Through a naturalist’s eyes
"Sleep and wakefullness are mutually exclusive, and one fascinating aspect of sleep research therefore is the discovery that some animals sleep with one side of the brain at a time. In this, known as unihemispheric sleep (US), one cerebral hemisphere becomes inactive, as shown by EEG activity and a low level of acetylcholine (which is linked with arousal), and one eye closes. And then the two sides switch. Among mammals, US is known to occur in cetaceans, manatees and eared seals."