Put simply, sleep is what you are doing when you are not awake. Generally, it refers to a reversible behavioral state during which we maintain relative disengagement from the external environment, a general unawareness of time and what is going on around us. Sleep is usually associated with a state of sustained immobility, and ordinarily (but not always) a recumbent, lying down posture is adopted, with most of us sleeping with our eyes closed.

Mammals vary enormously both with respect to the number of total hours they sleep as well as how many times during the day and night they sleep to achieve that number. For example, the nocturnal opossum can sleep (or rest) about 18 hours a day, in one long consolidated sleep bout,

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