Sharks rest in an approximation of what is termed sleep in land animals, though they do it with their eyes open. However, some sharks are nocturnal, which means they are active at night and rest during the day, and others rest when they are well fed and safe.

Sharks rest in various ways, depending on how they obtain oxygen. Some, such as nurse sharks and angel sharks, pump water into their mouths and through their gills in a process called buccal pumping. These can lie dormant on the ocean floor while resting. Others, such as great white sharks and dog sharks, need constant movement to get enough oxygen in a process called ram ventilation. These go into a sleep state while still moving through the water.