What Do Fish Do at Night? (Interesting And Surprising Answers!)

Are you curious about what fish do at night? This interesting, and surprising answer will satisfy your curiosity. Here is a detailed explanation of what fish do at night.

Diurnal fish species rest at night to get energy, while nocturnal fish species come out of inactivity to explore the tank or the natural water bodies for food, and other activities. What species the fish belongs to determines what it does at night.

So this is the short answer. Keep reading to learn more about what fish do at night, how they sleep, when they sleep, and more!

What Do Fish Do at Night? (Interesting And Surprising Answers!) image

What Do Fish Do At Night?

What Do Fish Do At Night? image

Fish do different things at night depending on whether they are diurnal or nocturnal species.

Diurnal Fish

Diurnal fish species rest at night, as many mammals do. But they do not close their eyes to sleep or snore in the water. They rest differently.

Two important ways they rest are by lowering their metabolism and reducing their activities.

Although fish lower their metabolism, and reduce their activity when resting at night, they remain alert to dangers from within, and outside the tank.

How they rest differs depending on the species. 

Research shows that some species rest by fixing themselves in the mud, sand substrate, coral, or other safe spots. 

Some others may find a hidden spot among live plants where they rest to save energy for the next day. 

It is also not surprising to find fish hiding behind appliances in the aquarium during their rest periods.

These rest periods are believed to help them recuperate their lost energy for the next day’s activities.

Nocturnal Fish

However, nocturnal fish species do not lower their metabolism or reduce their activities at night. 

The nighttime happens to be their “active” time. Fish that do not reduce their metabolism or activity at night include nocturnal fish like catfish species, knife fish, and freshwater eel species. 

These fish species usually come out to hunt because the low visibility gives them a good advantage over prey. The prey will hardly see them coming or only find out when it’s too late.

So some fish do not engage in activities as they would during the day as they rest with an eye open to alert them to any possible danger. But others come out at night to hunt because of the advantage the night offers.

Do fish sleep at night?

Fish do not sleep as mammals do, but you will often find them resting to gather strength for the next day’s activities. Diurnal fish typically sleep mostly at night, while nocturnal fish species sleep during the day.

However, when they sleep, they reduce their metabolism by sleeping or floating in place. They wedge themselves in secure spots, fallen tree branches underwater, or fibrous tree roots they find underwater. 

How do fish sleep?

Nocturnal fish usually “sleep” during the day. Recent research shows that catfish usually find a dimly lit area of the river or stream to “sleep.” 

They sleep by staying still, almost as if they are hypnotized. But while they “sleep,” their fins, and tails continue to move slowly to maintain their stationary position.

Interestingly, they hardly seem to be aware or bothered about the presence of other creatures in the water when they “sleep.” 

Other nocturnal species tend to have weird ways of sleeping. For instance, it is common to find newbies in the hobby freaked out when they find their ghost knife fish upside down. Knifefish are nocturnal, and they usually “sleep” in weird positions. 

Some have reported seeing them lay sideways and occasionally moving their fins and tails to maintain their stationary positions. Some other knife fish are known to “sleep” in a vertical stationary position. 

But the situation is different for diurnal fish. They stay active during the day, and “sleep” at night.

However, the “sleeping” pattern is usually the same. They find a safe spot where they can wedge themselves, and stay stationary. 

Fish sleep is like a normal nap that humans take. It helps restore recuperative functions for fish, as in humans.

What Time Do Fish Sleep At Night?

Most fish species rely on what is known as the circadian rhythm to tell when to sleep and when to be active. 

The circadian rhythm is an instinctive cycle of a wide range of behaviors in an animal’s body, that helps them stick to the sleep and wake cycle.

Like humans, fish’s circadian rhythm is usually sensitive to light. Fish rely on signals and cues from their environment to tell when to sleep and when to stay active. One of the main cues they use is light. 

Researchers discovered this fact in a study where they interrupted the sleep of a zebrafish with light exposure. 

Other external stimuli that can reduce or increase the amount of rest a fish gets at night include electrical or mechanical stimuli. This is especially true for fish that rely on electrolocation.

Since light cycles usually regulate circadian rhythms, diurnal fish usually “sleep” when the sun goes down. On the other hand, nocturnal fish will usually find a darker part of the river or reef to “sleep” when the sun comes up. 

This also applies to an aquarium. Many hobbyists find that they can trick their nocturnal pet fish into coming out for food by dimming the lights to imitate natural nighttime and vice versa. 

But in the wild, there are other fish species that don’t see light. For species like these, researchers believe that other environmental cues, such as food, may coordinate their sleep and wake cycle. 

The Mexican tetra, also known as the blind cave tetra, is a ready example. 

Do Fish Close Their Eyes At Night?

Fish generally do not have eyelids like humans and many other land mammals. They do not close their eyes at night when they “sleep.” 

But research shows that they engage in other behaviors that help them “sleep,” just as closing the eyelids helps humans and other land mammals sleep. 

Studies suggest that some fish, like sharks and tuna, do not close their eyes or stay stationary since they have to breathe. They swim all the time to keep breathing, but they are believed to sleep with half of their brains at a time. 

Research also shows that other fish species, such as dolphins, close one eye when sleeping. Scientists call this behavior unihemispheric sleep because the eye that is closed depends on which half of the brain is “asleep.” 

When the right part of the brain is resting, the dolphin closes the left eye, and vice versa.

