among others, seals and whales belong to the aquatic mammals. earless seals and eared seals have solved the problem of sleeping in water via two different methods. eared seals, like whales, show unihemispheric sleep. the sleeping half of the brain does not awaken when they surface to breathe. when one half of a seal's brain shows slow-wave sleep, the flippers and whiskers on its opposite side are immobile. while in the water, these seals have almost no rem sleep and may go a week or two without it. as soon as they move onto land they switch to bilateral rem sleep and nrem sleep comparable to land mammals, surprising researchers with their lack of "recovery sleep" after missing so much rem.