the consequences of falling into a deep sleep for marine mammalian species can be suffocation and drowning, or becoming easy prey for predators. thus, dolphins, whales, and pinnipeds (seals) engage in unihemispheric sleep while swimming, which allows one brain hemisphere to remain fully functional, while the other goes to sleep. the hemisphere that is asleep alternates, so that both hemispheres can be fully rested. just like terrestrial mammals, pinnipeds that sleep on land fall into a deep sleep and both hemispheres of their brain shut down and are in full sleep mode. aquatic mammal infants do not have rem sleep in infancy; rem sleep increases as they age.