Fishes sleep, but not all. Besides they sleep not in literal sense of this word. The fact is that to have a rest as if people, at them it will not turn out because of the physiological structure of an eye and absence at some species of fish of a swimming bubble.
How do fishes sleep?
To any person to fall asleep, it is necessary to close the eyelids. Fishes have no century which they could close, filling up with a sound and healthy sleep. However it does not prevent them to have a rest that is called "on - fish". Some of them in general lay down at the same time sideways. Actually they do not close an eye.It is impossible to tell that fishes are in unconsciousness as the person during sleep. Yes, their consciousness becomes dull, but not so. Physical functions for some time stop, and fish can not pay any attention to the events around. However at everything at the same time brain receptors are in combat readiness that allows it to leave the catalepsy instantly. The variety of species of fish on Earth is high. Many of them have certain periods of activity and rest. "To sleep", remaining afloat, they are helped by a swimming bubble. It should be noted that this body is available not for all fishes therefore some of them in general should sleep in the movement. All ground and deep-water species of fish concern them. "Dream" at them is a short respite during their continuous movement. For this reason it is possible to call such rest "dream" conditionally.
Without dream and without rest
One of bright representatives of the ground fishes who do not have a swimming bubble are, of course, sharks. Not to drown during sleep, these predators need to swim actively. Unlike the person and many land animals for whom swimming is quite short-term occupation sharks are forced to be involuntarily in the constant movement from the first day of the life to the last! The lack of a swimming bubble does not allow sharks to remain always in hover in the thickness of depths as many other species of fish can do it. If the shark at least for an instant stops the wavy movements by brawny fins and an outstanding tail, then the gravity of own trunk will inevitably pull her to a bottom! It is curious that for this reason dead sharks never emerge on a surface, and stoop down on a bottom. As it was already told above, in traditional understanding of a dream these fishes never sleep. They both in the afternoon, and are in the constant movement at night. However some of them manage to have a rest nevertheless. The special species of sharks living in coastal waters swim away in the small underwater caves located at small depths of the rivers or lakes. There they lie on rocky ledges or at the bottom. It is possible to draw a conclusion that in literal point of this understanding any fish does not sleep, however certain lucky persons whom God awarded with swimming bubbles, nevertheless are able to afford to have a rest somehow what you will not tell about ground and deep-water fishes.