Do Fish Sleep?

Fish do indeed sleep. It might not be how humans or other mammals sleep but most fish have regular periods of rest when they are less active, less responsive to various stimuli and their metabolism slows down.

When fish look like they are “zoning out” , floating in place without moving much or resting at the bottom of his habitat, they are sleeping. Fish do not have eyelids so it might not look like they are sleeping but they are!

How Do Fish Sleep?

Just like mammals, fish sleep in a variety of ways. Some fish hover at the surface or near the bottom,while others just drift with an occasional fin or tail flick to keep them steady. Some sleep in holes or under rocks and then there are fish that create nests in the sand to sleep in. Still others sleep in groups or schools, while some in the group look out for predators while the rest sleep.

Bizarre fact: Some parrotfish sleep inside a cocoon of mucus they secrete from glands inside their gills. The cocoon protects them from parasites and masks their scent from predators.

The sleep-wake cycles of fish are very similar to ours. Some fish are nocturnal but most are active during daylight and become still and unresponsive at night.

One major difference between fish and human sleep is that fish tend to remain alert to danger even while sleeping. This allows them to 

make a speedy escape if they feel threatened. One notable exception is the reef-dwelling Spanish Hogfish, which sleeps so soundly that it can be lifted all the way to the surface of the water before it “wakes up.”

Why Do Fish Sleep?

Similar to human sleep, fish rest periods seem to serve the same restorative function. It allows their bodies and brains to reset. However, fish brains are far less complex than humans so they do not cycle through the various sleep stages we do. For example, fish do not experience REM sleep.

Fish aren’t as reliant upon nighttime sleep as we are. Fish seem to be able to switch back and forth between nighttime and daytime sleep depending on factors like water temperature, food availability and the presence of predators. Although humans can adjust their circadian rhythms to accommodate work and daytime sleep, it is not an easy transition.

An example of this is the Atlantic salmon that is more nocturnal in colder waters. Although this is likely a safety precaution, cold fish usually are sluggish, and the darkness provides protection from predators.

Do All Fish Sleep?

Fish that swim continuously, like tuna and some sharks, don’t sleep. Though researchers aren’t exactly sure why, they theorize that one of the main reasons animals sleep is to process sensory input and form memories.

Since these fish are constantly swimming in deep ocean water, where the scenery doesn’t change much, they may not need to sleep. The theory is supported by the fact that blind, cave-dwelling fish don’t appear to sleep either.

Some fish that normally do sleep are able to go without it for long stretches when migrating, spawning, or caring for their young. And some fish don’t sleep until they reach adulthood. A study of Mozambique tilapia found that they didn’t show any signs of sleeping at all for the first 22 weeks of life.

Do Fish Suffer from Insomnia?

The zebrafish is a fish that has been the subject of extensive study. When it sleeps there are clear signs like it stops moving, it has a slower response to stimuli, and its breathing and heart rate slow down.

But when prevented from sleep by stimulating the zebrafish, researchers found they showed a rebound effect. When they were left alone in a dark environment, the zebrafish slept more to make up for their lost sleep; but they only did this if it was dark. The fish did not sleep if the lights stayed on. This shows clearly that sleep deprivation does not take the same toll on fish as it does on humans.

The studies conducted on zebrafish provide some insights into the sleep mechanism in humans and why sleep is so universal and it’s evolution.

A new, very interesting study on jellyfish is challenging the notion that you have to have a brain in order to sleep. This study conducted on upside-down jellyfish found that they experienced periods where they were inactive and less responsive to stimuli. They also showed signs of sleep deprivation when kept awake!

All of this shows that even the simplest living creatures have to sleep and that this behavior is much older than we previously thought. Even more research could help us understand the evolution of sleep which will prove very interesting!