All of us know what sleep is. We know how it feels. The meaning of the word is generally not questioned in ordinary conversation and we do not have to look it up in a dictionary. But no one really knows what happens when we sleep. Once we get beyond the simple level of recognizing sleep as a time of rest in which very little activity occurs, we find that most of the questions involving the nature of sleep remain mostly unanswered. What happens when we sleep? Why do we sleep? If some function of the brain takes place during sleep simply because it is easier to do so, could it also take place in the brain while awake? The questions are endless. But although many questions remain unanswered, scientists have learned a great deal about sleep and will continue to learn more as time goes on.
             Despite more than a century of scientific study of sleep, including almost three decades of modern intensive research, the function of sleep remains a mystery. There are many theories, but none of them have been proven due to the relatively small amount of data supporting each one. Several of these theories include: restoration and recovery, energy conservation, and ecological hypotheses of foraging and predator avoidance. First, the restoration and recovery theory simply states that sleep serves to renew all the damage or depletion done to the body during wakefulness. This hypothesis is the most widely accepted because it is a reasonable answer and because of what happens when a body is sleep deprived, i.e. it doesn't function properly. A second theory is called the energy conservation hypothesis, which states that "sleep serves to reduce metabolic rate and body temperature in warm-blooded animals during periods of rest to offset the high-energy costs of endothermy." This theory is supported by the fact that, during sleep, metabolic rate decreases by about ten percent and body temperature is reduced by about one to two d...