Do Fish Never Sleep?

When you think of sleep, you think of eyelids closing over your eyes. But most fish do not close their eyes, yet they do sleep.
Most fish sleep by keeping very still in the water. In this way ,their bodies rest, just as your body does while you are asleep. But when a fish sleeps, it is not the same deep, sound sleep that you are accustomed to, and a slight shake in the water will disturb it.