Why do fish not have eyelids? The main reason fish do not have eyelids is due to the fact that in underwater, fish do not need eyelids. Even if the fish do not having an eyelid, this does not mean that it cannot go to sleep.
Not having eyelids to shut out the light does not mean that fish can’t sleep and the recent study on zebra fish—a common fish kept as pets—shows that fish can fall asleep and when they’re sleeping they sink to the tank’s bottom and their tails droop down as they become more relaxed. Some zebra fish apparently even suffer from insomnia.