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a mode of sleep behavior, common for certain species of birds and aquatic mammals, in which one cerebral hemisphere is deeply asleep while the other is completely awake: Often, unihemispheric slow-wave sleep, with one eye closed and one eye open, is mingled with occurences of bihemispheric slow-wave sleep, but for cetaceans, including dolphins and whales, only unihemispheric sleep occurs. USWS