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 at night. 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.
Do Fish Sleep?
by: Virginia Wells
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. Photo by Beth Bianculli
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:
Sleep has a restorative function – It’s possible that sleep helps the body recover from all the work it did while it was awake.
Sleep has an adaptive function – It’s possible that they sleep in order to protect themselves or conserve energy. When fish move into their hiding places to rest, their body needs are lessened and they avoid getting eaten.
Different Strokes
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.
