An analysis of long-term menstrual cycle records kept by 22 women for up to 32 years shows that women with cycles lasting longer than 27 days intermittently synchronized with cycles that affect the intensity of moonlight and the moon's gravitational pull. This synchrony was lost as women aged and when they were exposed to artificial light at night. The researchers hypothesized that human reproductive behavior may have been synchronous with the moon during ancient times, but that this changed as modern lifestyles emerged and humans increasingly gained exposure to artificial light at night. While previous research suggests that women with menstrual cycles that most closely match lunar cycles may have the highest likelihood of becoming pregnant, lunar influence on human reproduction remains a controversial subject. To address this long-standing mystery, Charlotte Helfrich-Förster and colleagues examined long-term data on the onset of menstrual cycles spanning an average of 15 years, including records from 15 women age 35 or younger and 17 women over 35. To uncover any times during which the women's menstrual cycles occurred in sync with lunar cycles, the researchers displayed the data as raster plots (graphs that show time-based relations) along with fluctuations in the moon's cycles. The researchers found that most women's menstrual cycles aligned with the synodic month (the time it takes for the moon to cycle through all its phases) at certain intervals. Menstrual cycles also aligned with the tropical month (the 27.32 days it takes the moon to pass twice through the same equinox point) 13.1% of the time in women 35 years and younger and 17.7% of the time in women over 35, suggesting that menstruation is also affected by shifts in the moon's gravimetric forces. Furthermore, the researchers observed greater synchronization between lunar and menstrual cycles during long winter nights, when women experienced prolonged exposure to moonlight. While the moon's luminescence and gravimetric cycles each appeared to only weakly affect menstrual cycles on their own, the findings suggest that these cycles exhibit a stronger effect together, with menstrual cycles most in sync with lunar rhythms when the moon is nearest to the Earth. (Note: This study includes one author (Ignacio Spiousas) in common with the study by Casiraghi et al., highlighted in this same issue of Science Advances.)
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Journal
Science Advances