Fish sleep, but not all. In addition, they do not sleep in the literal sense of the word.
The fact that the rest, like people, they do not work because of the physiological structure of the eye and the absence of some species of fish swim bladder.
As fish sleep?
Any person to fall asleep,you need to close your eyelids. The fish do not have eyelids, they could clamp falling asleep strong and healthy sleep. However, this does not prevent them rest, as they say "in the fish." Some of them even go to while on its side. In fact, they do not turn a blind eye.
We can not say that the fish are inunconscious, as a person during sleep. Yes, their minds dulled, but not enough. Physical function for some time suspended, and the fish itself can not pay any attention to what is happening around. However, with all that brain receptors are in combat readiness, which allows it to quickly get out of his stupor.
A great variety of fish species in the world. Many of them have certain periods of activity and rest. "Sleep," staying afloat, helping them swim bladder. It is worth noting that this body has not all fish, so some of them do have to sleep on the move. They are all deep-sea species and demersal fish. "Dream" in them - this is a brief respite during their continuous movement. That is why the "dream" of such a vacation can be called conditionally.
No sleep and no rest
One of the brightest representatives of bottom fish are nothaving a swim bladder, are, of course, sharks. To avoid drowning during sleep, it is necessary to actively predator swim. Unlike humans and many land animals, for whom diving is a fairly short-lived occupation, sharks forced involuntarily to be in constant motion from the first day of his life until the last!
The lack of a swim bladder does not allowSharks always remain in "limbo" state deep in the depths, as it can do many other types of fish. If the shark at least for a moment stop its undulating muscular fins and prominent tail, the body's own gravity pull it inexorably to the bottom! Interestingly enough, that is why the dead sharks never float to the surface, and the falling rocks on the bottom.
As mentioned above, in the traditional sensesleep these fish never sleep. They both day and night are in constant motion. However, some of them still manage to have a rest. Particular types of sharks that live in coastal waters, swim in the small underwater caves, located in shallow waters of rivers or lakes. There they lie on the rocky ledges or on the bottom.
It can be concluded that in the literal sensethis understanding does not sleep, no fish, but some lucky that God gave the swim bladder can still afford to somehow relax, but not so on the bottom and deep sea fish.