The Earth formed 4.6 billion years ago, and the Moon appeared just 100 million years later. Since then, the satellite has exerted an enormous influence on our planet, giving it a balance conducive to life and subsequently affecting the existence of the living beings it helped make possible.
It has been shown that many fish, corals, and marine invertebrates synchronize their egg-laying with the phases of the Moon, ensuring that they all reproduce at the same time to increase their chances of reproductive success. This synchrony has also been observed in badgers and in certain breeds of cows in Venezuela and Japan. The new moon has been linked to higher fertility in macaques, a phenomenon which is believed to result from the cover of darkness that protects them from predators. Mountain gorillas, which have few natural predators, seem to exhibit this synchrony closer to the full moon.
Although the similarity between the lunar cycle (29.5 days from one full moon to the next) and the human menstrual cycle (between 26 and 32 days, with an average of 29) suggests that the Moon could influence female ovulation, scientific evidence has not allowed for clear conclusions. On Thursday, the journal Science Advances published a study that finds a certain lunar effect on the menstrual cycle and may also explain why many recent studies failed to detect it. The work, led by neurobiologist Charlotte Helfrich-Förster of the University of Würzburg in Germany, analyzed more than 11,500 menstrual records from 176 women collected over the past 70 years, some spanning more than three decades of observation.
Studies from the 1970s and 1980s already suggested that women with cycles closer to the 29.5-day average tended to menstruate around the full or new moon. However, in recent years, several studies found no such synchrony, and the authors of that research did not observe it, either.
Helfrich-Förster broadened the focus because she suspected that a technological change could have caused a shift. “I decided to analyze the data from before 2010 and after. Why? Because that’s when LED lights came to market, replacing traditional bulbs, and people were increasingly using screens that emit blue light, to which our eyes are highly sensitive,” she explains.
Her results show that in the records prior to 2010, many women’s menstruations aligned with the full or new moon, both individually and at the population level. After 2010, the synchrony almost completely disappeared, although the alignment is still detectable in January, when the Earth is closest to the Sun, and during periods when the Sun and Moon’s gravitational effects align.
Helfrich-Förster acknowledges that this gravitational effect, also identified in other studies with Indigenous people in Argentina or students in Seattle, is difficult to explain. “The moon has a significant gravitational effect on Earth; you see it with the tides, but we don’t know how a human being can feel that effect, and any physicist will tell you that just by climbing a staircase you experience more gravitational changes than the Moon can exert on us,” she says. “Perhaps we can feel something indirect that comes with gravitation, like changes in atmospheric pressure during the lunar cycle, but I can’t provide an answer.”
María de los Ángeles Rol de Lama, director of the Chronobiology and Sleep Laboratory at the University of Murcia, who did not participate in the study, recognizes the research’s value and significance, although “it goes against the scientific consensus that existed until now.” She also points out the difficulty in discerning the effects of lunar cycles in people who are going about their normal lives, and are exposed to a large number of circumstances can influence the menstrual cycle. “To measure this accurately, you would have to put women in a bunker for a month,” she argues.
This has been done in the past, although such experiments are difficult to replicate. In 1962, Michel Siffre, a 23-year-old French scientist, isolated himself in caves without clocks or natural light for months, and demonstrated that the human body maintains an internal rhythm close to 24 hours but loses synchronization with the solar day — an experiment that revealed the existence of an internal biological clock.
Claude Gronfier, who last year published an article suggesting an internal clock regulates the menstrual cycle and downplaying the influence of the Moon, considers that the newly published results and other recent similar studies indicate that the relationship between the lunar and menstrual cycles “is not coincidental.” However, regarding the hypothesis that smarphones and LED light are responsible for the loss of synchrony between menstruation and the Moon, he believes that “it would have been useful to be able to test this hypothesis using records of the light to which the participants in this study were exposed, in order to verify that the light intensity of the participants’ environment had actually changed before and after 2010.”
“This remains to be seen, as it cannot be ruled out that this weaker association observed over the past 15 years may be partly related to other environmental factors (increased temperatures linked to global warming, CO2, diet, etc.) or individual factors (aging of the study participants, more sleep disorders, etc.),” he concludes, urging caution when interpreting a result “based on a relatively small group of women.”
The mystery of menstrual synchrony with the Moon, like other possible effects of the satellite on human affairs, seems far from resolved. Cycles can lengthen or shorten due to stress, hormonal changes, restrictive diets, or obesity. Additionally, because the average menstrual cycle length is similar to — but not identical with — the lunar cycle, casual coincidences are easily mistaken for true synchrony. Many studies also rely on small samples or inaccurate self-recorded data collected by the women themselves, which further complicates analysis. As a result, just as with studies attempting to determine if women living together synchronize their periods, lunar effects sometimes appear in the data but are intermittent, weak, and difficult to replicate rigorously.
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