Although fish do not have eyelids, they can sleep. Some float, while others bury themselves or wedge themselves into tight places. Even though they are sleeping, they are still able to sense danger. Fish sleep for as long as they need, depending on how much energy they have exerted during the day. read more
The reason fish don't have eyelids is because, underwater, they don't need eyelids. Not having an eyelid doesn't mean that the fish can't go to sleep. I'm sure that many listeners have experienced falling asleep with the radio on, hopefully not during the Naked Scientists. read more
Since fish are literally swimming in water, there is little need for moistening. But eyelids can serve other functions, too. Many deep-sea fish, as well as less exotic fish like milkfish and mackerel, have a clear adipose eyelid, divided into several layers, covering much of their eyes. read more