Unihemispheric slow-wave sleep (USWS) is sleep in which one half of an animal's brain is at rest, while the other half remains alert. During USWS, only one eye is closed, allowing the animal to remain alert to activity in its environment. It has been observed in various species of birds, dolphins, seals, lizards, and manatees. This type of sleep probably resulted from environmental factors such as an abundance of predators or the need to continue moving while asleep. Some birds are able to rest both halves of their brains at the same time to remain active for longer periods of time than would be possible with the mental alertness normal sleep allows. In fact, this mode of sleeping is the most common, and terrestrial mammals generally do sleep with both hemispheres at the same time.
This phenomenon has been studied by, among others, behavioral neurophysiologists Niles/Niels C. Rattenborg, Steven L. Lima, Charles J. Amlaner and other colleagues of Indiana State University Department of Life Sciences. They have published content in Nature on the subject. USWS is possibly the first animal behavior which uses different regions of the brain to simultaneously control sleep and wakefulness.