Biorhythms. Abstract illustration of human bio- logical time, showing a clock face superimposed on a woman's face, and planets of the solar system. Many biological processes are believed to take place under the direction of an internal clock mechanism. They vary in a rhythmic way (for example the menstrual cycle, which repeats itself approximately every 28 days in fertile women and has often been associated with the moon's cycle). Most biorhythms, 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 mechanism is poorly understood and is a subject of current (1990's) research.