Do fish sleep at night?

According to Live Science, most fish species do sleep, although their sleep cycles are different from human or mammalian sleep. Certain shoaling species that never stop swimming may never sleep.

Sleeping fish usually appear motionless and move only to occasionally twitch their fins, which keeps them suspended in the water. Most fish that sleep appear unresponsive. Blue-headed wrasses can even be lifted out of the water without reacting while they are asleep. Some fish suffer from sleep disorders. For instance, zebrafish with a mutated brain receptor are known to sleep 30 percent less than normal zebrafish under the same conditions, indicating insomnia.