Sleep is risky. By abandoning temperature control of our bodies, as we do in sleep, we make ourselves vulnerable to being frozen, or cooked. By abandoning waking watchfulness, we expose ourselves to surprise attack. By abandoning controlled consciousness, as we do in dreaming, we risk committing errors of perception, logic, and judgment. These are the dangers of entirely normal sleep. In addition, the system that organizes sleep may make errors that emerge as sometimes fatal disorders.
Why does nature take such risks? Or, to put it more scientifically, how do the benefits of sleep outweigh the risks, given the evolutionary success of animals that sleep? Two complementary answers have emerged from recent research on sleep. First, by lowering our metabolic rate in sleep, we conserve energy during the coldest part of the day. Second, because the senses are not gathering new information, during sleep we can reorganize and more efficiently store the information already in the brain.
After these two functions have been served by sleep, we are better prepared to handle the demands of our waking hours. After sleep, our bodies are able to meet a widely fluctuating set of energy demands as they generate more or less energy depending on such factors as the outside temperature and the effort required to obtain food and shelter. At the same time, our brains are better able to both gather data and guide our behavior.
More generally, we can say that sleep serves the functions of both homeostasis (or constancy) and heteroplasticity (or change), which are required for successful adaptation. Homeostasis refers to the tendency of physiological systems to resist change and maintain a constant set of internal conditions. Heteroplasticity refers to the capacity to change in response to new circumstances. While homeostatic systems are mainly concerned with energy regulation and heteroplastic systems are mainly concerned with information processing, the two functions can overlap. Most scientists are convinced that both energy conservation and information processing are served by sleep, but they do not yet have an objective physiological measure of our subjective sense of restoration and improved mental alertness after a night of sound sleep.