Dolphins aren't like fish, which can breathe underwater.
They're mammals and they have to get to the surface to breathe air periodically.
And a lot of them like everyone's favorite, the bottlenosed dolphin can only hold their breath for seven minutes or so.
So how's a poor cetacean supposed to get some shut eye? Their brilliant answer? Shut one eye at a time.
While sleeping, dolphins let one hemisphere of their brains nod off while the other half keeps an eye out for trouble. Literally. If the left brain is sleeping, the right eye stays open and vice versa.
So they always know when it's time to surface and breathe and when to get to the heck out of dodge.
They seem to switch off every two hours or so until they get a full eight hours a day.