A Full Night's Rest?

Don’t ever mess with dolphins when they’re asleep: They sleep with one eye open! They rest in a state of unihemispheric sleep, which means that only half of their brain sleeps at a time while the other half stays awake. Throughout a sleep cycle, dolphins will alternate sleep between both sides of the brain, opening and closing each eye whenever the change occurs. Part of them has to be conscious at all times because dolphins are conscious breathers. If they lost complete consciousness during sleep, like humans do, they would suffocate or drown. When they do sleep, they float at the surface of the water with — amazingly enough —one eye wide open.