do fish sleep?

Yes, fish do sleep, though it looks quite different from how humans sleep.

Because most fish do not have eyelids, they cannot close their eyes, which makes it appear as though they are always awake. Instead of drifting into a deep, unconscious state, fish typically enter a period of "suspended animation" or a resting state. During this time, they become less active, lower their metabolism, and react more slowly to their surroundings while remaining alert enough to avoid predators.

Some fish, like certain species of sharks, must keep swimming to push oxygen-rich water over their gills; these fish likely rest parts of their brains at a time or have ways to "shut down" while remaining in motion. Others, like parrotfish, secrete a mucus bubble around themselves at night to mask their scent and protect them from parasites while they rest.