Fish do not sleep like humans do, unaware of their environment. They do have rest periods that seem to function similar to sleep. While at rest, a fish reduces his activity and slows his metabolism. However, he is conscious of his surroundings at all times.
Fish do not all rest in the same way. Some simply float at rest, while others hide in mud, vegetation, coral or nests. Certain fish, such as bass, rest at night, while others slow down during the daytime. Minnows busily swim in units - schools - during the day, but, at night, they separate and rest quietly. Sharks also rest, but some species must keep moving. In order to "breathe," they need water to circulate through their mouths continually.