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."