Circadian Rhythm: the natural pattern of physiological and behavioral processes that are timed to a near 24-hour period. These processes include sleep-wake cycles, body temperature, blood pressure, and the release of hormones. This activity is controlled by the biological clock located in the hypothalamus in the brain. It is highly influenced by natural dark-light cycles, but will persist under constant environmental conditions.
Brandon Peters, MD http:// sleepdisorders.about.com
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