Do Fish Sleep?

Do Fish Sleep?

Diving at night allows us to see the underwater world in a different light (literally!)... Sorry, I couldn’t resist. At night fish take one of two approaches. One is to use this time to nestle away in the rocks and coral to sleep off a long day of swimming and grazing. The other, is to hunt the fish trying to catch a bit of shut eye!

Working the Night Shift?

Many humans work night shifts meaning that they are awake and working through the night and sleep through the day. The transition to this way of life is not often the easiest for us. We feel too awake to rest/sleep enough through the day and in turn, feel shattered through our shift at night. The sleep patterns of fish are much more flexible than ours. Unlike humans, they do not seem to cycle through sleep stages. This is believed to be due to the fact that the brain of a fish is less complex. Fish are able to adjust their sleeping patterns based on life dependant factors such as food availability, water temperature and predator presence.

How do Fish Sleep?

Only sharks have eyelids and even then, these are only used to protect the sharks eyes when attacking prey. This means that the eyes of a fish are always open. There is no ‘one size fits all’ when it comes to how fish sleep. Some fish are happy to hover in midwater whilst others drift in the current, only flicking a fin or their tail occasionally to keep themselves steady and upright. Not all fish like to be out in the open, some choose to take themselves to a 'bedroom' in the form of a hole in coral, between the rocks or bury themselves in the sand. Both the sleeping pattern of a fish and how/where it sleeps (if it sleeps at all) is completely dependent on the species. One bizarre example of this is seen with the Parrot Fish. Some Parrot Fish belch out their very own sleeping bag, made of mucus, every night. There are many theories and possible explanations for these cocoons but the most popular theory is that it protects the fish against predators. This is a theory that dates back to 1959 and is included in a citation aptly named 'Differential Food Selection by Moray Eels and a Possible Role of the Mucous Envelope of Parrot Fishes in Reduction of Predation' by Howard Winn and John Bardach. The results of their experiments show that three species of mucus-weaving Parrot Fish are less often eaten by Moray Eels than a fourth species that sleeps uncovered. This being said, the pair did not consider other differences between the four species which may also have played a part in the findings. Read the citation HERE if you are interested.

Do all fish sleep?

The simple answer is No. Many pelagic fish species (residing neither close to the bottom nor near the shore) such as Tuna, Atlantic Mackerel, Bonito, and some sharks, swim continuously and do not show signs, behavioural or otherwise, of sleep. It has been argued that one function of sleep is to allow the brain to consolidate into memory the things it has learned during the animal's normal period of activity. As the pelagic zone is often fairly boring with mile after mile of open water with not much going on it is believed that the sensory input for these fish is so low that they do not require this function of sleep. Memory formation could take place as the fish swims meaning that the fish has no need to fall asleep in the traditional sense of the word. Most fish that do sleep, sleep lightly. This is due to the fact that although they need to rest, they also need to remain alert for predators. Many are also known to stay awake for long stretches when migrating, spawning or caring for their young. It's not only humans who are deprived of sleep when the little ones turn up! This (again) is not one rule for all. There are several species of Wrasse that fall into such a deep sleep that you could lift them out the water without them waking up at all!
Fact or fictionMarine life

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