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Dear Jessica,
Most fish spend part of every day asleep. Being asleep can mean different things to different fish. Some fish (like herring and tuna) are motionless in the water during the night. Other fish, like rockfish and grouper, don't appear to sleep at all. They rest against rocks, bracing themselves with their fins. Also, some freshwater fish (like catfish) swim up under a log or river bank for shelter during the day. By the way, fish can't close their eyes when they sleep; they don't have eyelids.
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.
It all depends on what you mean by sleep. My dictionary says that sleep is a period of rest in which the eyes are closed and there is little or no thought or movement. That is, sleeping means closing your eyes and resting.
The first thing we notice is that most fish don't have eyelids (except for sharks). Also, while some deep ocean fish never stop moving a great many fishes live nearly motionless lives and many do so on a regular diurnal/noctural cycle, some active by day others by night..
So we can't generalize and say that all fish sleep like we do. But most fish do rest. Usually they just blank their minds and do what we might call daydreaming. Some float in place, some wedge themselves into a spot in the mud or the coral, some even build themselves a nest. They will still be alert for danger, but they will also be "sleeping."
Another way of saying it is most all fish spend time in an energy-saving state that can be called "rest", and we might even call their behavior "sleep", though it is probably different than "sleep" in most land
animals. Many fish, like bass and perch, rest on or under logs atnight. Coral reef fish active in the day, hide and rest in crevices
and cracks in the reef to avoid being eaten at night.
The resting behavior of fish is very different from their behavior the rest of the day. Many minnows, for example, which are very active in
schools during the day, scatter and remain motionless in shallow water at night. Many fish "rest" or "sleep" during the day and are
active at night instead, but almost all fish sleep.
There are some animals that never stop swimming, like many species of shark, however, they HAVE to keep moving to push water through their mouths in order to breathe, and they may still sleep while moving, we just don't know yet.
Write in again soon!
(This was by me, not robbed off the internet.)
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