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Oct17-07, 04:02 PM
Sci Advisor
HW Helper
P: 8,953
It depends wether you sleep to replenish your brain (ie. allow chemicals to be built up or waste products to be removed ) or it is purely a low power state to save energy.

Cats sleep most of the time - to save energy, you might as well ask why they haven't evolved to chase and eat prey 24hours a day.