People believe that fishes don’t sleep, but they do. They need sleep like all animals do. Deprive them of this and their health — just like ours — soon goes downhill.
It is, however, usually difficult to establish whether fishes are asleep or awake, since they don’t possess eyelids and are forced to sleep with their eyes open. In some species, their colors change when they doze off. In marine parrotfishes, for example, the mucus cocoon they secrete each night is the giveaway, but in most other species the differences between both states are subtle, often difficult or impossible to detect.
So what has this got to do with clown loaches? Well, in this species, it is extremely easy to determine whether an individual is asleep or awake. When awake, it will often rest on the bottom or scuttle around. However, when it decides to take a nap, it does a very unfishlike thing: It lies on its side!
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