During night dives, we, divers, get the chance to see the underwater world from another perspective from which we can learn! Fish take this time to either hunt or sleep. Have you ever wondered about fish sleeping patterns?

The physiology of their sleep

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

Fish sleep patterns are more flexible than ours. While humans can accommodate nighttime work and daytime sleep, this transition is not that easy, However, fish can adjust their sleep depending on several factors, such as presence of predators, food availability and water temperature.

How do they sleep?

A parrot fish sleeping

There are as many ways to sleep for fish as there are species. Some simple drift and occasionally flick their tail or fin to keep them steady. Others snooze under rocks, in holes or in a nest . Some hover at the surface or near the bottom. In a school of fish, some sleep while the others keep an eye out for predators. Parrotfish, on the other hand, secretes a bunch of mucus that surrounds it as it sleeps. This cocoon protects them from parasites and masks their scent from predators

How deep do they sleep?

First we have fish, like tunas, that in fact don’t sleep! Since these fish are constantly swimming in deep ocean water, where the scenery doesn’t change much, they may not need to sleep.

However, most of the fish sleep but they are never completely asleep as they need to be alert for danger. Dolphins, for example, only keep one half of their brain asleep while the other one remains awake so they are always alert.

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

There are some exceptions, like the reef dwelling Spanish Hogfish, that fall into such a deep sleep that they can be lifted out of the water without waking up at all.

Interesting fact: Zebra fish and even jellyfish can suffer from sleep disorders or sleep deprivation such as having trouble drifting off.

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 sleeping! 😉

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