Unihemispheric slow-wave sleep (USWS) is the ability to sleep with one half of the brain while the other half remains alert. This is in contrast to normal sleep where both eyes are shut and both halves of the brain show reduced consciousness. In USWS, also known as asymmetric slow-wave sleep, one half of the brain is in deep sleep, a form of non-rapid eye movement sleep and the eye corresponding to this half is closed while the other eye remains open. When examined by low voltage electroencephalography (EEG), the characteristic slow-wave sleep tracings are seen from one side while the other side shows a characteristic tracing of wakefulness. The phenomenon has been observed in a number of terrestrial, aquatic and avian species.
Of all the cetacean species, USWS has been found to be exhibited in the following species
Amazon river dolphin (Inia geoffrensis)
Beluga whale (Delphinapterus leucus)
Bottlenose dolphin (Tursiops truncates)
Pacific white-sided dolphin (Lagenorhynchus obliquidens)
Pilot whale (Globicephala scammoni)
Porpoise (Phocoena phocoena)