Best of all there are birds, who can rest part of their brain to sleep while, yes, flying. Most animals sleep — and in all sorts of strange ways that, on the surface, are quite baffling. Dolphins have been documented sleeping and swimming with their open eye facing their pod, suggesting they’re trying to stay with the group. For birds, the ability to half sleep could serve similar functions. “Now we fully recognize that even in the human brain, sleep is often a local phenomenon,” Rattenborg says.