Do people have biorhythms?
(Lansing State Journal, December 29,1993)
You are probably aware of a number of normal biological cycles. Migrations of birds are yearly, menstrus in women is monthly, and sleep patterns are more or less daily. Many more go largely unnoticed. Your internal temperature varies with your sleep wake cycle, being lowest when you are in deep sleep. In fact, it is sometimes harder to wake up on a cold morning because your body temperature is one of the physiological cues to wake up. The concentration of various hormones in your body also vary on a daily basis. For example growth hormone is generally highest in the middle of the night.
These rhythms are greatly influenced by environmental cues, including light and eating schedule. These cues help to set our internal clocks, but can cause physiological confusion when present in unexpected ways. Consider light. Nature provides a daily cycle of light every day, but human activity goes on around the clock thanks to electric lighting and intercontinental air travel. When we shift to very different schedules, say after a trans Atlantic flight, there is a period of adjustment, in which appetite and sleep schedules are affected. This is the basis for what is commonly referred to as "jet lag". After several days the stability of the new cues, that is the new time zone, realigns one's internal clocks, and the traveler feels fine.
Real biological rhythms are in synch with natural physical cycles: day and night, the changing seasons. The major exception to this rule is human menstrus, which is iremarkably independent of external cues. Despite this, the claims of some that so called "biorhythms" with 29-22 day cycles, affect our lives are false. Critical investigation of these super monthly cycles has shown no evidence that they exist. Very popular in the '70's the appeal for this unscientific fortune telling has waned, but machines and computer programs can still be found that claim to predict your daily physical, emotional and intellectual state.