Sleep
Paragraph 1: Why do we sleep? It can be dangerous to lose awareness of the external world. If nothing else, sleep might seem to take away from time that could be spent on productive, or even necessary, activities. But people cannot override indefinitely the desire to sleep; the body shuts down whether we like it or not. There are three general explanations that describe the adaptiveness of sleep: restoration, circadian cycles, and facilitation of learning.
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