Have you ever sat in front of your aquarium and wondered, do fish sleep? If so, you are not alone. Wondering if fish sleep is a common curiosity and has been a question for many ages. How can an animal living underwater possibly sleep?

Sleep plays an essential role in overall health and how things function. But does the same apply to things like fish? Continue reading to learn more about fish, fish sleeping habits and sleep patterns, and other things research has to say about fish sleep.

Do Fish Sleep?

The answer to this question is a bit more drawn out than a simple yes or no. Just like humans and other animals, fish do experience sleep. Although, to say that they sleep the same way that most animals do is inaccurate.

Humans and most animals have a state of altered consciousness and reduced bodily functions that help characterize their sleep. Fish sleep differs from this type of sleep. Researchers often refer to fish sleep as rest because fish technically cannot sleep for different reasons.

Why can’t fish sleep like other animals?

The first reason that fish cannot sleep like other animals is simply because of a physical trait. Fish do not have eyelids like mammals, so they are unable to close their eyes. Some people may believe that because of this, fish do not sleep or rest at all. It’s important to understand that fish do not rest in the same way that humans and most animals do.

Another reason why fish are unable to sleep like other animals is because of their anatomy. The neocortex is a part of the brain from which cognitive function originates. It also helps with senses and emotions. When mammals fall asleep, their neocortex shuts off, making it easy to classify their rest as sleep. On the other hand, fish do not have a neocortex in their brain.

Because of these factors, fish technically do not sleep. Scientists refer to fish sleep as resting instead.

How to classify sleep

To further understand how fish sleep, let’s break it down into two categories:

Brain activity

Sleep is classified by brain activity. Scientists and sleep experts recognize sleep in mammals, reptiles, and birds as specific patterns in brain waves. Brain waves in the neocortex go through periods. This includes slow waves and a pattern associated with rapid eye movement (REM).

Although fish do not have a neocortex, researchers have found similar patterns in their dorsal pallium. Some fish seem to cycle between some sort of slow-wave sleep and a similar state of sleep to REM sleep.

Behavior

Sleep can also be categorized as a behavior. Under this category, sleep is considered a period of inactivity and rest. This is usually temporary and cycles with more awake behaviors. This is why it is more accurate to say that fish rest.

Just like every other animal, fish have cycles of rest that allow them to remain alert. This will help them to sense dangers or threats in their environment.

Stages of Fish Sleep

Every living thing that has the ability to sleep will experience sleep in stages. Humans experience sleep in 4 stages:

• 2 stages of light sleep
• Deep sleep or non-REM sleep
• REM sleep

For fish, there are only two types of sleep. The type of sleep that fish experience does not come in stages as human sleep. Instead, fish get one of the two:

Alert rest

This type of sleep is pretty self-explanatory. During alert rest, the brain is sleeping but the senses are still active. This is how fish are able to remain alert during dangerous situations, like being attacked by predators. When in this type of sleep, fish are still able to take action when it is necessary.

Actual sleep

When fish are actually sleeping—or resting, as research would have it—there is no alertness. All bodily functions during this sleep are temporarily put to a stop to allow for complete sleep. You can consider this a standby type of rest.

How Do Researchers Know That Fish Sleep?

It may be difficult to tell when a fish is sleeping because they pretty much look the same when they are awake. It’s not like fish can close their eyes or snore like humans. Although it may be a bit tough to recognize if a fish is sleeping, there are some noticeable physical changes you may see.

Fish become less responsive to things when they are in a state of rest. Maybe you’ve seen an aquarium fish or a pet fish in a home. Knocking against the glass to see if your fish moves doesn’t do much in this situation. You may see some fish near the surface of the water when sleeping. Although the fish may appear to be too unresponsive or a bit too still for your comfort, it’s OK to assume that this is how most aquarium fish sleep.

How Do Fish Sleep?

Whether you call it sleep or rest, fish do have periods of inactivity. While this is true, it’s important to understand that not all fish sleep in the same way. Some fish simply stop moving and remain suspended in the water. During this time, breathing slows down. Some species of fish burrow into the mud at the bottom of the lake or river to take a break. Some other fish may wedge into crevices in rocks or coral to stay in place or hide under rock ledges for security.

Parrot fish have an entirely different way of sleeping. Many fish are attacked at night by predators that are out hunting. Before going to sleep each night, parrot fish spend about an hour building a transparent, mucus cocoon. This cocoon is made from mucus that secretes from the glands in the fish’s gills and acts as a sleeping bag for them. This helps protect the parrot fish from parasitic things, like isopods that attach themselves to fish while they are sleeping.

While this cocoon itself is not strong enough to protect the fish from attacks from bigger predators, the smell of the mucus may discourage animals like sharks and moray eels.

What Time Do Fish Sleep?

First, let’s map out the different times that animals sleep.

