Sleep in Aquatic Mammals

The sleep of aquatic mammals is different from sleep in terrestrial mammals. These differences are determined by physical properties of the aquatic and terrestrial environments. Dolphins and porpoises (cetaceans) have only one sleeplike state, called unihemispheric slow-wave sleep (USWS). It has been suggested that USWS allows vigilance and motion and facilitates breathing controlled by the waking hemisphere. Semiaquatic pinnipeds can sleep both on land and in water. The pattern of sleep in fur seals and sea lions (Otariidae) is similar to that in terrestrial mammals when they are on land but similar to the sleep in cetaceans when they are in water. Sleep in "true seals" (Phocidae) and in the walruses (the only representative of the Odobenidae family) is similar to that in terrestrial mammals both on land and in water, with the exception of long apneas, which allow them to sleep in water at depth and survive predation and harsh freezing conditions.