Robert Poh rpoh at
Wed Mar 1 07:03:05 EST 1995

ST004131 at (Justin Baker) wrote:

> 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,
> will likely save itself from probable death.
> There may be obvious gaps in this theory


One gap in the theory that sleep is, in evolutionary terms, a mechanism for 
predator-avoidance is the fact that cetaceans sleep - one hemisphere at a
time - in open water.  Another similar gap is that grassland ungulates 
also sleep in the open.  Predator-avoidance does not explain sleep in those
groups, and so the theory is not general enough to explain the phenomenon as
it is observed.  In my opinion, any theory of ultimate causation is premature
if it precedes an understanding of proximate function.

>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.

And, one small point of logic: If a species did not sleep at night, it would 
not be a diurnal species.


More information about the Neur-sci mailing list