As the average length of women’s menstrual cycles matches the moon’s 29.5-day waxing and waning cycle, many cultures associated the moon with fertility. The celestial body’s influence on humans biology had largely been dismissed as myth, but several recent studies have linked lunar phases with sleep and moods. In a study published January 27 in Science Advances, researchers analyzed long-term data from women and found that for some their periods synced with lunar light and gravity cycles at certain times in their lives.

“[The study] has not completely settled the debate,” says Kristin Tessmar-Raible, a chronobiologist at the University of Vienna who was not involved with the research. “But it’s really cool that this puts fresh spirit into the whole discussion: is the moon—yes or no—[affecting] human biology.”

Charlotte Helfrich-Förster, a chronobiologist at Julius-Maximilians University of Würzburg in Germany and the lead author of the study, says she was initially “skeptical” of a link between lunar menstrual cycles. “On the other hand, it is very interesting that the [menstrual] cycle length is more or less the moon cycle length, and it’s known from many studies that animals—at least marine organisms—rely on the moon for synchronizing their reproduction,” she says. To examine whether moon cycles influence human menstrual cycles, Helfrich-Förster and her colleagues examined 22 women who recorded the date their period started for five to 32 years. 

As the moon makes its 27.3-day journey around the Earth, it exhibits three different lunar cycles: the luminance cycle, the perigee-apogee cycle, and the lunar standstill cycle. The position of Earth’s natural satellite in relation to the sun changes during its orbit, causing the familiar luminance cycle between the new and full moon every 29.5 days. This celestial circuit is elliptical, thereby altering the moon’s gravitational tug as it swings from perigee, the closest point on the loop around Earth, to apogee, its most distant, every 27.5 days. Additionally, this orbit is tilted in relation to the Earth’s axis, causing varying gravitational effects on the Southern and Northern hemispheres across the 27.3-day lunar standstill cycle.

The researchers found that menstrual rhythms varied greatly among the women and over time within individuals. Evaluating the six women who kept records for the longest—between 19 and 32 years—they found that five of these women’s periods intermittently coupled with the moon. When dates for an individual woman were combined, there was a significant association of menses onset with the full moon and new moon, but not with other parts of the luminance cycle.

In the eight women who recorded their periods for a shorter duration when they were young, six showed intermittent coupling of their period start date with the full or new moon, and when all menses for individual women were combined, three of these women’s menses onsets were significantly associated with the full or new moon—but not with other phases of the luminance cycle.

Similar to other studies, they found that the average duration of women’s periods was 29.4 days in women younger than 35 and 26.3 days in women older than 35. Because younger women’s cycles are closer in length to the moon’s 29.5-day luminance cycle, the researchers suspected that their periods would couple with the moon more often than those of older women would. Combining the younger women together, they found that, sure enough, younger women’s menses onsets synced with the new or full moon of the moon’s luminance cycle 23.6 percent of the time, on average. Older women synced with the new or full moon just 9.5 percent of the time on average.

The researchers also found that periods’ start dates lined up with the perigee-apogee or lunar standstill cycles­ 13.1 percent of the time in younger women and 17.7 percent of the time in older women. When the records from all women were combined, menses onsets coupled with the full moon, the new moon, and the perigee more often than would be expected by chance. The researchers say that these findings suggest that both moonlight and the moon’s gravity influence menstruation, although it’s unclear how humans sense these fluctuations.

Picking up on lunar cues

There’s growing evidence that the moon influences human biology. In a study published in Molecular Psychiatry, Thomas Wehr, a psychiatrist at the National Institute of Mental Health and a coauthor on the current study, found that manic-depressive cycles in patients with bipolar disorder oscillated with the moon’s gravitational cycles. In a study published alongside the latest one in Science Advances, a different group of researchers found that sleep duration was shorter near the full moon and longer during the new moon. Because the findings were similar in people living without electricity and those living in a large city where urban light sources obscure the influence of moonlight, the researchers propose that the moon’s gravity could explain these effects.

Most women in the menstrual cycle study lived in relatively rural areas—where moonlight is more visible—suggesting that they may be able to perceive changes in the moon’s brightness.

