What does the Moon do in menstrual cycles? A study points to an occasional but significant association

This work, however, does not allow us to attribute a possible role to the Earth’s natural satellite in the rhythm of menstruation.

Does the Moon influence menstruation? This question is the subject of abundant pseudo-scientific literature, which attributes to this star a supposed influence on the course of our lives. In the case of periods, it is the similar duration of the two cycles which is often highlighted. An international research team bringing together Inserm, CNRS, and Claude Bernard Lyon 1 University looked into the regularity of menstrual cycles in order to understand their mechanism. Their work published in the journal Science Advances on Wednesday, April 10, made it possible to show that the rules were regulated by an internal clock and to identify an association punctual with the lunar cycle.

Chronobiology, a growing discipline, studies biological rhythms in the body. Most biological functions are in fact subject to a “circadian” rhythm, which designates a cycle lasting approximately 24 hours (sleep, hormone production, alertness, blood circulation, etc.). The circadian rhythm is called “endogenous”, because it is managed by the body itself, from an internal clock located in the hypothalamus, with electrical activity that oscillates between 11:30 p.m. and 24:30 p.m. depending on the individual (24:10 p.m. on average), explains Inserm on its site.

An internal clock regulates menstrual cycles

Logically, we should all live offbeat. But life in society is nevertheless possible, fortunately, because this cycle is constantly smoothed over 24 hours under the effect of several external elements. Among these exogenous factors, light is the most powerful “synchronizer”, which explains the recurring debate on the possible consequences of the transition to wintertime. But menstrual cycles have a rhythm greater than 24 hours (known as “infradian”). A typical ovulatory cycle lasts on average 29.3 days, with variations in length from one person to another and from one cycle to another in the same person.

Would they therefore be deprived of a mechanism to ensure their regularity? To find out, the authors of the study analyzed data on 32,000 monthly cycles in 3,000 European and North American women. Their results are clear: the cycles can last more or less long, but the cumulative duration of several successive cycles is generally stable. “Cycles tend to oscillate around their meanexplains to franceinfo René Ecochard, a doctor at the Hospices Civils de Lyon (HCL). If there are longer cycles, there will also be shorter cycles, as if the body is remembering that it has fallen behind.”

“We find a correlation between a cycle and the one that precedes it by three or four months, which is impressive. The cycles are not independent. There is a clock, a memory. –“René Ecochard, first author of the study and doctor at the Hospices Civils de Lyon at franceinfo

“This rhythmicity is very precisely controlled with long-term corrections”, continues Claude Gronfier, co-author of the study and chronobiologist at Inserm. At this point, however, no one knows whether the clock involved is the same one guiding the circadian rhythm from the hypothalamus. The hypothesis is, however, plausible. “Certain animals can only be impregnated at one time of the year and give birth at another, however, observes the researcher. And we know that their seasonal cycle is controlled by the circadian biological clock.”. Second hypothesis: this is another cycle inherited from ancient species, why not maritime ones, and subject to the tides?

A statistical link between the Moon and periods

The authors then wanted to check if the Moon could play a role as a “synchronizer” of menstrual cycles, as light can do in circadian rhythms. The existence of a link between the natural satellite of the Earth and the intimacy of women, until now, has never really been resolved. “There is no consensus on the question underlines Claude Gronfier. A third of the studies showed a link, another third did not show one, and a final third of the studies were too poorly done. Most of the time, studies have focused on very small populations, with an insufficient number of cycles and without long-term follow-up.”

This time, the authors were able to apply their statistical methods to a large dataset, with the collaboration of American colleagues. And surprise: associations then appeared “occasional” but “significant” between menstrual cycles and the Moon cycle.

“The menstrual cycle drifts at its own speed. The lunar cycle drifts at its own speed. And, at some point, the two will intersect. This association will last between one and three days, depending on the woman.” Claude Gronfier, chronobiologist at Inserm at franceinfo

Main lesson of the study: this brief temporal correlation exists in all women. “There is a link with the Moon. It is modest, but it exists. There is no doubt”, summarizes René Ecochard. At this stage, however, this observation does not allow us to say that the Moon “synchronizes” the menstrual cycles regulated by an internal clock. “We do not know what this association can be used for and we cannot explain it for the moment”, explains Claude Gronfier. To add to the mystery, cycles begin more often at the waxing Moon for European women, and at the full Moon for American women.

“We discover that there is an internal clock for menstrual cycles. And that it is externally sensitive, like the nycthemerus [cycle biologique de 24 heures].” —René Ecochard, doctor at the Hospices Civils de Lyon at franceinfo

Further study is needed to know if the Moon really plays a role in the regularity of menstrual cycles. And even if this is true, it will still require a complex statistical model, with a much larger data set, to assess the importance of this element in the harmonization of cycles. This will undoubtedly be modest, warn the authors.

“The weekly effect is much stronger than a possible effect of the Moon, warns René Ecochard. People don’t see it too much, but periods most often come on Thursdays and Fridays. I am working on the way, and in particular on sleep data. When it is dark, in fact, we secrete melatonin, which acts on the ovarian follicle. All this work may one day make it possible to implement innovative treatments against ovulation and fertility disorders.