Unihemispheric slow-wave sleep
Unihemispheric slow-wave sleep (USWS) is sleep where one half of the brain rests 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 unconsciousness. 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 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.
A young house sparrow (Passer domesticus) exhibits unihemispheric slow-wave sleep.
Polysomnogram demonstrating slow-wave sleep. High amplitude EEG is highlighted in red.
Common swift
Slow-wave sleep (SWS), often referred to as deep sleep, is the third stage of non-rapid eye movement sleep (NREM), where electroencephalography activity is characterised by slow delta waves.
Polysomnogram demonstrating SWS, stage four. High amplitude EEG is highlighted in red.