A restless sleep
Fishes need to get some rest but this is really hard to notice as they never close their eyes ! They don’t have eyelids as living under water their eye cannot get dry. Some marine animals stay in motion, such as pelagic sharks, which have to constantly swim during their resting time in order to keep breathing thru their branchial arches. It is really difficult to know if a fish is awake or sleeping. Nevertheless, some technics and behavior related to sleep can be noticed.