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.[1] The phenomenon has been observed in a number of terrestrial, aquatic and avian species.