For centuries, scholars and writers have speculated about the possibility of a link between lunar cycles and menses. And in 2021, it seems that the potential synchronicity between the two continues to fascinate.

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Menstruation is a cyclical process, as are the phases of the Moon — from new moon to waning crescent. Little wonder, then, that poets, philosophers, and scholars have, over the centuries, drawn parallels between the two, suggesting that they might be connected.

The mystique of the Moon and that of female bodies — at a time when medicine was in its infancy — led Greek philosopher Aristotle to claim, in the 4th century before the common era, that:

“[T]he menses tend to occur naturally during the waning moon […]. For this time of the month is colder and more humid because of the wasting and disappearance of the Moon.”

Age-old parallels between the menstrual cycle and the phases of the moon have likely also led to some females referring to their periods as “moon cycles” to this day.

Is there really a link between lunar cycles and menstrual cycles? In this Special Feature, we investigate.

Popular belief and many works of literature suggest that there may be some synchronicity between menses and the phases of the Moon.

That may be based on the similarity of duration between menstrual cycles and lunar cycles.

One full revolution of the Moon around the Earth takes 27 days, 7 hours, and 43 minutes. A moon phase cycle, during which the amount of Moon surface that we are able to see from Earth waxes and wanes, takes 29.5 days.

The length of menstrual cycles can be in the range of 25–30 days, with the median duration of a menstrual cycle being 28 days.

One 1986 study — which Sung Ping Law, from the Department of Gynecology at the Canton Traditional Chinese Medical College in Guangzhou, conducted — did seem to find a link between menstrual and lunar cycles.

The research, which appears in the journal Acta Obstetricia et Gynecologica Scandinavica, studied the cycles of 826 female participants, aged 16–25 years, over “4 lunar months in different seasons.”

The study concept, the author writes, “was based on the concept of traditional Chinese medicine that human physiological rhythms display synergism with other natural rhythms.”

Law found that, in the study cohort, “a large proportion” of menstruations occurred around the new moon. This led the researcher to deduce that ovulation periods tended to coincide with the full moon.

However, more recent research contradicts the notion that menstrual cycles often synch with moon phases.

For example, a year-long retrospective study from 2013 — which appears in the journal Endocrine Regulations — found no “synchrony of lunar phases with the menstrual cycle.”

This study monitored 980 menstrual cycles in 74 females of reproductive age over a calendar year. The authors say that the findings came “in defiance of traditional beliefs.”

A more recent study, which the company who program the period tracking app Clue commissioned in 2016, also concludes that synchrony between menstrual and moon cycles is a “myth.”

This research, which analyzed over 7.5 million menstrual cycles, suggests that periods most likely do “not sync with the lunar cycle.”

The researchers collected data on menstrual patterns from 1.5 million Clue users. Clue data scientist Dr. Marija Vlajic Wheeler analyzed them.

“Looking at the data, we saw that period start dates fall randomly throughout the month, regardless of the lunar phase,” says Dr. Wheeler.

Clue’s raw data and subsequent analysis are not available to the public.

A new study in the journal Science Advances, however, suggests that there may be more to the idea of synchrony between lunar phases and menstrual cycles than previous research may have indicated.

This small-scale study analyzed the menstrual patterns of 22 participants who had kept track of their period onset for up to 32 years.

“Together, we had recordings of 15 women aged [35 years and younger] and of 17 women aged [over] 35 years,” the researchers write.

Their study found that those whose menstrual cycles were longer than 27 days had intermittent synchrony with two of the Moon’s cycles: the luminance cycle and the gravimetric cycle.

The luminance cycle refers to the Moon’s different light intensity as its position in relation to the Sun changes and it passes through its different “phases,” from new moon to full moon.

The gravimetric cycle refers to the cyclical difference of the Moon’s “pull” on the Earth as it orbits around our planet. Since the Moon’s orbit is elliptical, sometimes it is more distant from the Earth, and sometimes it comes closer.

Its cycle from perigee (when it is closest to the Earth) to apogee (when it is farthest from the Earth) lasts 27.5 days. Depending on where it is in its orbit, the Moon exerts a different gravitational pull on different parts of the Earth.

A third lunar cycle — the tropical month, or the mean time of the Moon’s revolution from any one point in its orbit back to that same point — also seemed to be linked to period onset, though “to a lesser degree,” according to the study authors.

