannot be overstated. Close your eyes and stay still for as long as possible when you’re trying to get some shut-eye. We may become less conscious of our surroundings, and our breathing may slow down as a result of this state. It takes a LOT to get someone who is a heavy sleeper to get them up in the morning!

In the ocean, fish don’t have eyelids since they don’t have to deal with dust. Fish, on the other hand, continue to slumber. Others just wake up once a night while others sleep throughout the day and stay awake all night (just like you and I).

Do Fish Sleep?

Many fish appear to sleep, however fish sleep is different from the sleep we normally associate with fish. Fish sleep is generally referred to as rest by researchers because they haven’t been able to measure the recognized brain wave patterns that characterize human sleep and the sleep of many other animals in most fish. Furthermore, fish without eyelids are unable to close their eyes while resting.

When fish are asleep, they appear to be more attentive than humans, which may allow them to react more quickly to possible hazards in their surroundings. Even so, fish’s metabolism slows down when they’re resting, much like ours does. Additionally, they’ve slowed physically, with a few fish merely floating in place.

How Do Researchers Know That Fish Sleep?

It can be tough to tell if a fish is sleeping or awake because they don’t seem significantly different. 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.

Fish that are more easily preyed upon at night may be diurnal, or active during the day and asleep at night. According to research, the first two hours after sunset are the deadliest hours for some fish species, indicating that these fish are dozing off.

It is common for fish to be sleepy, but researchers have discovered other sleep indicators that fish share with humans and other animals. Melatonin, for example, has been shown to influence zebrafish sleep-wake cycles in experiments. Similar to humans, it appears that zebrafish go through stages of sleep and have a circadian rhythm.

The Stages of Fish Sleep

People sleep in four stages, each of which includes two stages of light sleep, a stage of slow-wave, deep sleep, and a stage of rapid eye movement sleep (REM). 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.

Fish appear to be vulnerable to the effects of sleep deprivation if they do not get enough sleep. A sleep rebound occurs in zebrafish following a night of sleep deprivation.

Like other animals, fish are able to retain a little amount of sleep since this resting condition is so important to them. To put it another way, if fish are subjected to an experiment where their sleep is disrupted, they will finally fall asleep and get at least 5 percent to 10 percent of their typical amount of sleep in microsleeps.

Do Fish Have a Circadian Rhythm?

Like humans and other animals, many fish have a regular sleep-wake cycle. Fish’s circadian cycles appear to be light-sensitive, like our own, making humans more alert in the daylight and more tired at night. The fish’s sleep was disrupted in one study by exposing them to light or applying an electrical or mechanical stimulus. The fish were awakened by both sorts of interruptions, but only exposure to light shortened their sleep duration for days at a time.

A common way that animals regulate their circadian rhythms is through the use of light-dark cycles. The cavefish, on the other hand, are unable to perceive light. Cavefish, which live so deep in the ocean that they are unable to see, have evolved as a result of this. Although they exhibit behaviors associated with a circadian rhythm, researchers believe that additional environmental cues, including as food, may assist govern their sleep-wake cycles.

Why Do Fish Sleep?

We don’t know why we evolved to sleep, according to scientists. The origins of sleep can be traced back to two different theories.

Restorative Sleep Hypotheses

Those who believe in the restorative sleep theory claim that sleep helps the body physically restore itself, strengthens the immune system, and makes new memories. According to this notion, sleepy fish have a decreased metabolic rate as well as a shift in their heart rate while they are asleep.

Adaptive Sleep Hypotheses

According to the adaptive sleep theory, animals’ ability to sleep has evolved in response to their environment, allowing them to avoid danger and conserve energy when necessary.

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

Smaller fish, such as rainbow wrasses, three-spot wrasses, and zebrafish, are diurnal, which means they sleep throughout the daytime. Zebrafish sleep for long periods of time before being jolted awake by any sort of external stimulus. Predators may be fooled by their shifting coloration. 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

Some fish species may require less sleep than others because they live in dark environments. Cave-dwelling Mexican cavefish, for example, have smaller eyes and sleep more frequently than their sighted counterparts who live in rivers closer to the surface.

How do fish know when it’s bedtime?

In order to determine whether or not a fish is dozing, simply observe how still and immobile it is. They are either slow to react or do not respond at all to what is going on around them (see some sleeping catfish here). Their gills will tell you that they’re breathing very slowly if you pay attention.

People who own fish tanks at home know that the fish are less active when the lights go out at night. You can notice how still they are if you switch on a light in the middle of the night.

When it comes to activities like sleeping and eating, fish also have an internal clock, much like humans. As a result, even if you forget to turn off the lights at night, the fish may eventually go off to sleep.

Fisheries that live in dark caverns have been investigated for sleep patterns. Sleep-like periods can be found in even the most active species. In caverns, of course, there is no sunrise or sunset, therefore the fish’s rhythm is different from that of the fish who dwell on the surface in the strong sunlight.

Some fish, such as tuna and some sharks, must constantly swim to obtain oxygen. Like dolphins, these fish probably sleep with half their brains active at a time.

At night, parrotfish form a mucus cocoon around themselves, which may act as a defense against parasites that prey on their vulnerable state.

Fish may dream like people do!

Whether fish have dreams while they sleep is a question that has intrigued many people. Until now, we don’t know, but recent video footage of a sleeping octopus has shown it changing colors, which suggests it was dreaming about hiding from a predator or sneaking up on its own prey (which is why octopuses change color when they’re awake).

Although it may seem strange, the sleep of fish is being investigated in an attempt to learn more about human sleep. Zebrafish are commonly used in these investigations, which investigate the consequences of sleep deprivation, insomnia, and circadian rhythms (sleep cycles).

