During night dives aboard our vessels we, divers, get the chance to see the underwater world from another perspective. And it’s a perspective from which we can learn! As with animals on land, fish can use the night to either hunt or sleep. But, have you ever wondered, how do fish sleep?

The physiology of sleep

Sleep seems to have the same restorative function on fishes bodies and brains as it does for us humans. However, as fish brains are less complex than ours, fish don’t seem to cycle through the sleep stages as we do. Thus, fish don’t seem to experience REM sleep.

Fish sleep patterns are also more flexible than ours. While humans can accommodate night-time work and daytime sleep, this transition is not as easy as you might think it is. However, fish can adjust their sleep more easily, depending on several factors. These include the presence of predators, food availability, and water temperature.

So, how do fish sleep?

Firstly, to be clear, not all fish sleep. Believe it or not, many oceanic fish, such as tuna, don’t actually sleep! This sounds counter-intuitive, but a large function of sleep is to cut down sensory input in order to form memory (hence aiding learning). Since these fish are constantly swimming in deep ocean water, where the scenery doesn’t change much, this seems to negate need to sleep.

For fish that do sleep, there are as many ways to sleep for them as there are in land animals. Some simply drift and occasionally flick their tail or fin to keep them steady. Others snooze under rocks, in holes, or even in nests. Some hover at the surface and some near the bottom.

A parrot fish sleeping in its bubble
A parrot fish sleeping in its bubble

Within schools of fish, some sleep while the others keep an eye out for predators. Parrotfish, on the other hand, secrete a bunch of mucus that surrounds them as they sleeps. This cocoon protects them from parasites and masks their scent from predators, such as moray eels.

How deep under are they?

We are not talking water depth, more how ‘well’ do they sleep. Most species of fish that sleep are never completely asleep as they need to be alert for danger. Dolphins (yes yes, we know, not a fish.. but they live in the ocean), for example, only keep one half of their brain asleep while the other one remains awake so they are always alert.

A dolphin, who may or may not be asleep
A dolphin, who may or may not be asleep. Photo: Martin Reiser

Some fish can stay awake for long stretches when migrating, spawning, or caring for their young (a new mom is a new mom after all!). And some fish don’t sleep until they reach adulthood.

There are some exceptions. The reef dwelling Spanish Hogfish, for example, will fall into such a deep sleep that they can be lifted out of the water without waking up at all. I’m sure we’ve all had nights like that! Speaking of which, zebra fish and even jellyfish can suffer from sleep disorders, such as having trouble drifting off. It’s also known for them to suffer from sleep deprivation.

So, next time you do a night dive or encounter a fish during the day that is not really reacting to you… it’s probably because it’s having a snooze! 😉

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