Sharks, like other marine animals, don’t seem to fall into a deep sleep like humans do. Sharks need to keep water moving over their gills to receive necessary oxygen. Not all sharks need to move constantly, though. Some sharks have spiracles, a small opening behind their eyes, that force water across the shark’s gills so that the shark can be still when it rests. Other sharks do need to swim constantly to keep water moving over their gills and their bodies, and have active and restful periods rather than undergoing deep sleep like we do. They seem to be “sleep swimming,” having parts of their brain less active while they remain swimming.