Metallica lyrics notwithstanding, most humans would find it impossible to sleep with one eye open. Dolphins, on the other hand, can't sleep any other way. That's because, unlike us, they don't breathe automatically and have to remain conscious in order to do so actively. The solution to this predicament is called unihemispheric slow-wave sleep, in which one half of the brain rests while the other remains awake. When the right half of a dolphin's brain is sleeping, its left eye is closed, and vice versa; they periodically alternate which side of the brain is asleep in order to rest fully while retaining consciousness. While sleeping, dolphins may rest motionless or swim slowly and steadily near the water's surface.
Dolphins aren't the only species with strange sleeping patterns. A number of their fellow sea creatures, from beluga whales to sea lions, also exhibit unihemispheric sleep. On the extra adorable end of the spectrum are otters, who float belly-up and often hold hands with one another so as to not drift apart while snoozing. Great Frigatebirds, meanwhile, sleep in 10-second bursts while remaining airborne for as long as two months at a time. And, like dolphins, they do it with one eye open.
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