The moon's influence on menstruation is a controversial subject, with most believing there is no correlation. However, a study conducted by Helfrich-Förster and colleagues show that the lunar cycle does influence the menstrual cycle.
Researchers hypothesized that reproductive behavior was in synchrony with the moon. However, modern lifestyle factors and exposure to artificial light have altered this and reproductive physiology and behavior have changed.
Unlike previous studies, which investigated large numbers of women over a short period of time, Helfrich-Förster and colleagues analyzed records from a small number of women over a long period of time. This approach had not been used previously.
Their results showed that all women respond to the lunar cycle, but only during a time interval in their life. This could be a few months or a few years. The study also suggested that menstruation is affected by shifts in the moon's gravitational pull. In addition, the researchers observed greater synchronization between lunar and menstrual cycles during long winter nights, when women experience prolonged exposure to moonlight.
Several studies have shown that artificial night light shortens the length of the menstrual cycle. This reduces the probability of synchronization with the lunar cycle. Ongoing synchronization is only possible if the length of the menstrual cycle is close to that of the lunar cycle.
Another study suggested, the fact that the period comes slightly before the full moon is notable because at that time, the Moon shines brightly during evening hours when women are still awake. Consequently, this makes women more likely to be exposed to it. In addition, they are more likely to be exposed to moonlight for a longer period at that time because sleep onset occurs at its latest times during the nights leading up to the full moon.
Many scientists and studies reject the belief that the lunar cycle and menstrual cycle are related, but these studies show a correlation.
- C. Lin, D. F. Kripke, B. L. Parry, S. L. Berga, Night light alters menstrual cycles. Psychiatry Res. 33, 135–138 (1990).