OSCAR BURRIEL / SCIENCE PHOTO LIBRARY OSCAR BURRIEL / SCIENCE PHOTO LIBRARY
Biological clock. Abstract image depicting human biorhythms, showing a clock super- imposed on a woman's face. Many biological processes are believed to take place under the direction of an internal clock mechanism. These processes vary in a rhythmic way (for example the menstrual cycle, which repeats itself about every 28 days in fertile women and has often been associated with the moon's cycle). Most bio- rhythms, 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. The fertile lifespan of a woman is also sometimes portrayed as a biological clock.
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