Is the menstrual cycle synced with the moon?
New research suggests that it could be…
The background.
The speculation that menstrual cycles might be synchronised with the moon has been an idea that has been going around for centuries. Although some might say it is just a myth and not logical… there may be some truth to it after all.
Firstly, the thought that the menstrual cycle could be linked with the lunar cycle is not so extraordinary, as both cycles have a similar length. The menstrual cycle (which you can read more about here), can range between 21 and 35 days in length, from the onset of menstruation to the next cycle. However, the average (and textbook) menstrual cycle length is 28 days, which is very close to the time it takes for the moon to complete its phase cycle, from the new moon through to the full moon, and back to another new moon (29.5 days). The fact that both the menstrual and lunar cycles are so similar in length is intriguing, especially given that this is not the case for our evolutionary close primate relatives, such as chimps, which have an average menstrual cycle of around 35 days (Lacreuse et al. 2008).
There is then the question of course, as to whether the typical menstrual cycle length was favoured for the human species, and whether it provided an evolutionary advantage of some kind (maybe moon-related?). Or is it just a complete coincidence that the lengths of the two cycles are similar, and that’s that. But this potential coincidence was and still is not something that could be dismissed, even to this day. There have been numerous studies that have been carried out to try to prove (or disprove) that the menstrual cycle and moon cycle are indeed synced with one another – but with contradictory results.
The Scientific Research.
The common belief of those that believe the moon and menstrual cycle are linked, is that menstruation tends to happen during a new moon. Well, in 1986, one study found this to be the case, with a large proportion of the participants experiencing menstruation at the same time as a new moon (1). More recent research from 2013, nonetheless, then went on to contradict this and show that there is no connection between the time of menstruation and the phase of the moon (2). Sigh…
Given that for those menstruators with an average menstrual cycle length of 29.5 days, there is a 50% chance that they will have their period around the time of either a new moon or full moon, the menstrual cycle lengths being analysed in these studies are an important factor to take into consideration. Any small variation in the days of the cycle lengths could influence the results, and therefore the outcome of the study. The study from 1986 did not state what the menstrual cycle lengths were that they analysed, and the 2013 study included people who had different menstrual cycle lengths, which could be why they saw no connection. Unfortunately, from these two and all the other studies carried out trying to decipher whether there is a connection or not, we still actually don’t have a conclusive answer.
Period-tracking Apps weigh in.
Nevertheless, with the advent of period-tracking Apps and the massive amounts of data that can now be collected and analysed as a result of these… it could be that now we can finally get an answer!
Clue, a well-known period-tracking App, analysed over 7.5 million menstrual cycles (and lunar cycles) in 2016. They stated, that they did not find a correlation between the lunar phases and the menstrual cycle/period start date (data & analysis not available) (3). They state that “the period start date falls randomly throughout the month, regardless of the lunar phase”. They also pointed out that statistically speaking, for those with an average menstrual cycle length that is close to that of the lunar cycle, the cycles will more or less correlate. But, of course, not everyone has a 29-day menstrual cycle, and the length can vary due to several factors such as age, race and ethnicity, and stress. As a result, some people will probably never have a menstrual cycle that correlates with the lunar cycle.
So, that’s that then… we now know that the moon cycle and menstrual cycle are not in sync, right? It is just a coincidence for some people.
Erm, not quite.
One study published in 2021 suggests that menstrual cycles may have once been synchronised with the lunar cycle, in the past, but not now in the present time (4), which has been proposed by others before (5). So, how could the moon have affected menstrual cycles?
One study showed that light exposure mimicking that of moonlight during the moon cycle, influenced menstrual cycle lengths, making them more regular (6). Light exposure has been proposed to affect menstrual cycle regularity in an unrelated study, where night shift workers were more likely to experience irregular menstrual cycles than those menstruators who work during the day (and are exposed to more light) (7). This is thought to be a result of the amount and duration of light exposure, affecting the hormones that are responsible for regulating the menstrual cycle. Nevertheless, one older study suggested that it could even be the electromagnetic radiation that is emitted from the moon that affects the menstrual cycle (8). But the question is then, why is this big data set from Clue apparently showing that menstrual cycles and lunar cycles are not in sync with one another, today if the moon does have this effect?
It could be that in the modern world, lifestyles and artificial light sources are affecting the synchronisation of menstrual cycles with the lunar cycle, for the majority of people. This theory has yet to be extensively studied, but it is postulated by some scientists, that this could be a result of the impact on circadian rhythms, sleep and melatonin levels, and their subsequent effect on menstrual cycles; although more research is needed for more conclusive findings here.
So, from all of the mentioned above, it seems that maybe menstrual cycles used to be in sync with the lunar cycle, but now with modern-day life, they are not for the majority of people – as proven by the big data set analysis by Clue. So why, in 2024, is suddenly everyone talking about menstrual cycles being synced with the moon again? (at least on my Newsfeed).
New insights.
On January 10th 2024, a study by René Ecochard was published where data from the period-tracking App, Natural Cycles, actually did show a slightly significant association between the onset of menstruation and the phase of the moon (9). In this study, 311,064 menstrual cycles were analysed, for 3 years. Nearly 36,000 menstruating individuals, took part, all of whom were instructed on how to track their period data correctly, and categorised based on their age and their menstrual cycle lengths. From this analysis, the authors showed that a circalunar menstrual rhythm (menstrual cycles being linked to lunar cycles) was statistically significant and therefore apparent across the whole data set, for all ages and menstrual cycle lengths. Interestingly, this did not seem to vary with the menstrual cycle lengths, and the alignment was that of a small predominance of menstruation during the phases around the full moon.
Of course, there are some limitations to the study, such as the fact that menstrual cycles and menstruation dates were self-reported, premenstrual spotting could have very slightly hindered the results, and the participants were all European or North American, so it may not be representative of all menstruating individuals. With this being said, the results are intriguing…
What could also be deduced from this study, which was more apparent than the correlation between the moon and menstrual cycles, was that menstruation seemed to start more often on a Thursday or Friday, especially for those with a 27-29-day menstrual cycle length. Given the above points on the possible link between the menstrual cycle and the circadian rhythm, and the known effects of other factors on the menstrual cycle, such as stress, the authors discuss that this phenomenon could be explained by the weekday working life and weekends, and the changes in routines between the two. The authors also point out that the possibility of an internal clock regulating such things is not inconceivable. Now, whether this internal clock could be controlled by the moon, well that’s still open. And so is whether the phase of the moon has anything to do with the menstrual cycle, but at least from this latest study, it seems that there could be truth to it, after all.
So the answer really is… it could be, but more evidence is needed.
What do you think? Do you think the menstrual cycle is correlated with the moon cycle? Have you noticed that you are usually menstruating during a full moon? If you’re interested in this, the best thing you could do is just track your menstrual cycle in relation to the moon cycle…
All we know is that the majority of us don’t go out and howl at the moon whilst menstruating… or maybe you do, and that’s okay.
2) Do lunar phases influence menstruation? A year-long retrospective study - PubMed (nih.gov)
3) The myth of moon phases and menstruation (helloclue.com)
5) Knight. C. (1995) Blood relations: Menstruation and the origins of culture. Yale University Press.
7) Rotating shift work and menstrual cycle characteristics - PubMed (nih.gov)
8) Lunar and menstrual phase locking - ScienceDirect
9) The menstrual cycle is influenced by weekly and lunar rhythms - ScienceDirect
Further reading:
Does the moon affect the menstrual cycle? (naturalcycles.com)
