How do fish sleep?
Sleep is defined by Merriam-Webster’s* as “the natural periodic suspension of consciousness during which the powers of the body are restored” and “a state of torpid inactivity.” Looking at sleep in this way, rather than saying that sleep requires an animal to lay down and close its eyes, then we can say that fish sleep.
Most fish (sharks being the notable exception) do not have eyelids. So sleep for a fish does not include the closing of eyelids.
Some fish, such as sharks, never stop moving. The movement of a shark necessary in order to force water into the mouth and out through the gills, so that the shark can breathe. Other fish live their lives in an almost motionless state, “sitting” on the ocean floor, on a lake bed, or under some sort of cover such as a log, waiting for prey to catch and eat.
Other fish move (swim) and remain motionless, or nearly so, on a regular cycle. Some are active diurnally (in the daylight) and are motionless nocturnally (at night). Others reverse this, and are nocturnally active, and diurnally motionless.
For those fish that have cycles of activity and motionlessness, the time of motionlessness is the period of “rest.” The time of rest occurs in different ways. Some fish float in place. Some will settle to the bottom of the body of water in which they live, and “hide” themselves in the mud. They may find a calm hidden place in a coral reef, under a log or a rock or a ledge. They may build something that could be considered a nest. The mind of the fish will become relatively inactive, something of a blank, not really thinking any fish thoughts at all.
During this period of time, the fish’s body is resting, regaining strength. Looking at the fish, we might not know it is sleeping, as it does not have closed eyelids. It will be nearly motionless. Part of its mind will be alert to danger, but most of its focus will be on rest.
Fish sleep. Just not in the ways that would be familiar to us as humans, and what we recognize as sleep in many other animals with which we are familiar. Dogs, cats, lizards, farm animals all get their rest in sleep much as humans. Fish rest and relax, just not in the familiar way of closing eyelids and lying down.
*Merriam-Webster’s Collegiate Dictionary, Eleventh Edition (2003, Springfield, MA)