Here’s an interesting video on the subject of animal sleep, especially that of 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 have distinct physiologic correlations in these species, such as a slowing of the heart rate or a lowered tone in the muscles.

Typically, these sleep-related brain waves are seen in the neocortex of most mammals. 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

Sleep is defined as a period of inactivity, particular body postures, and a decreased attentiveness to the outside world under the behavioral definition of sleep. 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. Several fish species, such as blueheads, Spanish hogfish, and wrasse, also show times of diminished responsiveness where they can be easily handled and moved by scientists. Similar to goldfish and perch, sleeping nurse sharks only respond to stronger-than-usual stimuli when they’re sleeping.

How Do Fish Breathe During Sleep?

To push water over their gills, several species of bony fish, shark, and ray use their mouths to breathe. Allows them to float for long periods of time, breathing as they sleep.

The water must flow over their gills if other species of fish are to survive. According to studies, they may sleep in currents, keep their fins moving, or sleep with only a portion of their brain functioning to keep them breathing as they sleep depending on the species.

Sleep in Fish vs. Land Mammals

REM and slow-wave sleep are clearly separate in mammals. Despite the lack of research thus far, studies on zebrafish imply that they rotate between two distinct sleep periods. Slow-wave sleep, which is common in mammals, is characterized by regular pulses of nervous system activity in the first of these states. The second is more like REM sleep.

Researchers are still trying to figure out if fish are able to sleep in the REM stage of sleep. Rapid eye movement occurs during REM sleep in mammals; the closed eyelids move rapidly, and the brain’s activity increases. Many of the same traits have been discovered in zebrafish, although without the rapid eye movements.

There is evidence that zebrafish and humans may share some of the same hormones and neural processes that control sleep in both mammals and birds.

Where Do Fish Sleep?

Some fish slumber in the water, others float motionless, and yet others hide from predators in the shadows. For instance:

  • The Rainbow Wrasse burrows beneath the sand at dusk and remains there until the sun rises.
  • For security, these fish float beneath coral and build their own cocoons of mucus.
  • Sharks may hide out in caves or on the ocean floor if they feel threatened. Whitetip reef sharks, on the other hand, prefer to spend their nights in a group rather than by themselves.

How Do Fish Sleep?

There is a wide range of sleep patterns among fish species, just like there is among other animal species. It’s possible that fish’s sleep needs have developed based on their surroundings, according to researchers. The surface cavefish, for example, sleep substantially more than their deep-sea cousins.

Fish can be either nocturnal or diurnal, with the latter indicating that they are active at night and the former indicating that they sleep at night. Necropsies like nocturnal sharks, for example, sleep during the day and hunt at night.

Swimming While They Sleep

To stay alive, fish must move water against their gills, which is how they obtain oxygen. It is possible for some fish to receive oxygen while remaining practically immobile by ignoring the stream, remaining floating, and moving a fin every so often. Others, such as sharks and rays that need to move more water over their heads, require bigger movements. These larger fish are able to sleep while swimming due to unihemispheric sleeping.

When a fish goes into unihemispheric sleep, it shuts down one side of its brain at a time. The fish can still swim, albeit at a slower pace than when they are awake, because the other half of their brain is still working. Having eyelids would allow them to sleep with one eye open, like many birds and whales do, while the other eye would remain open.

Estivation vs. Sleep

Eestivation is a state of suspended animation that occurs in dry rather than cold climes. During times of food or water scarcity, fish, reptiles, and amphibians may estivate. Their metabolism slows dramatically during estivation, allowing them to live for a dry season or even years.

Cavefish don’t go into estivation, but they do alter their sleep requirements according on the availability of food in their environment. When they are deprived of food, they sleep more. It’s possible that they’ll sleep less if there’s a lot of food around.

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

There are fish that sleep in cycles, and the majority of the time, they nap at night. Fish, like humans, create hormones that govern sleep cycles and adjust internal body clocks, according to a study from Stanford University School of Medicine.

Fish living in dark caverns, even those species that have evolved to have no eyes, can sleep in a regular daily rhythm, according to Webster.

As an interesting side effect of overexposing fish to light, researchers have found that they sleep longer the following day. After a hard night, “like us, they love a lie in.”

A surprising number of fish species are capable of changing their sleeping habits on the fly. The Canadian Journal of Zoology showed that white sucker fish, for example, can become nocturnal when away from their school. Why? Even though swimming alone is risky during the day, predators are significantly less likely to target them at night.

Humans’ sleep cycles often suffer in the early stages of motherhood, as you may be aware. Sticklebacks, on the other hand, make it seem like a little inconvenience. He must constantly fan oxygenated water over the eggs in order to keep them from suffocating, Webster adds. Fish daddies die from weariness all the time!

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.

Many fish have been observed sleeping on their sides at the bottom of the aptly named seabed, as well as in aquariums and rivers. However, some species, such as the loach, have been observed to sleep with their heads in the clouds (and cause any owner a lot of undeserved panic).

Some people take it a step farther. Damselfish and other coral fish use the reef branches to cling to in an effort to escape being washed away by the ocean current. Even though they’re snuggled away, they can’t afford to lie completely still while they sleep because they need to beat their fins twice as fast as normal to generate an oxygenated water flow across their gills.

Other fish have been known to construct their own beds, which is interesting, but hardly something you’d want to unwind on after a long day at the office. According to Webster, species like the parrotfish create a mucus sack around themselves before they go to sleep.

In addition to making prey more difficult to detect, this cocoon also protects them from parasites like lice. “It’s a brilliant idea, but I don’t think it’s safe for humans to try at home.”

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