Do Fish Ever Sleep?
Yes, fish 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 lie on the bottom, resting upright or on their sides. And a few others wriggle into the sand on the water’s floor.
In this way, fish rest their bodies as you and I do when we sleep. Fish don’t sleep as soundly as we humans do, and the slightest disturbance will wake them. If you happen to see your pet fish lying quietly in the water, it is very likely that it is sleeping.
Research shows that fish may reduce their activity and metabolism while remaining alert to danger. Some fish float in place, some wedge themselves into a secure spot in the mud or coral, and some even locate a suitable nest. These periods of “suspended animation” may perform the same restorative functions as sleep does in people.
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.