Do fish sleep?
Yes, most fish rest in a sleep-like state. They do not close their eyes, because they have no eyelids, but they slow down, become less alert and often hover or hide.
Scientists define sleep by behavior: reduced activity, slower response to the surroundings and a regular daily pattern. Many fish show all of these. Some float in place, some rest on the bottom and others wedge themselves into coral or rocks.
Some reef fish, such as parrotfish, wrap themselves in a bubble of mucus at night, which may protect them from parasites and hide their scent from predators. Zebrafish, a common lab fish, show sleep patterns similar to those of mammals, and researchers use them to study sleep.
Fish that never stop
Some fish keep swimming while resting. Certain sharks need water flowing over their gills to breathe, so they rest while moving slowly. Newborn dolphins and orcas, which are mammals, not fish, barely sleep at all in their first weeks.
Did You Know?
Blind cavefish that live in total darkness sleep much less than their relatives living near the surface, which helps scientists understand how sleep evolved.