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Biological clock
Biological clock. Conceptual computer illustration depicting human biorhythms, showing a woman's face with foetuses and a clock. Many biological processes take place under the direction of an internal clock mechanism controlled by hormones. Examples include the 40 weeks of pregnancy, as well as the menstrual cycle, which repeats itself roughly every 28 days in fertile women. Other examples are processes such as wakefulness and tiredness, which are based on a daily or circadian (24 hour) cycle regulated by the hormones melatonin and cortisol.
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