Throughout history, humanity’s daily life has been intertwined with changes in the sky, such as the varying “faces” of the moon. One of the many legends surrounding the moon suggests that the lunar phases influence our daily moods and feelings. Notably, women’s menstrual cycles typically last between 28 to 35 days, which closely resembles a lunar cycle or a lunar month lasting approximately 29.5 days. Is this a coincidence, or is it a systematic phenomenon?
According to science writer Luis Villazon, an expert responding to questions on BBC Science Focus, women’s menstrual cycles align with the lunar cycle in duration but do not correspond to any specific lunar phase.
He references the renowned biologist Charles Darwin, who suggested that the human menstrual cycle lasting 28 days is evidence that our ancestors lived near coastal areas and needed to synchronize with the tides. It is certain that the phases of the moon influence the behavior of many animal species.
The fiddler crab (a type of crab found in swampy and mangrove areas) is more active during the full moon and new moon because higher tides make their burrows more easily detectable. Night-feeding birds tend to avoid foraging on full moon nights due to the increased light making them more vulnerable to predators. However, the human menstrual cycle, while matching the lunar month in length, does not coincide with any specific lunar phase.
A frequently cited study published in the American Journal of Obstetrics and Gynecology in 1980 found some evidence of this coincidence, but the effect was weak. In a sample of 312 women, 244 had cycles longer than 29 days or shorter than 27 days, and only 70% of the remaining women actually started their periods within two weeks of the full moon.
(Image: vatlythienvan.com)
If linking our reproductive cycles to the lunar month is advantageous, one might expect other animal species to do the same. Orangutans and marsupials have 28-day cycles, but our closest relatives, chimpanzees, have a 35-day cycle. Non-primate mammals have estrous cycles that operate differently from menstrual cycles, but none of these species have reproductive processes that align with the phases of the moon.
Nevertheless, ancient myths are beginning to be substantiated by science.
According to Space, a new study published in the journal Science Advances at the end of January this year suggests that the fairy tales about the moon have partly become reality. Researchers argue that although all the myths surrounding the correlation between the lunar cycle and menstruation may not be entirely true, there could still be some connection between the lunar cycle and menstrual cycles.
In this study, scientists analyzed long-term data on the start dates of menstruation over an average period of 15 years from records of 22 women both under and over the age of 35. By comparing this data with lunar cycle fluctuations, researchers found that in women with menstrual cycles longer than 27 days, there was “an intermittent alignment with cycles affected by lunar brightness.” This correlation gradually diminished over time as the participating women aged, and the association decreased with increased exposure to artificial light.
Menstruation is also affected by changes in the moon’s gravitational forces. (Image: Health Journal)
“The menstrual cycle corresponds with the tropical month 13.1% of the time in women aged 35 and younger, and 17.7% of the time in women over 35, indicating that menstruation is also affected by changes in the moon’s gravitational forces,” the study concluded.
A tropical month lasts 27.321 days, or 27 days, 7 hours, 43 minutes, and 4.7 seconds. The tropical month is based on the vernal equinox and refers to the time it takes for the moon to return to the same longitude.
An average lunar month (synodic month) lasts 29.5 days, which is the time between two full moons or two new moons—the duration it takes for the moon to return to its original position in the sky relative to the sun. The lunar month is longer than the tropical month because as the moon orbits the Earth, the Earth is also revolving around the sun, requiring the moon a few additional days to return to the same position in the sky relative to the sun (as seen from Earth).
The moon affects not only menstrual cycles but also our sleep patterns.
Another study published in Science Advances in January showed a correlation between sleep cycles and the lunar cycle. This research, independent of the previous study, involved a team of scientists from Washington University, Yale University (USA), and the National University of Quilmes (Argentina).
The findings indicated that during the days leading up to the full moon, we tend to go to bed later and sleep a few hours less.
In this study, participants included university students from Seattle, Washington, and individuals living in indigenous communities in northern Argentina, two different environments that vary in access to electricity, as artificial light might affect participants. Ninety-eight individuals from three indigenous Toba-Qom communities in Formosa, Argentina, and 464 university students in the Seattle area wore wristbands to monitor their sleep.
The results showed that the connection between sleep cycles and the lunar cycle was present among individuals living in off-grid communities in Argentina as well as in electrified areas in the USA, with the connection being more pronounced in off-grid locations. The nights before the full moon were when people slept the least and went to bed the latest, coinciding with brighter skies as the crescent moon became more illuminated.
“We observed a clear adjustment in sleep patterns according to the moon, with fewer hours of sleep and later bedtimes in the days leading up to the full moon.” “While this effect is stronger in off-grid communities, it also appears in electrified communities, including among university students in Washington,” stated lead researcher Horacio de la Iglesia, a biology professor at the University of Washington.
“We hypothesize that the observed patterns are an innate adaptation that allowed our ancestors to take advantage of natural nighttime light at specific times in the lunar cycle,” said Leandro Casiraghi, a postdoctoral researcher in biology at the University of Washington.
