Do fish sleep? Yes and no. First fish have no eyelids (except for sharks) and they do not operate like us humans with regular sleep hours. Fish do no appear to have the characteristic brain waves of REM either.
Their sleeping appears to be more like daydreaming or a trance like state. They are not fully awake nor are they fully asleep. You have probably seen fish in tanks floating motionless or nestled into seabed/coral. Fish become less aware of their surroundings with slower reactions times, yet their brainwaves do not change. Their metabolic rate slows down and their movement decreases.
Some fish practice a hibernation like state called estivation. Estivation is a state of dormancy practiced by certain fish to protect themselves from heat and dehydration. Fish in tropical areas perform this act during the summer months when rivers and swamps dry up. The African lungfish buries itself in the mud on the riverbed to survive the dry season. During the winter carp bury themselves in lake mud.
Do you wonder what happens to fish in outdoor ponds that freeze in the winter? The fish practically freeze yet are awake and alive come the warm temperatures.
Like humans who have a nightly ritual to prepare for slumber, the parrotfish has quiet the routine. Making its way into a reef crevice, the parrot fish begins to ooze a jelly-like mucus over its entire body before it rests for the night.
Overall the answer varies from fish to fish. Fish will sometimes float motionless in the open water, while others hide under a log or the river bank. Fish do take on some form of a resting state whether they are moving or not, in the open or hiding. Often a little disturbance or ripple will wake the light sleepers. Fish sleep in their own special way that significantly varies from how humans sleep.
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