• Nocturnal animals tend to sleep during the day and are awake at night. Some examples of nocturnal animals are owls, raccoons, opossums, foxes, and moths.
• Crepuscular animals are active during twilight. That is, during morning and evening hours. These animals usually have short bursts of energy in between rests. Deer, some dogs and cats, and elephants are a few examples of crepuscular animals.
• Diurnal animals are active during the day and sleep at night. This is also the same type of sleep that humans get. Squirrels, bears, and some fish are some known examples.

So, do fish sleep or rest at night like humans and diurnal animals do? Like other animals, some fish are nocturnal, sleeping during the day. Others may be diurnal, sleeping at night. Sharks are nocturnal animals and do their hunting at night.

Knowing animals and the way they sleep is one way researchers have been able to confirm that fish do receive some sort of sleep. Most fish species are diurnal. When predatory, nocturnal animals, like sharks, are out hunting, diurnal fish become the number one prey. A sleeping fish is easier to catch than one that is awake and alert. Researchers realize the death rate in fish increases during this time.

Do Fish Need to Move to Breathe?

It is a popular belief that all fish need to move in order for them to breathe and survive. This is a common misconception that is only true for some fish. Fish breathe through their gills. Water enters their mouths and moves past the surfaces of the gills that are rich in blood vessels. They absorb dissolved oxygen from that water and release carbon monoxide.

When some fish are at rest, their mouths open and close to keep water flowing across their gills. Other species use the pharynx or small openings called spiracles to breathe when they are not swimming.

Some shark species are able to rest and continue breathing. Others must keep moving to breathe. In these species, the spinal cord controls the movement while the brain rests. You may see sharks like these floating at a steady pace underwater.

Can Fish Have Sleep Disorders?

Although a sleeping fish is classified as a resting one, fish can still suffer from sleep disorders. Fish can have problems with insomnia and sleep deprivation. When a fish feels unsafe in its environment, it may eliminate sleeping altogether. Maybe a predatory fish is lingering around or the fish is in an uncommon environment.

For some fish, sleep disorders may not be related to danger at all. California scientists discovered that zebrafish, a common fish used in an aquarium, have a mutant gene that disrupts sleep patterns in a similar way to how insomnia affects humans. Fish with this mutation sleep 30 percent less than fish without it. When they were finally able to sleep, mutated fish could only stay asleep for half the time that normal fish slept.

Just like humans, when fish do not get enough sleep, they have difficulty functioning.

Can Light Affect Fish Sleep?

Light is one of the main determining factors when something or someone is trying to rest. The more light there is, the more difficult it may be to relax. Light is known to slow down melatonin production, which is the main hormone necessary for sleep. In the same way light affects the sleep of humans, it can affect fish.

Leaving the aquarium light on throughout the day and at night can disrupt a fish’s sleep. Humans prefer sleeping in dark areas because it helps them feel more relaxed—the same goes for fish. Too much light when a fish is trying to sleep can cause stress because they require a period of darkness to do so. You may realize that most fish naturally go to darker areas of their environment when they are resting.

Too much light in an aquarium can also cause other problems, like an overgrowth of algae. This leaves your fish in a dirty environment and a big hassle for you to clean.

How to Make Sure Your Fish Gets Enough Rest

If you have a fish and you are worried about how much rest they are getting, there are a few things that you can do to help. First things first, be sure that you are knowledgeable about what type of fish you have. Knowing your fish’s biological makeup can help you determine the sleep habits you should introduce to them.

Give fish day and night.

If fish live in an aquarium, it is your responsibility to make sure that they are experiencing life in the most natural way possible. Giving fish a day and night schedule can help with things like making sure they get enough rest.

Fish getting a day and night schedule can also help with their appetite. Unrested fish may sometimes refuse to eat. Other times, you may notice that they fight a lot with other fish if they live together.

If you have a hard time remembering to turn off the lights in your aquarium, getting a timer is an easy and cost-effective way to get the job done. These timers will help you turn the lights on and off for whatever time you set them.

Make fish feel at home.

Maintaining a comfortable sleep environment helps aid sleep. From the bedding and pillows to the lighting, room temperature, and sound, all of these factors can affect the quality of sleep you get. The same thing goes for fish.

As stated earlier, fish that feel unsafe or stressed can easily have trouble with sleep disorders. Adding things to your aquarium can help create a comfortable environment for your fish. Some ideas of things you can add include coral, plants, and small caves.

Many species of fish share the same type of environment, but be sure to only add things that are acceptable for that species and its functionality.

Choose the best time to feed them.

Humans are advised not to eat close to their bedtime for many reasons. Consuming food late at night can keep you awake. Digestion problems occur from having a full stomach and trying to sleep shortly after. It’s important to know the time your fish sleeps based on their species.

If your fish is diurnal, it’s best to feed them at the start of the day and just before you turn off the lights at night.