Is women’s menstrual cycle linked to that of the Moon? A new study may provide answers

By Roberta Attanasio

In the rush of modern life, we often fail to notice the link between our biological rhythms and those of the natural world. In many animal species, the reproductive cycle is synchronized with different Moon phases—most likely because reproductive success is facilitated when different individuals within a species behave in the same way at the same time. The moon phases act like a guide to synchronize courtship, mating, and other reproductive behaviors.

But what about women? To some people, a connection between the menstrual cycle and the Moon cycle is apparent because their length is very similar—about 29 days. Others argue that there is no connection, as the length of the menstrual cycle varies a bit among women. In addition, some says that if there was a connection all women would have fertile days at the same time. Now, thanks to a new study, we might be able to side more with one of these two opinions as compared to the other.

A team led by Charlotte Helfrich-Förster, a Chronobiologist at Julius-Maximilians University of Würzburg in Germany, set on a study to find out whether Moon cycles influence human menstrual cycles. The researchers examined the long-term menstrual cycle records kept by 22 women for up to 32 years and constructed a temporal raster plot—a graph that shows time-based relations—to point out any times during which the women’s menstrual cycles occurred in sync with the Moon’s cycles. Although the researchers found that menstrual rhythms varied greatly among the women and over time within individuals, they also found that most women’s menstrual cycles aligned, at certain intervals, with both luminosity and gravitational pull cycles of the Moon. Thus, the study results (published in the journal Science Advances on January 27, 2021) suggest that both cycles cooperate to influence reproduction in humans and, when together, have a stronger effect.

The researchers also found that on average, in women under 35 years of age, menstruation coincided with the full moon or the new moon in about a quarter of the recorded time. For women over 35, it occurred on average in about one tenth of the time. Furthermore, concurrence of Moon and menstrual cycle not only decreased with increasing age, but was also dependent on the extent of exposure to artificial light at night. Therefore, the researchers hypothesize that, in ancient times, human reproductive behavior was in sync with the Moon phases, but modern lifestyle may have changed reproductive behavior and also biological aspects of reproduction.

The researchers recognize that their study included only a relatively small number of women, so the results are not conclusive. Therefore, they’re thinking to develop a mobile phone app to collect data—menstrual cycles and exposure to artificial light at night—for a new larger study that will involve women from all around the world. The researcher will then analyze the app data to uncover the relationship between Moon cycles, exposure to artificial light at night, and menstrual cycle.