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 of them 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.
Even though they are asleep, fish are still alert for danger. And their normal sleep patterns fall by the wayside during moments like migration, spawning.
They can suffer from sleep disorders or sleep deprivation just like humans. Take zebra fish, which naturally drop their tails low and settle in at the bottom of tanks or just beneath the surface of the water to sleep.
Zebra fish that are lacking something called hypocretin receptors display classic signs of insomnia (having trouble drifting off and not sleeping for as long once they do). Those same fish, if kept awake when they normally would be sleeping, catch up on their sleep as soon as they stop being bothered.