Sleep is a natural state of rest that is as essential to a person's well being as food and water. Without enough sleep, the ability to perform even simple tasks declines dramatically. Sleep is easily distinguished from other sleeplike states, such as a coma, because it is easily interrupted by external stimulation.

The sleep process is governed by a complex architecture and characterized by five stages. During the first stage, muscles relax and a person's brain waves become irregular and rapid. But the official onset of sleep doesn't begin until stage two, when eye movements cease and brain activity intensifies. Eventually, brain activity subsides and the person drifts into a profound slumber. The next stages, three and four, are marked by large, slow brain waves. A person who awakens during these stages of deep sleep will usually feel groggy and disoriented. About 75% of sleep is spent in these...