Summary Of Grolier Multimedia Encyclopedia Advanced Search Button

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Turn Read Aloud On Grolier Multimedia Encyclopedia Advanced Search button Browse Articles buttonBrain Jam buttonNews buttonToday is...buttonResearch Starters buttonTimelines buttonEvent Planner buttonQuizzes and Games buttonAbout/Help Web Links button Periodicals button Sleep Table of Contents How to Cite This Article Sleep is a complex form of resting state that is observed in animals that have highly developed nervous systems. Birds and mammals, including humans, do not simply become inactive and slow to react during their daily rest periods. Their nervous systems shift into modes that differ from but are not necessarily less active than wakefulness. The Nature of Sleep Humans and nearly all other animals vary their level of activity according to the time of day. Daily rhythms are not simply responses to daylight and darkness or to other signs of the time of day. They are generated by the animals themselves. This is shown by experiments in which animals live in artificial surroundings that do not vary with time of day. They experience no daylight, no darkness, no day-night swings of temperature, and no contact with other animals that "know" the time of day. Under such conditions, the animals continue to exhibit near-daily, or circadian, rhythms of activity and rest. These rhythms must be generated by the animals ' own inner "clocks." Such biological clocks are normally reset every day, mainly by the cycle of daylight and darkness. An animal 's activity and rest

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