All kinds of tired

Donkeys sleep about three out of each 24 hours. Certain reef fish spend the night moving their fins as if swimming in their sleep. Some biologists argue that all animals sleep in some form or another. But identifying sleep can get complicated. Insects have brain architecture so different from humans’, for example, that electrophysiological recordings during “sleep” won’t match human patterns. The real problem may be that researchers haven’t agreed on what sleep does for people, so it’s hard to agree on the animal equivalent. Studying animal sleep, though, offers the prospect of discerning evolutionary patterns in sleep pointing to some ancient function. — Susan Milius