Dolphins sometimes rest on the seabed, swimming to the top occasionally to breathe. At other times, dolphins have been found to lie motionless on the water’s surface so that they can rest, and breathe simultaneously. 

But they only close their eyes for a short nap at night because the night is when they get the opportunity to hunt squids and other prey rising from below. 

Parrotfish, on the other hand, do not have eyelids, so they cannot close their eyes. But they envelop themselves in a mucus cocoon before zoning out. This mucus cocoon is a gross, sticky protective cover that prevents parasites from attacking as they enjoy a good night’s rest.

So it generally depends on the fish species whether they close their eyes at night.

Can Fish Sleep With Lights On In An Aquarium?

Fish generally prefer to sleep in the dark. But nocturnal species usually stay hidden and inactive when the aquarium is lit.

However, keeping your aquarium lights on for more than 12 hours is not advisable. Keeping the lights on in the aquarium for more than 12 hours can affect fish behavior.

Fish require up to 12 hours of rest, but since rest is generally possible for diurnal species in the dark, it is necessary to ensure that you turn off the aquarium lights. 

Fish need some darkness to rest peacefully. Too much daytime can lead to stress and distort their internal clocks. You need to balance the day and night to ensure that fish enjoy a healthy life. 

Algae blooms are another reason why it’s not a good idea to leave aquarium lights on all the time. Leaving your aquarium lights on all the time can spur the growth of the green algae that normally grows on the aquarium’s wall.

You want to avoid spurring algae to bloom in your tank because too much algae will decrease how much dissolved oxygen is available for your fish. A shortage of oxygen in your aquarium will inevitably lead to an increase in the fish death rate in the aquarium. 

If your aquarium is in a corner of the house that is constantly exposed to light, cover it with a black sheet or cloth to ensure it gets up to 12 hours of darkness.

Do all fish sleep at night

All fish do not sleep at night. Fish are usually diurnal or nocturnal. Diurnal fish usually “sleep” at night. 

Diurnal fish stay active during the day and sleep at night. Many freshwater species are diurnal. Some of the commonest examples in the hobby include Bettas, Angelfish, Neon Tetras, Discus, and Goldfish

But nocturnal fish are fish species that stay awake and active during the night. They usually spend the day resting in dimly lit corners and spaces of rivers, reefs, or aquariums. 

During the night, they hunt for food and engage in other activities. Common examples in the aquarium hobby include different species of catfish, Rainbowsharks, Gobbies, etc.

Can Fish See At Night?

Most fish can see at night. Fish species living in the deep sea have eyes well adapted to the dimly lit environment to help them find prey. 

Some fish species can adjust their sight from brightness to cope with a dark environment. 

How do they do this? 

A systematic adjustment of cones, rods, and pigment granules occurs in the fish’s eyes. In well-lit conditions, the pigment granules will move to the external part of the rods and cones. The cones will advance to permit color-vision for the fish, while the rods will be withdrawn for safety from harsh light rays.

But in dimly lit conditions, the rods will advance to meet any available light while the cones will be withdrawn. This permits the fish to adjust to the dim conditions of the night. 

However, fish vision is almost always useless for most fish at night. Fish mostly rely on special sensory organs to “see” at night. 

In many fish species, “seeing” at night is only possible with the help of their lateral line. 

The lateral line is a row of pressure-sensitive organs on each side of the fish’s body. The function of this organ is to help the fish detect movements from the difference in water pressure.

Other fish species, like electric eels, use an electric field to “see.” The elephant nose fish also relies on receptors to pick subtle electric currents that animals around them emit. 

These lateral lines and electrolocation organs help fish to also navigate the water at night. Although some fish may have some level of night vision, they mostly rely on these sensory organs to “see” at night.


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Do All Fish Sleep On Their Sides?

Different fish species sleep differently. Bettas sleep on their sides. Many whale and dolphin species sleep vertically.

West African lungfish bury themselves in the mud before sleeping. Sand-sleeping Wrasses bury themselves in the sand before sleeping. There is no one way for fish to sleep.

Can I Keep Nocturnal and Diurnal Fish In The Same Tank?

You can keep nocturnal fish with diurnal fish. Having some fish stay active in the tank at every point in time is a great idea. 

But ensure that the nocturnal fish are not predators of the diurnal fish. Otherwise, your nocturnal fish might have your diurnal fish for dinner during their rest.

Can I Feed Fish At Night?

Feeding fish during the day is the best. If you keep nocturnal fish like Catfish, and Loaches, feed them before turning the lights off for the night. 

Use sinking pellets, as this is the best for carnivorous species. They will sink to the tank’s base for them to find as they explore the tank.


Fish that are generally active during the day usually sleep at night. But nocturnal fish see the night as a time to escape inactivity and explore the water. 

Also, irrespective of the species, when fish sleep, they usually lower their metabolism and reduce how much activity they engage in. 

If you are keeping or plan to keep fish in your aquarium, the insights in this article, especially about how much light time is enough and how much is too much, will help you take better care of your fish.





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Prathmesh Gawai
He is the main author and editor at Aquagoodness.com. And he loves to share helpful information on aquarium and/or fishkeeping hobby. Prathmesh has over five years of aquarium and/or fishkeeping experience. Currently, he has a Betta fish tank. He has written hundreds of articles on various aquarium fish species and on fish tank maintenance over the last five years. Connect with him on YouTube here. Learn more about him here.

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