Do fish sleep?

Everyone needs sleep and every night the average person performs a sleep ritual. We change into our pyjamas, crawl into our soft comfy beds, close our eyes and enter into a restful state. Our hearts slow down, we breathe slowly and regularly, and our muscles become relaxed. Once or twice an hour we roll over, but we are no longer tuned in to our environment. We spend about 8 hours a day sleeping – that’s one third of our lives.

Sleep means different things to different forms of animal life, so fish sleep – but it’s not sleep as we know it….

Firstly apart from Sharks,most fishes don’t have eyelids to close and none show the characteristic brainwave patterns like REM sleep seen in humans. Fish however do have a period of reduced activity and metabolism which seems to perform the same restorative functions as nocturnal sleep does in humans. Most sharks and a few other species have to keep swimming in their sleep as this is how they breath, some fish species will just float at a particular place, whereas others are more obvious about it and actually rest on the bottom or in coral crevices. Some fish even build a nest for resting, they remain alert for any sign of danger, but at the same time are able to rest peacefully, somewhat similar to cats…. At night many Parrotfish secrete and then sleep inside a mucous cocoon. This cocoon protects it from parasites and helps it warn about the presence of predators, it also adds a horrible taste and smell to the parrot fish thus keeping the predators at a safe distance. Certain species of fish have a sleep cycle which is similar to hibernation but not as drastic as the type of hibernation experienced by certain mammals.  Fish will enter a sedentary state, in which their metabolic rate greatly decreases and their bodily functions slow.  Hibernating fish sometimes retreat to a small shelter, or may simply reside in a low area of their water habitat. Then there are some special species of fish that enter a period of estivation during dry seasons in order to prevent from becoming dehydrated. This is an extremely interesting phenomenon, since it allows a fish to essentially survive outside of water for a period of drought or extreme heat. In Africa, there is a species of fish called the African Lungfish – Protopterus annectens that cocoons itself inside a casing of mud in order to spend several months in a dormant state. This is similar to the behavior of Carp, who bury themselves in lake mud during the cold winter months in order to prevent themselves from freezing.

So, yes fish do sleep although I’ve absolutely no idea if they dream or not…..

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