Everyone needs sleep. Every night average people perform a sleep ritual: We change into pajamas, 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. The Saunders Comprehensive Veterinary Dictionary sums it up pretty well: A period of rest during which volition and consciousness are in partial or complete abeyance and the bodily functions partially suspended; a behavioral state marked by characteristic immobile posture and diminished but readily reversible sensitivity to external stimuli.
Most animals have some daily pattern of rest and activity, and in many species these daily cycles are similar to people running around during the day then lying down at night and doing nothing or sleeping. It is believed that fish are no different, although it is a controversial subject. Some fish keep very still, experiencing a quiet period (quiescence) that you might call sleep. Scuba divers often handle reef fish in the middle of the night without startling them and can even lift some species out of the water before they awaken. Tropical freshwater fish in home aquaria appear to be resting immediately after turning the lights on in a room that has been darkened for several hours. Unfortunately, fish have no eyelids so it is difficult to tell whether they are asleep or not.
Why We Sleep
No one knows for sure why creatures sleep. But there are two basic theories:
Being asleep can mean different things to different fish. Some fish and amphibians reduce their awareness but do not ever become unconscious like the higher vertebrates do. Fish have time periods when they become less aware of their surroundings but their brain waves do not change, and they do not exhibit REM sleep. They aren't quite asleep but they don't seem to be fully awake either.
Some fish undergo a yearly sleep cycle. They hibernate and their metabolic rate slows down. Although they do not hibernate like mammals, as environmental temperatures fall, their metabolic rate and activity decrease, and they go into a stupor and stop feeding. They usually adopt a position towards the bottom of the pond.
Some fish practice estivation, a state of torpor or dormancy in which they spend time during hot, dry periods to protect themselves from dehydration. The African lungfish buries itself in mud and survives the dry season protected by a cocoon of mud in the riverbed. Carp spend the winter partly buried in lake mud, and in tropical countries many fish sleep, or estivate, through the summer months when swamps and rivers dry up. Walking perch and lungfish bury themselves in mud, leaving only an airhole open, and breathe by means of their lungs. One of the gobies of the Ganges River delta digs a burrow and sleeps through the dry months with only the tip of its tail touching the water. It apparently breathes through its tail.
Some fish make elaborate preparations for sleep. In David Feldman's bookWhen Do Fish Sleep?, a scientist describes the nightly ritual of a tired parrotfish that lives in reefs near shore. The parrotfish squeezes into a crevice on the reef. Once settled in, it begins oozing a jelly-like mucus, which forms a protective membrane over his body, and then he nods off into a deep sleep.
Some fish are motionless in the water during the night, while other fish, like rockfish and grouper, don't appear to sleep at all. They rest against rocks, bracing themselves with their fins. Some freshwater fish, like catfish, swim up under a log or river bank for shelter during the day.
Finally, some fish don't hide the fact that they take an occasional nap. One of the favorite habits of the clown loach, which has alarmed most new clown loach keepers in the past, is that of resting on the bottom of the aquarium on their sides. They appear as though they are dead or sick, but this is just one of the positions that they adopt when resting.
It's probable that fish do sleep in some form, whether slowing down or coming to a complete stop, whether hiding or doing it right in the open. But when they sleep the slightest ripple in the water will disturb them. Nevertheless, in some way they rest, just as we do.
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