Do Fish Sleep? They Have to ... Right?

sleeping fish — Do fish sleep like you and me? Marina Avgust/Shutterstock

It's not unreasonable to ask, "Do fish sleep?" After all, when was the last time you actually saw one of these little swimmers taking a snooze?

It's hard to tell when a fish is sleeping because they don't have eyelids (or they lack ones that close, if in the case of sharks). But scientists know that, while they don't engage in deep sleep, fish rest, and some have some very interesting ways of doing it.

Contents

How Do Fish Catch Z's?
Fish Have Biological Clocks, Too
How Fish Sleep Differs From Land Mammals' Sleep
Sleeping Patterns Vary Among Fish Species
Impact of Light and Environment on Fish Sleep

How Do Fish Catch Z's?

Fish don't sleep the same way that you, your dog — or any mammal for that matter — does. Most don't experience REM cycles (though research on zebra fish suggests they have similar brain activity to rapid eye movement (REM) sleep).

Still, scientists have observed that many species of fish rest. This is a period of restoration when their metabolic functions slow and they are less active.

Some fish, like reef sharks, lay still at the bottom of the ocean or inside caves when they sleep. These sharks have a unique anatomical feature called a spiracle that forces water out of the shark's gills so they can continue to breathe.

Some species of sharks and tuna must keep swimming to breathe because they don't have spiracles to keep water flowing over their gills. In their cases, scientists hypothesize that they shut off half of their brains like dolphins do. They slow their breathing and move more slowly but are still somewhat responsive to their environment.

Fish Have Biological Clocks, Too

clownfish — Clown fish (like Nemo) often sleep near sea anemones, which protect them from predators. ©RAZVAN CIUCA/Getty Images

Like humans, fish have biological clocks that tell them when to be alert and awake and when to rest, so not all fish sleep at night. Some fish are nocturnal, and some are diurnal (awake during the day, like many humans).

These tendencies primarily evolved based on when they are more or less likely to be attacked by predators or what the best times are to find a meal.

How Fish Sleep Differs From Land Mammals' Sleep

Most fish don't have eyelids and do not fall asleep in the same way land mammals do. Many fish enter a state of rest where their metabolic rate slows, but remaining alert to their surroundings is crucial for their survival in the wild.

Land mammals, on the other hand, typically require a safe place to get enough sleep without interruptions. Their sleep patterns involve cycling through different stages, including light sleep, deep or slow wave sleep and REM.

Sleeping Patterns Vary Among Fish Species

Each species of fish likely has a slightly different ritual to grab some shuteye than any other. Here are just a few distinct types of fish and their unique sleeping patterns.

Cavefish: Cavefish populations have adapted to environments with little to no light and can exhibit sleep loss without the adverse effects seen in other fish.
Clown fish: Clown fishes bury themselves in sea anemones to protect themselves from predators while they sleep.
Loaches: Loaches sleep like the dead. These fish float to the top of the water and remain completely still in suspended animation.
Parrot fishes: The parrot fish creates enough mucus to form a cocoon around themselves at night when they stop moving. This mucus blanket is like a sleeping bag.

Impact of Light and Environment on Fish Sleep

Most fish need very little aquarium light to sleep well. Fish in brightly lit tanks may experience sleep deprivation because their natural circadian rhythms are disrupted.

Providing a dark environment during their natural rest periods is important for their health and well-being. After all, the ocean and most natural bodies of water get pretty dark!