Dolphins sleep with only half their brain at a time

Because dolphins are conscious breathers who must surface for air, they can't afford to fully lose consciousness the way land mammals do. Instead, one brain hemisphere sleeps at a time — complete with a closed eye on the opposite side — while the other hemisphere stays alert enough to keep swimming and breathing.

— Lev M. Mukhametov, Unihemispheric Slow Wave Sleep in the Bottlenose Dolphin — Brain research studies, 1970s-80s