Unihemispheric Slow-wave Sleep

From USFWS National Wildlife Refuge System:

Did you know that dolphins can sleep with one eye open? Iguanas, dolphins, fish, some birds and ducks, seals – they sleep with only half their brain at a time. It’s called unihemispheric slow-wave sleep. It rests part of the brain while the other part keeps an eye open, literally, for predators and other dangers. You can learn about it here http://www.radiolab.org/story/91529-one-eye-open/ and you can see the dolphins at Merritt Island National Wildlife RefugePhoto by NASA