[on why it’s not sleep that needs the explanation]

Being awake is relatively costly, compared with lying dormant… Mother Nature economizes where she can. If we could get away with it, we’d “sleep” out entire lives. That is what trees do, after all: all winter they “hibernate” in deep coma, because there is nothing else for them to do, and in the summer they “estivate” in a somewhat lighter coma, in what doctors call a vegetative state when a member of our species has the misfortune to enter it.

— Daniel Dennett, Darwin’s Dangerous Idea, 1995