The study notes that the young women whose cycles did not synchronize with the moon’s luminance cycle at all were “night owls,” hinting that exposure to lots of artificial night at light could override the potential effects of moonlight.

As for detection of the moon’s gravity, Helfrich-Förster says it’s “extremely unlikely” that people can sense these changes. Instead, she says she thinks that humans may indirectly pick up the moon’s influence on other variables. “I can only speculate,” she says. “Maybe it’s atmospheric pressure . . . or perhaps it has to do electromagnetic fields, which are also influenced by the moon.”

“I can only speculate,” she says. “Maybe it’s atmospheric pressure . . . or perhaps it has to do electromagnetic fields, which are also influenced by the moon.”

—Charlotte Helfrich-Förster, Julius-Maximilians University of Würzburg

“I think the study is very plausible,” says Tessmar-Raible, who collaborates with Helfrich-Förster, although she notes that a limitation of the study was its small sample size of just 22 women.

Virginia Vitzthum, a biological anthropologist at Indiana University who was not involved with the research, is less convinced. In an email to The Scientist, she says that because the study found that synchronization was intermittent and not shared across most women, “it is not a compelling case that biologically meaningful synchrony is occurring.”

Two studies in the 1980s similarly found that women with cycle lengths of about 29.5 days had menses onset that coupled to phases of the moon. But a handful of other studies—including a non–peer-reviewed analysis of more than 7.5 million menstrual cycles—found no correlation between menstrual and lunar cycles. Most of these studies did not consider women’s age or cycle length, and a bulk analysis of many women’s menstrual cycles over a short chunk of time could miss patterns because individual women’s cycles vary across their lives, says Anna Wirz-Justice, a chronobiologist at the Psychiatric Hospital of the University of Basel who was not involved with the study.

“The uniqueness [of Helfrich-Förster’s investigation is] long-term, individual data sets,” says Wirz-Justice. “The advantage of this longitudinal [data] set is that they’ve looked at it in amazing detail. I just think it’s astonishing.” She says that this approach “reveals secrets you just don’t see in averages.”

Evolution of lunar synchronization

Helfrich-Förster and colleagues speculate that synchronization of human reproduction with lunar cycles may have been stronger in ancient times, but exposure to artificial light in modern life has dulled the moon’s influence. A recent study found that ovulation occurs, on average, 12.4 days before menses onset. If menstruation starts near the full moon, women’s most fertile phase would occur near the new moon. Because it may have been dangerous to go out at night without the moonlight, humans stuck in the safety of their shelters who spent this time making babies may have had an evolutionary advantage, says Helfrich-Förster. Another study found that badgers mated mainly during the darkest moon phases.  

Vitzthum says she thinks that this hypothesis is “unlikely.” In an email to The Scientist, she says that her and other researchers’ studies of nonindustrialized populations in Bolivia and Mali suggest that before modern contraceptives, “women spent most of their adult reproductive years either pregnant or breastfeeding—which suppresses ovulation/cycling—and generally had perhaps only 40–50 cycles in a lifetime—which means there was not much opportunity for natural selection to favor syncing with the moon.” 

Synchronization of reproduction with the moon’s phases is well documented in ocean creatures such as plankton, crabs, fish, and corals. “Life evolved in the ocean,” says Helfrich-Förster. “When life evolved on Earth, the moon was much closer to Earth. So probably the forces of the moon on Earth were also much larger.” Ancient organisms were likely more influenced by lunar cycles, and she says that if “the adaptation to this is still in our genes,” human responses to the moon could be a relic from our evolutionary past.

Vitzthum doesn’t find this is a likely hypothesis either. A few days before a woman’s period begins, production of the hormone progesterone ceases, and low levels of this hormone trigger menstrual bleeding. She says that for a woman’s menses onset to sync with the full moon, the progesterone-producing structure would need to receive a signal in advance of the full moon. “For such a complex set of signals to evolve and be maintained over evolutionary time, there would need to be some reproductive advantage—I can think of none.”

C. Helfrich- Förster et al., “Women temporarily synchronize their menstrual cycles with the luminance and gravimetric cycles of the Moon,” Science Advancesdoi:10.1126/sciadv.abe1358, 2021.