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How Do Fish Sleep?

For humans, sleep is essential for survival. This is the same for other mammals, reptiles, amphibians, and other animals as sleep help the body recharge. But is it the same for fish? Do fish sleep?

If you own a pet fish or an entire tank full of them, then you must be curious about their sleeping habits. 

  • Do fish need sleep?
  • How do I know if my fish is sleeping?
  • How long does a fish sleep?
  • How do fish sleep?

These are just some of the most common questions fish owners have about their pets. However, they’re not the only ones curious about the sleeping habits of fish. Fish sleep is being studied by scientists to better understand sleep in humans too. 

In this article, you’re going to find out whether fish sleep or not, how they sleep, when they sleep, and how to know whether your pet is asleep or not.

Let’s get started.

Do Fish Sleep?

Yes, fish do sleep. Some species are nocturnal or they sleep during the day and move at night, while others are diurnal or they sleep during the night and move about during the day. 

Fish also don’t have eyelids as they don’t need them underwater which makes it challenging to tell whether they’re sleeping or not.

Note that most fish do not sleep in the same way that humans do. 

Humans and other mammals go through periods of REM (rapid eye movement) sleep, during which we are mostly paralyzed and the brains are quite active. Fish, on the other hand, enter a state of reduced activity called “tonic immobility.” During tonic immobility, fish are less responsive to their environment and their brain activity is reduced. You can see this condition in sharks as well.

There are a few different theories about why fish enter this state of reduced activity. One theory is that it helps them conserve energy. Since they aren’t moving around much, they don’t need as much energy as they would if they were swimming. 

Another theory is that tonic immobility helps fish avoid predators. When a fish is not moving, it is less likely to be noticed by a predator.

There is some evidence to support both of these theories. Studies have shown that fish are more likely to enter a state of tonic immobility when they are in an environment with low oxygen levels. This suggests that tonic immobility may help fish conserve energy. 

It’s not clear why some fish enter a state of tonic immobility more often than others. It is possible that different fish have different strategies for conserving energy or avoiding predators. It’s also possible that some fish are just more prone to entering a state of tonic immobility than others.

Whatever the reason, it is clear that tonic immobility is a common state for fish. 

How Do I Know If My Fish is Sleeping?

There are a number of ways to know whether your fish is sleeping or not. It’s important to note that most aquarium fish are diurnal so you should catch them sleeping at night.

Here are some signs that a fish is asleep:

  • They haven’t been moving for a long period of time
  • They’re at the bottom of the aquarium
  • They don’t move or are slow to respond even when the things around them are moving
  • They do these things every time the light is turned off

How Long Does a Fish Sleep?

Group of fishes under the ocean
Photo by Francesco Ungaro

Sleep duration depends on the species and how big the fish is. Most aquarium fish sleep anywhere from 8 to 12 hours.

Do Fish Stay Still Asleep?

Not all fish stay still when sleeping. Most fish do stay motionless but other species, like sharks, need to keep moving to be able to breathe properly and ventilate their gills. 

Sleeping habits do vary in aquatic creatures and fish. For example, some species of wrasses and parrotfish need to enclose themselves in a mucus cocoon to be able to sleep.

Do Fish Sleep at the Bottom?

Many species of fish tend to sleep at the bottom or close to the bottom of the tank. However, some tend to float head up, some choose to sleep while floating head down, whereas others can be seen sleeping on their sides.

Do Fish Sleep With Their Eyes Open?

There are a variety of different sleep behaviors exhibited by fish, and whether or not they keep their eyes open during sleep varies depending on the species. Some fish, such as sharks and clownfish, do sleep with their eyes open.

While we may not know exactly why fish sleep with their eyes open or closed, it is clear that sleep is an important part of their lives. So the next time you see a fish swimming lazily through the water, remember that they might just be taking a little nap.

How Do Fish Know When It’s Time For Bed

Fish know when it’s time for bed in a variety of ways. Some species of fish are able to sense changes in the level of light and will begin to head towards their hiding spots when the light starts to dim. 

Other species of fish may have an internal clock that tells them when it’s time to sleep, and they will start to become less active as the night goes on.

There are also occasions when other fish may simply follow the lead of their tank mates and will start to sleep when they see the others doing so. Regardless of how they know, though, once it’s time for bed, most fish will head to the bottom of the tank and find a place to rest until morning.

Where Do Fish Sleep?

How do fish sleep
Photo by Kova Krasilnikov

Fish sleep in a variety of different locations and positions, depending on the species. Some fish, like sharks, sleep in midwater. Other fish, like catfish, sleep on the bottom of a lake or riverbed. Still, other fish, like tuna, sleep by swimming slowly next to another fish or object.

Most fish sleep by suspending themselves in the water. This means that they remain in one spot, usually near the bottom of a body of water, and do not swim around. 

For them to be able to remain in one spot, they use a variety of different methods. Some fish, like catfish, anchor themselves to the bottom with their fins. Other fish, like eels, tie themselves in knots to keep from moving. There are species like flounder, that bury themselves in the sand.

However, there are some species that sleep in groups. This is especially common among schooling fish, like herring or mackerel. When these fish sleep, they often do so with their heads pointing in the same direction. This allows them to quickly swim away if a predator comes near.

Make Sure Your Fish Gets Enough Sleep

Unlike in the wild where fish can easily tell what time of day it is, tanks or aquariums tend to be in a more controlled environment. 

You should make sure that your pet is getting enough sleep by giving them a consistent day and night sleeping schedule.

Like turtle tanks, fish tanks need their own light especially when the room isn’t well lit. The light should be set on a timer so that it automatically turns off at night. This will let the fish know that it’s nighttime and time to sleep. 

Also, provide adequate cover around your aquarium so that the inhabitants would feel comfortable and safe.

Like all living creatures, fish need enough sleep to replenish lost energy during the day so make sure that you provide them with the proper pet care and environment for them to lead healthy and long lives.

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