The results of the article published in the prestigious journal Science Advances, after undergoing the peer-review process that attests to the soundness of the study, add to scarce scientific evidence on the various factors that may converge in the regulation of the menstrual cycle. Although we know about the influence of the hypothalamus-pituitary-ovary axis in the cyclical hormonal fluctuation that allows the succession of the different stages of the ovarian/uterine cycle, we don't fully understand the other factors that, by acting on this axis, could be involved in the coordination of a physiological process that is vital for our survival as a species. 

In general terms, this study analyses two large epidemiological databases from Europe and North America, with information on the menstrual cycles of women between 18 and 50 years of age without related pathologies. The aim of the study was twofold: on the one hand, to determine whether variations in the monthly rhythmicity of the menstrual cycle, i.e. its duration, could be governed by some kind of unknown circadian clock (as is the case with other internal clocks known to govern our daily sleep/wake cycles, blood pressure, temperature, enzyme and hormone production, etc.); and on the other hand, studying whether these variations could be synchronised with an external process, the lunar cycle, that plays a decisive role in other natural events.  

By applying methods for the numerical analysis of circadian rhythms, the authors concluded that their results supported the existence of this hypothetical endogenous clock. This would help to adjust for discrepancies in the length of the menstrual cycle that occur over a given period of time, i.e. the alternation between shorter and unusually long cycles, to compensate for oscillation around an internally marked periodicity. As the researchers point out, the mechanism could be similar to the one that our organism sets in motion to correct the decompensation in the sleep/wake rhythm after an intercontinental trip, the famous jet lag.  

Regarding the synchronising role of the different phases of the lunar cycle with the onset of the menstrual cycle, the association between the two variables was moderate. It was observed that European women started menstruating more frequently in the crescent moon, while North American women started menstruating more frequently in the full moon. Among possible explanations, this phenomenon is suggested to be reminiscent of our evolutionary process from aquatic species that, as is the case today, depend on the tides to reproduce. Both conclusions are very interesting and represent a turning point in the advancement of knowledge about female reproductive physiology. 

Among the limitations, which the authors themselves highlight, is the difference in the time periods in which the databases analysed were collected (between 1960-1990 for the European database, as opposed to 2000 for the North American database). This could influence the results obtained, particularly the geographical differences in the synchronisation of lunar and menstrual cycles. Our lifestyle habits (diet, physical activity, socialisation, sun and artificial exposure, etc.) influence the functioning of our bodies, including reproduction, and have changed a lot in the last 60 years. It would therefore be advisable to carry out further research to corroborate the evidence obtained using current big data, which is available thanks to the mobile applications used by millions of women around the world to monitor our menstrual cycle.  

Finally, it is worth highlighting the clinical application of this evidence, particularly in relation to fertility problems and potential therapeutic approaches from a chronobiological perspective, which has been shown to be effective against other pathologies such as cancer, sleep disorders or depression. In this way, we could advance in the emerging field of personalised circadian medicine. 

For chronobiologists and chronobiology outsiders alike, it is striking that at this point in our history as a species we still do not know in detail the reproductive physiology of women, and in particular the influence of various factors, both internal and external, on the regulation of the menstrual cycle. This only highlights the need to strengthen current research in the field of women's health, with the aim of moving away from the androcentric vision that has prevailed in modern medicine, and towards Sustainable Development Goals 3 and 5 - 'Good Health and Well-being' and 'Gender Equality', respectively - promoted by the World Health Organisation. 

The study by Ecochard et al. published in Science Advances shows a high methodological quality, as it includes a large number of data points (more than 3,000 women collectively, providing data from more than 30,000 menstrual cycles). In addition, the authors apply specific statistical techniques for the analysis of chronobiological patterns.  

However, the study is limited to participants living in Europe and North America and most of the data was collected before 2000. Firstly, it would be appropriate to conduct the analysis with participants from, if not all continents, at least both hemispheres and different latitudes. These geographical differences, as well as the different seasons, could provide interesting information on other external mechanisms that could be interfering with the process, beyond the lunar phases. In addition, as the authors point out, light pollution has increased in recent decades with greater night-time illumination and increased use of screens and backlit devices. Therefore, it would be interesting to also collect data on participants' light exposure, among other lifestyle variables. 

During routine ovarian stimulation treatments, substances are administered to pharmacologically control the functioning of the ovaries, independently of the internal control of the process, so this would have a no direct impact on clinical practice. However, if a mediator (e.g. melatonin or light therapy) were to be discovered that could play a role in regulating ovarian function, this could be useful to improve ovarian response, for example in treatments with low doses of hormones. 

However, this is an emerging field and much work remains to be done to confirm the results and describe the molecular mechanisms involved. Once this knowledge has been obtained, it will be time to test the effectiveness of different strategies to exploit the control of the menstrual cycle rhythm to improve the results of fertility treatments.