Sharks do not sleep in the same manner most other animals do, instead having split periods of rest and activity, according to the Florida Museum of Natural History. Many species of shark must maintain movement in order for their gills to extract oxygen from the water.
Some species, like the nurse shark, however, have structures called spiracles that force water through the gills, allowing the shark to remain stationary. Sharks in this state still have open eyes, and their pupils track the motion of animals in front of them, so it is not a true sleep state. As examples of evolutionarily fit animals, sharks have changed very little in their 400 million years of existence. All sharks have skeletons composed of cartilage and rows of replaceable teeth. The hide of a shark features toothy projections called denticles, giving the sensation of sandpaper when touched.
Sleep-swimmers such as the spiny dogfish continue to move while unconscious. In spiny dogfish, swimming is not a higher brain function, so they can continue to swim regardless of whether they are mentally alert.