The fact that most people sleep almost exactly the same amount each day under free-running conditions, despite large day-to-day variations in physical and mental activity provides strong support for the dominance of

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The fact that people sleep almost the same amount each day despite variations in activity suggests that sleep is regulated by internal, biological factors rather than external factors such as physical or mental exertion or environmental conditions like light or temperature. This is most likely due to circadian rhythms, which are internally generated 24-hour cycles that regulate sleep and other physiological processes in the body.

The Fact That Most People Sleep Almost Exactly the Same