The menstrual cycle is like a biological calendar that a woman’s body follows each month and in which a series of events are repeated to prepare the female body for a possible pregnancy. It is a natural process that lasts on average 28 days – although it can vary from one woman to another and from one month to another – in which hormonal changes take place. It is as if nature said: “We are ready for life!” However, we still do not know in detail the reproductive physiology of women, nor how various factors, both internal and external, influence the regulation of the menstrual cycle.
Now, an international group of scientists bringing together Inserm, the CNRS and the Claude Bernard Lyon University 1 has investigated the female ovarian cycle and specifically whether its monthly rhythm is controlled by an internal mechanism similar to a clock, or by external factors, as well as as its relationship with the lunar cycle. Analyzing two large epidemiological data sets that include cycles of European and North American women, the researchers found evidence that the menstrual cycle is likely regulated by an internal mechanism.
Additionally, they have discovered a slight but significant influence of the 29.5-day lunar cycle on the menstrual cycle, although this influence varies between European and North American populations. These findings suggest the possibility of optimizing the ovulatory cycle through chronobiological approaches to treat fertility problems in women. Their findings have been published in Science Advances.
How circadian rhythms influence menstruation
The menstrual cycle, with an average duration of 29.3 days, is a biological rhythm that exceeds 24 hours. Although it is known to have three phases (latency, fertile window and postovulatory), the question of whether an internal clock regulates its duration has remained unanswered. The authors assumed that there is some evidence to suggest that each of the three phases of menstrual cycles may be under the influence of the circadian synchronization system, and that alteration of circadian rhythms is associated with alterations in menstrual function.
Previous studies have shown that air pollution has an effect on the timing of ovulation, or on the duration of the post-ovulatory phase, while others have described an effect of light exposure and shift work. However, to date, there is no strong or even moderate evidence to support the existence of an endogenous timing system that drives the entire menstrual cycle. The new study used advanced statistical analysis on menstrual cycle data from women in Europe and North America (26,912 cycles from 2,303 European women and 4,786 cycles from 721 North American women) to explore this question.
The findings open the door to future research into how chronobiology can be applied to improve female fertility.
The researchers observed that the duration of the menstrual cycle shows a regulation reminiscent of the duration of previous cycles, suggesting the existence of an internal timing system of endogenous origin. Additionally, they identified a connection between menstrual and lunar cycles, although this relationship is weak and varies between the populations studied. This could indicate that the menstrual cycle, and therefore ovulation, could be directly influenced by the lunar cycle or be a holdover from our evolutionary past.
The study also mentions that the length of the menstrual cycle has a genetic basis, related to the regulation of follicle-stimulating hormone, and is influenced by clinical factors such as age, the number of pregnancies and births, and the age of menarche.
This in-depth analysis suggests that the menstrual cycle is not only regulated by an internal mechanism that acts like a clock, but is also slightly synchronized with the lunar cycle, although inconsistently between different populations. These discoveries open the door to future research into how chronobiology can be applied to improve female fertility, suggesting that it might be possible to optimize the ovulatory cycle to increase success rates in the treatment of subfertility, the authors conclude in their article.
“Confirming the existence of an internal clock that coordinates the menstrual cycle, as well as the mechanisms involved in its synchronization, could allow the application of personalized medicine approaches called ‘circadian’, already used in oncology and for the treatment of diseases, from sleep or depression, for example, to issues such as ovulation and fertility disorders,” concludes Claude Gronfier, researcher at Inserm.
Address fertility problems from a chronobiological perspective
For Cristina Carrasco, substitute professor and researcher at the Department of Physiology, Faculty of Medicine and Health Sciences of the University of Extremadura, this study – in which she has not participated – had a double objective: “on the one hand, to determine if the Variations in the monthly rhythmicity of the menstrual cycle, that is, in terms of its duration, could be governed by some type of unknown circadian clock (as happens with other internal clocks that are known to govern our daily wakefulness-sleep, blood pressure, temperature, production of enzymes and hormones, etc.); on the other hand, if these variations could be synchronized with an external process as decisive for other natural events as the lunar cycle,” she explained in statements to Science Media Center Spain.
“Thus, 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 serve to correct discrepancies in the duration of the menstrual cycle that occur over a certain period of time, that is, the alternation between shorter cycles with unusually longer ones to compensate for the oscillation around an internally marked periodicity. . As the researchers point out, the mechanism could be similar to the one our body sets in motion to correct the decompensation in the sleep/wake rhythm after an intercontinental trip, the famous jet lag.”
“Regarding the synchronizing role of the different phases of the lunar cycle with the beginning of the menstrual cycle, the association between both variables was moderate. It was observed that European women began to menstruate more frequently on a crescent moon, while North American women did so on a full moon. Among the possible explanations, this phenomenon is pointed out as reminiscent of our evolutionary process from aquatic species that, as happens 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 periods in which the analyzed databases were collected (between the years 1960-1990 for the European database, compared to 2000 in the North American one) . This could influence the results obtained, particularly geographical differences regarding the synchronization of the lunar and menstrual cycles. Life habits (diet, physical activity, socialization, sun and artificial exposure, etc.) influence the functioning of our body, including reproduction, and have changed a lot in the last 60 years. Therefore, it would be advisable to carry out more research to corroborate the evidence obtained using current big data, available thanks to the mobile applications that millions of women around the world use to monitor our menstrual cycle.”
“Finally, it is worth highlighting the clinical application of such evidence, particularly in relation to fertility problems and its potential therapeutic approach 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 the emerging field of personalized circadian medicine,” concludes the expert.
