Menstrual Cycle: A Scientific Study Establishes a Link with the Moon

Due to their cyclic rhythms and similar durations, a link between the menstrual cycle and the lunar cycle has often been assumed, but until now, science has not been able to provide solid evidence.

To better understand the origin of the rhythmic regularity of the menstrual cycle, an international research team involving Inserm, the CNRS, and Claude Bernard University Lyon 1 compared a large number of cycle data collected in European and North American studies. Their results show that the menstrual cycle may be finely regulated by an internal clock, which is occasionally influenced by the lunar cycle.

This work, to be published in Science Advances, argues for the importance of further studying this potential link using larger datasets, to identify any potential benefit of chronobiology in the treatment of fertility disorders.


A typical ovulatory cycle, or menstrual cycle, in humans lasts an average of 29.3 days, with variations in length from one menstruating person to another and from one cycle to another in the same individual. It begins on the first day of menstruation and consists of three phases, each dedicated to carrying out a specific process related to ovulation, which occurs around the 14th day of the cycle.

Some studies have suggested that each of these three phases could be influenced by an internal clock, whose rhythm disturbance would be associated with irregularities in the menstrual cycle.

In humans, the best-known internal clock is the circadian clock, which is very close to 24 hours, and it maintains the wake-sleep cycle and all physiological rhythms. It is synchronized with the day-night cycle under the influence of light. When the circadian clock is disrupted - as in the case of jet lag, for example - it takes a few days to recalibrate to its usual rhythm by resynchronizing with the new day-night cycle.

In the case of menstrual cycles, the involvement of an internal clock could manifest in a similar way: the length of the cycle would usually be highly stable in the same individual and, in case of disturbance, adaptation mechanisms by synchronization with external conditions would come into play to restore the optimal rhythm.

So, what could this "external synchronizer" be? A recurring theory suggests that the lunar cycle could play this role, but scientific evidence has been lacking to date.

An international research team led by Claude Gronfier, Inserm researcher at the Neuroscience Research Center in Lyon (Inserm/CNRS/Claude Bernard University Lyon 1), explored the potential existence of an internal biological clock regulating the menstrual cycle that could be synchronized with the lunar cycle. Thanks to a large database of menstrual cycles collected in European and North American studies, they compared a total of nearly 27,000 menstrual cycles in 2,303 Europeans and about 4,800 cycles in 721 North Americans.

Initially, the researchers examined the stability of the menstrual cycle from one cycle to another at the individual level by comparing the lengths of successive cycles. They observed a general stability in the average cycle length of each participant, even though, across a series of successive cycles, some lasted indeed longer or shorter than her "standard" cycle.

"These observations suggest the existence of a mechanism that could correct the difference between the length of the current cycle and that of a typical menstrual cycle for the concerned individual," explains René Écochard, the lead author of the study, a doctor at the Hospices Civils de Lyon and a professor at Claude Bernard University Lyon 1. "A few shorter cycles could thus compensate for a series of a few longer cycles so that the total length of the cycle oscillates around the usual duration of the menstrual cycle. The length of a cycle could therefore depend on the length of previous cycles."

""The observation of this phenomenon argues in favor of the existence of an internal clock finely regulating menstrual cycles, itself synchronized by an environmental cyclic event," adds Claude Gronfier.

Subsequently, the research team looked at potential relationships between the onset of menstruation in the studied cycles and the phases of the moon at the time of data collection.

They observed an occasional but significant association between the menstrual cycle and the lunar cycle. However, without these studies being able to determine the cause, a major difference was observed between the European cohorts and the North American cohort: in Europeans, the cycle most often began during the waxing phase of the moon, while it more often began at the full moon in the North American cohort.

"Despite this surprising difference that we cannot explain at the moment, the identified links in this work between lunar and menstrual cycles, due to their proximity to certain phenomena we observe in chronobiology, suggest that the periodicity of menstruation and ovulation could be modestly but significantly influenced by the lunar cycle," clarifies Claude Gronfier.

These results therefore argue in favor of an internal clock system with a quasi-monthly rhythm, weakly synchronized by the lunar cycle. However, they need to be deepened and confirmed through more extensive laboratory studies and epidemiological studies.

"With the advent of cycle tracking apps for smartphones, the emergence of large databases containing cycle information from several hundreds of thousands of women could provide new study opportunities," indicates René Écochard.

"Confirming the existence of an internal clock coordinating the menstrual cycle, as well as the mechanisms involved in its synchronization, could allow for the application of so-called "circadian" medicine approaches, personalized - already used in oncology and for the treatment of sleep disorders or depression, for example - to issues like ovulation and fertility disorders," concludes Claude Gronfier.