When imagining the evolution of a species in a dangerous environment where food is scarce, being asleep for a third of the time seems counterproductive and dangerous. At any time during this period of unconsciousness (if we can call it that), the animal could be attacked and ripped to shreds by a predator. And don't forget: humans are also animals. It’s debatable that we evolved to conserve energy. Actually, there is a lot of evidence to suggest that most animals burn almost just as many calories as when they're awake!
It seems that modern technology has been able to reveal a lot about how we sleep (how we dream, what the brain does during sleep, etc.), but hasn't been able to answer the why to many questions surrounding sleep. To discover more about the mysterious world of sleep, click through this gallery.