[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