The team also notes that, while menstrual cycles intermittently synched with the Moon cycles, the likelihood of synchrony faded as the participants got older.

Overall, the researchers observed that the Moon’s light intensity cycle seemed to be the most influential lunar cycle in terms of its effect on menses onset.

“We hypothesize that in ancient times, human reproductive behavior was synchronous with the Moon but that our modern lifestyle, notably our increasing exposure to artificial light, has changed this relation,” they explain.

Medical News Today spoke to first study author Prof. Charlotte Förster, from the Neurobiology and Genetics Biocenter at the Julius-Maximilians University of Würzburg in Germany.

We asked her what spurred her research interest in the possible synchrony between moon cycles and the menses.

“As a chronobiologist, I am interested in all kind[s] of rhythms, and I was always fascinated by the coincidence of the lunar cycle length and the menstruation cycle length,” she told us.

“At the same time,” she went on, “it was very clear that most women appear not to be in synchrony with the moon — at least not permanently. This brought me to the idea to investigate whether menses onset couples intermittently to the moon, and I started to ask women about long-term records of their menses onset.”

Although her and her colleagues’ study may be small-scale, sourcing the necessary data to conduct it was no mean feat.

“It took more than 10 years until I had collected the data from these 22 women,” Prof. Förster explained.

Their study taps into much debated questions regarding how and to what extent human circadian rhythms — or our “body clocks,” which regulate our biological patterns — relate to cycles from our natural environment.

Previous studies, including a recent one in the Journal of the Endocrine Society, have looked at fluctuations in melatonin levels in the blood throughout the menstrual cycle.

These have shown that levels of melatonin — which is a hormone key to regulating circadian rhythms, and especially the sleep-wake cycle — peak just before the onset of menses and decline, overall, the closer a female gets to menopause.

There is also some evidence to suggest that a full moon influences sleep, essentially disrupting sleep duration and quality. One 2013 study in the journal Current Biology suggests that participants slept less on a full moon night, and their melatonin levels also decreased.

Furthermore, there is some evidence to suggest that artificial light can disrupt sleep-wake cycles and have a negative impact on sleep duration and quality.

However, is there any evidence in support of Prof. Förster and colleagues’ hypothesis that exposure to artificial light has affected females’ natural synchronicity with lunar cycles over time?

“Why and how might exposure to artificial light affect menstrual cycles?” MNT asked Prof. Förster. “This is a difficult question, and I cannot answer it yet,” she replied.

Despite existing evidence that suggests that artificial light affects various aspects of circadian rhythms, research into how it might interfere with menstrual cycles is lacking, she explained.

“What we know from circadian rhythms is that their period is strongly affected by light. Depending on the species, [this] period becomes shorter or longer with increasing light intensity. Obviously, this also applies to monthly rhythms, such as the menstruation cycle. In circadian rhythms, light interferes with the molecular mechanisms that generate them. For monthly rhythms, the molecular mechanisms are not yet known. Therefore, I also don’t know the mechanisms [underlying] how light affects them.”

– Prof. Charlotte Förster

The intermittent synchrony between menstrual cycles and lunar cycles is not coincidental, though, the researcher maintains. “We performed sophisticated statistical tests that revealed that the intermittent synchrony does not occur by chance,” she told us.

Although this new study may have opened up new avenues for research into menstrual patterns, much more work is necessary to confirm whether or not there is a synchrony with moon cycles and, if so, what biological mechanisms might be at play.

Further research should include a larger and more inclusive cohort, the investigators note in their study paper. Aside from the limited number of participants in the recent study, there was also a dearth of variance in participant diversity.

“This cohort is not at all representative of the global female population, because the majority of women are white and stem from Europe,” Prof. Förster told MNT.

Using a more diverse cohort is important, as previous research has suggested that menstrual cycle length may vary by race or ethnicity.

Prof. Förster believes that learning more about the environmental factors that may influence menstrual cycles could come in handy under certain circumstances.

“What applications might this and further studies on this topic have, in the context of women’s health?” we asked her.

“I think that it is still too early to [draw] conclusions. There are so many factors that influence health, and the absence of a synchronization with the moon is for sure a minor contributor to health problems,” she told us.

“Nevertheless, women who have difficulties [in getting] pregnant and could exclude all other medical reasons might wish to consider a more ‘natural life’ without too much artificial light at night,” she suggested.