Do Fish Sleep?
Yes, fish do sleep. Because fish have no eyelids, they cannot close their eyes, and it may look as though they never sleep. Many fish sleep by simply remaining very still in the water. Others rest on the bottom of the pond, ocean, or aquarium where they live. Still others wriggle into the sandy bottom.
Some of them even produce a gooey covering as a protection against other fish who might be looking for an easy meal. In the dark many fish have a lower rate of metabolism, and that is much like sleep. In this way, fish rest their bodies as you and I do when we sleep. If you happen to see a fish lying almost motionless in the water, it is probably asleep. Fish don’t sleep as soundly as humans, and the slightest disturbance will awaken them.
Sometimes people see fish when they’re sleeping and assume they’re awake because they’re still moving. Most fish need to keep moving even when they’re sleeping, so that they keep a constant flow of water moving past their gills to maintain a proper oxygen level in their bodies.
For some larger fish, like sharks, this can take the form of swimming at a slower rate when sleeping. Smaller fish might be able to do nothing more than occasionally move their fins. Certain types of fish are known for sleeping in ways that look more like traditional sleep to humans. For example, fish that live near a coral reef might stay active all day long and then hide in crevices in the reef at night to avoid predators.