Is Sleep Essential?

Is Sleep Essential?
Chiara Cirelli, Giulio Tononi
PLoS Biol 2008;6(8):e216.

Everybody knows that sleep is important, yet the function of sleep seems like the mythological phoenix: “Che vi sia ciascun lo dice, dove sia nessun lo sa” (“that there is one they all say, where it may be no one knows,” Wolfgang Amadeus Mozart and Lorenzo da Ponte [1790], Così fan tutte). But what if the search for an essential function of sleep is misguided? What if sleep is not required but rather a kind of extreme indolence that animals indulge in when they have no more pressing needs, such as eating or reproducing? In many circumstances sleeping may be a less dangerous choice than roaming around, wasting energy and exposing oneself to predators. Also, if sleep is just one out of a repertoire of available behaviors that is useful without being essential, it is easier to explain why sleep duration varies so much across species. This “null hypothesis” would explain why nobody has yet identified a core function of sleep. But how strong is the evidence supporting it? And are there counterexamples?