Whether sharks sleep or not, is still a mystery, as it is very difficult to monitor them 24/7. Many sharks need permanent movement in order to breathe. They swim close to the water surface, go into a resting position and slowly sink again for some time. Hereafter, they repeat the process by slowly swimming up again in order to get into the resting position … This is their way of saving energy and getting rest – a tad similar to sleeping.
Other smaller, more bottom oriented species, who don’t need to be swimming in order to breathe e.g. the nurse or catfish sharks, simply go crash on the bottom without moving.
Sharks also have no eyelids. Their eyes are always open.