September 24, 2025

expert reaction to study looking at the correlation between menstruation, the moon and artificial light

A study published in Science Advances looks at the impact of the moon and LED light on menstruation cycles. 

This comment was provided by our friends at SMC Spain:

Claude Gronfier, Senior Research Associate and Prof of Circadian Biology at the National Institute of Health and Medical Research (Inserm), Lyon Research Centre in Neurosciences (France), said:

“This is a fascinating article that looks at the association between the menstrual cycle and the lunar cycle. The study was conducted among 176 mainly European women who recorded their menstrual cycles for an average of 5 years and 8 months. The authors observed five main findings:

“1- A statistically significant association between the lunar cycle (light) and the menstrual cycle. The same authors had observed a similar association in their previous study of a small group of 22 women, and this result was confirmed with a high level of evidence by René Ecochard and his colleagues in Lyon in the same journal in 2024 in a study of 3,000 women (Ecochard R et al. SciAdv 2024 https://www.science.org/doi/10.1126/sciadv.adg9646). This result indicates that this association, which is similar in three well-conducted studies (Helfrich-Förster C et al, Sci Adv 2021 – 22 women, Ecochard R et al. Sci Adv 2024 – 3,000 women, Helfrich-Förster C et al. Sci Adv 2025 – 176 women), is not due to chance. The level of evidence is high.

“2- The menstrual cycle is controlled by a circadian clock with a limited training period. This result is interesting. It suggests that a clock with a period (rhythm) close to that of the lunar cycle (light) controls the menstrual cycle. This is entirely conceivable, as certain phenomena observed in the data are reminiscent of those observed with other biological clocks. [My] team in Lyon made a similar hypothesis in 2024, proposing that the circadian clock that governs our daily physiology (our sleep-wake cycle, for example) could be responsible for this rhythm of the menstrual cycle (Ecochard R et al. Sci Adv 2024 https://www.science.org/doi/10.1126/sciadv.adg9646). Further studies are needed to validate this hypothesis and determine the mechanisms involved, as the level of evidence is still modest.

“3- The association between the lunar cycle (light) and the menstrual cycle has been found to be weaker over the last 15 years than previously. The authors suggest that this is linked to light pollution. This hypothesis is interesting because it highlights an environmental risk that is still largely overlooked, but for which health effects have been observed in connection with light pollution. It would have been useful 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 in the participants’ environment did indeed change between before and after 2010, but this could not be done. This remains to be done, as it cannot be ruled out that this weaker association observed over the last 15 years could be partly linked to other environmental factors (increased temperature linked to global warming, or CO2, or diet, etc.) or individual factors (ageing of study participants, more sleep disorders, etc.). Caution should be exercised when interpreting this result, which is based on a relatively small group of women.

“4- The association between the lunar cycle (light) and the menstrual cycle is strongest every ~18 years (long-period gravimetric lunar cycles). This result is difficult to interpret because there is no consensus on the possible role of lunar gravimetry on human biology, nor are there any known mechanisms to explain it. It cannot be ruled out, but it is intriguing and must be treated with caution, as it is based on a relatively small group of women.

“5- A Google Trends search shows more queries for the words ‘painful periods’ during perihelion (~in January) in Germany and Australia, and the authors suggest that this seems to confirm the fourth result of their study, which suggests an association between gravimetry and the menstrual cycle. It is difficult to interpret this result, which is based on data from Google searches, which are known to be highly sensitive to the environment, sociology, politics, seasonal rhythms, cultural factors and media effects. This is particularly true given that the same Google search does not show any association in other countries (Italy, France, New Zealand). That said, this result cannot be ruled out, but the level of evidence is modest. Overall, this article is fascinating but should be read with the caution required when reading any scientific article. The first two findings are very convincing and consistent with other studies, reinforcing the idea that the lunar cycle and the menstrual cycle are temporally linked and that a biological clock may be involved. The other three findings—the hypotheses of a role for lunar gravimetry, light pollution, and its confirmation by Google search profiles for ‘painful periods’—present a modest level of evidence. However, they are interesting because they raise questions that should not be dismissed but, on the contrary, can and deserve to be investigated in other studies and by other investigators, using other approaches and larger populations.”

‘Synchronization of women’s menstruation with the Moon has decreased but remains detectable when gravitational pull is strong’ by Charlotte Helfrich-Förster et al. was published in Science Advances at 19:00 UK time on Wednesday 24^th September. 

Declared interests

Claude Gronfier: Declares having no conflict of interest.