Synchronization of women’s menstruation with the Moon has decreased but remains detectable when gravitational pull is strong

To increase reproductive success, many species synchronize reproductive behavior with a particular phase of the lunar cycle. The human menstrual cycle has also a period close to that of the lunar cycle, and recent studies suggest a temporary synchrony between menstrual and lunar cycles. Nevertheless, lunar influence on human reproductive behavior remains controversial. Here, we analyzed long-term menstrual records of individual women from the past 24 years and compared them with records from the past century. We show that women's menstrual cycles recorded before the introduction of light-emitting diodes in 2010 and the extensive use of smart phones significantly synchronized with the Moon, while those after 2010 coupled to the Moon mostly in January. We hypothesize that the high gravimetric forces between the Moon, Sun, and Earth every January are sufficient for this coupling, while the increasing exposure to artificial light at night impinges on synchrony at other times.