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Sleep, also known as sleep, is a natural resting state common in mammals, birds, and fish. In addition, it is also found in invertebrates, such as flies. Characteristics of sleep include: reducing active body movements, weakening responses to external stimuli, enhancing assimilation (production of cell structure), and reducing levels of dissimilation (decomposing cell structure). In humans, mammals, and many other animals that have been studied, such as fish, birds, mice, and flies, regular sleep is a prerequisite for survival. Unlike coma, sleep is more likely to be interrupted and return to waking state. Wake up from sleep is a protective mechanism and a necessity for health and survival. For people, sleep accounts for one-third of life.