VICTOR DE SCHWANBERG / SCIENCE PHOTO LIBRARY VICTOR DE SCHWANBERG / SCIENCE PHOTO LIBRARY
Biological clock. Abstract image depicting human biorhythms, showing a clock superimposed onto a woman's head. Many biological processes are believed to take place under the direction of an internal clock mechanism. Processes such as wakefulness and tiredness are based on a daily or circadian (24 hour) cycle regulated by hormones such as melatonin and cortisol. Another example is the menstrual cycle, which repeats itself roughly every 28 days in fertile women.
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