OSCAR BURRIEL / SCIENCE PHOTO LIBRARY OSCAR BURRIEL / SCIENCE PHOTO LIBRARY
Cycle of sleep. Abstract image of a clock superimposed over a woman asleep and waking, depicting the biological cycle of sleep. Many biological processes are believed to take place under the direction of an internal body clock mechanism. Sleeping and waking, for example, are based on a daily or circadian (24-hour) cycle and are regulated by hormones such as melatonin and cortisol.
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