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Re: Evolution of sleep



don ohmes wrote:
> 
> To expand the question:
> Assuming sleep correlates with sharply reduced
> alertness/awareness, what is the across-the-board
> selective advantage of unconsciousness that prevents
> at least some vertebrate animals from evolving always
> on status relative to mental awareness? Or if there
> are some, why are they rare?

Sleep may simply be a way of conserving precious energy while an animal
is inactive. Few vertebrates are able to do what they do best 24 hours a
day. A species with poor eyesight and no other keen senses to fall back
on (like us apes) will be virtually useless on a dark night. To remain
active during those hours when they can't forage for food would burn up
energy uselessly. Remaining still in a safe place may also reduce the
chances of a predator discovering you at a time when your senses aren't
at their best.

Us larger-brained vertebrates may also sleep more than other species due
to the tremendous amounts of data our brains have to process. Go for too
long with out sleep and information may start to back up, forcing the
brain to take cognitive shortcuts in order to deal with the back log
(hence the halucinations).

Dolphins (and probably whales) can't breath automatically, so complete
unconsciousness would lead to drowning. Hence they rest in a state of
semi-unconsciousness, where half the brain is still active enough to
keep breathing and stay at the surface, while reducing brain activity
enough to prevent 'cognitive burnout' (a halucinating dolphin would be
an interesting sight, though!).

-- 
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Dann Pigdon
GIS / Archaeologist         http://www.geocities.com/dannsdinosaurs
Melbourne, Australia        http://heretichides.soffiles.com
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