Do Fish Sleep?


Though they are in a restful state, many fish are still acutely alert for danger – sleeping, in essence, with one eye open.  Sleep may have evolved as a means to “reset” brain circuits through phases of unresponsiveness to sensory inputs.  Organisms with brains that are capable of processing massive amounts of sensory information also need to be able to refresh memory circuits for infrequently used functions.   Sleep serves as a time for fish to disengage from their surroundings and gives them a time for circuit refreshment.

Some fish actually lie on the bottom at night. Some of them even produce a gooey covering as a protection against other fish who might be looking for an easy meal. In the dark many fish have a lower rate of metabolism (their body machinery slows down), and that is much like sleep. But there is one thing you do in your sleep that fish do not do. You close your eyes. Fish can’t do that. They don’t have any eyelids.

Fish that swim continuously and blind cave fish do not sleep in a traditional sense.  Researchers hypothesize that these types of fish may have less need to process sensory information, particularly visual information, and, as a result, their brain does not need to rest and reset in the same way that other fish do.    For example, sharks and tunas that are obligate ram ventilators must swim to breathe so they cannot rest or sleep.  These sharks and tunas also live in open water pelagic habitats, with very few visual features to register. 

Fishes don't have the same degree of neocortical development as mammals and thus don't display these brain-wave patterns associated with sleep. So, as far as brain-wave patterns go, fishes don't sleep.

If, however, you define sleep as a combination of a reduced metabolic rate, slowed physical activity, lowered response to stimuli and the assumption of a resting posture, then many fishes do sleep. Perhaps the best-known 'sleepers' are the parrotfishes (family Scaridae). Many parrotfishes find a suitable spot on the seafloor and secrete a mucus envelope in which they spend the night.


Fishes don't have eyelids like reptiles, Birds or mammals do. So naturally, they don't (can't) blink. But, they can sleep. They reduce the amount they move about when asleep, breathing also reduces. If sudden stimuli (a hard tap, light, motor suddenly switched on etc) are given, they give out jerks just like we do.


With their inability to give us obvious clues such as closing their eyes, sleep analysis in fish has been conducted slightly differently. Variations in physiological mechanisms such as breathing and heart rate are recorded, and both of these show a drop in activity suggesting a reduced metabolic rate. It conserves energy and is believed to allow fish to recharge in much of the same way as people.

With the body slowing down, the responses to external stimuli also become more sluggish. Studies on cave fish showed a slower reaction in fish that had been inactive for 60 seconds beforehand than those that remained on the move, suggesting a sleep like state. Unlike animals, it is not a deep sleep which is handy when you have to be aware of predators trying to snap you up.


Posted by: Lusubilo A. Mwaijengo

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