Do Fish Sleep?

Florrie Byrd
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The short answer is yes — most fish have regular periods of rest when they become less active, less responsive to stimuli, and their metabolism slows down.

If you have a fish as a pet, you may have noticed times when he seems to “zone out,” resting at the bottom of his habitat or simply floating in place without moving much. Since fish have no eyelids, it might not look like he’s sleeping — but that’s exactly what he’s doing.

How Do Fish Sleep?

Fish sleep in a variety of ways, just like mammals do. Some simply drift, with the occasional tail or fin flick to keep them steady. Others hover at the surface or near the bottom. Some snooze under rocks or in holes. Some fish even create “nests” for themselves in the sand. Still others sleep in schools, with some in the group resting while others keep a lookout for predators. 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.

Fish sleep-wake patterns are a lot like ours. While some fish are nocturnal, most are active during daylight hours and become still and unresponsive at night. One key difference between fish and human sleep is that fish tend to remain alert to danger even while sleeping, so they can make a quick 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?

While different from human sleep in a lot of ways, fish rest periods seem to serve the same restorative function that sleep does in humans. It’s a chance for their bodies and brains to reset. Because fish brains are far less complex than human brains, fish don’t seem to cycle through sleep stages like we do. For instance, fish don’t experience REM sleep.

Fish also aren’t as locked into nighttime sleep as humans are. While humans can adjust their circadian rhythms to accommodate nighttime work and daytime sleep, it’s usually not an easy transition. Fish, on the other hand, seem able to switch back and forth between daytime and nighttime sleep depending on factors like the presence of predators, water temperature, and food availability.

One example is the Atlantic salmon, which becomes more nocturnal in colder waters. This may be a safety precaution — cold fish tend to be sluggish, and 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.

Can Fish Have Insomnia?

One fish that’s been the subject of extensive study is the zebrafish. The zebrafish shows clear signs when it’s asleep — it stops moving, it is slower to respond to stimuli, and its breathing and heart rate slow. When researchers stimulated zebrafish to prevent them from sleeping, the sleep-deprived fish showed a rebound effect — meaning once they were left alone in a dark environment, they slept more to make up for their lost sleep. However, they only did this if it was dark. If the lights stayed on, the fish didn’t sleep. So clearly sleep deprivation doesn’t take the same toll on fish that it does on humans.

Studying zebrafish sleep is providing some insights into the mechanism of sleep in humans, and further research may help explain why sleep is so universal and how it evolved.

To that end, new research on jellyfish is challenging the notion that you have to have a brain in order to sleep. A study conducted on upside-down jellyfish found that they experienced periods where they were inactive and less responsive to stimuli. What’s more, the jellyfish showed signs of sleep deprivation when they were kept awake.

If even the simplest living creatures sleep, that suggests sleep as a behavior is much older than we previously thought. More research could help us understand why sleep evolved in the first place.

Ever wondered how cows can sleep standing up, or if sloths really are the sleep champs of the animal kingdom? We’ve got more fascinating facts about animals and sleep.