Sleep is in general defined cross-species as a state in which an
organism has a higher level of threshold stimulus for arousal. In
other words, an asleep animal is less responsive. From what I've
seen, that's pretty much a working definition for most. In general,
organisms often take a characteristic posture during sleep -- a posture
which is species-specific (for humans, I suppose it's just lying down).
I imagine that rates of metabolism also decrease.
In regards to purposes, no one knows for sure, as I suspect you have found.
An interesting theory, which IMHO is correct, is that sleep evolved as a
basic defense mechanism.
At night, a diurnal animal is vulnerable to predation. Right. If it is
out and about, it will get eaten. If it is hiding away in a warm place,
expending little energy, it will likely save itself from probable death.
In fact, if one considers the probabilities of predatory attack for
a diurnal species at night, it soon becomes obvious that by sleeping,
the species increases its evolutionary potency a good bit.
There may be obvious gaps in this theory, but overall I find that is a
useful one. It also has some interesting preadaptive implications,
which may or may not be obvious. I will not pursue those here.
I would appreciate any comments.
JTB
