Menstrual cycles are temporarily synchronized with lunar cycles: study

ONs The average length of women’s menstrual cycles corresponds to the 29.5-day wax and waning cycle of the moon. Many cultures have linked the moon to fertility. The influence of the celestial body on human biology has been largely dismissed as a myth, but several recent studies have linked moon phases to sleep and moods. In a study published January 27 in Advances in scienceThe researchers analyzed long-term data from women and found that some of their periods were synchronized with moonlight and gravity cycles at specific times in their lives.

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

Charlotte Helfrich-Förster, chronobiologist at Julius-Maximilians-Universität Würzburg in Germany and lead author of the study, said she was initially “skeptical” about a connection between lunar menstrual cycles. “On the other hand, it’s very interesting that the [menstrual] The cycle length is more or less the lunar cycle length, and it is known from many studies that animals – at least marine organisms – rely on the moon to synchronize their reproduction, ”she says. To investigate whether lunar cycles affect human menstrual cycles, Helfrich-Förster and her colleagues studied 22 women who recorded the date of their menstruation for five to 32 years.

During the moon’s 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 the natural earth satellite in relation to the sun changes during its orbit and causes the well-known luminance cycle between new and full moon every 29.5 days. This celestial circle is elliptical and thus changes the moon’s gravitational tug as it swings every 27.5 days from perigee, the closest point on the earth’s loop, to the most distant apogee. In addition, this orbit is inclined with respect to the earth’s axis, which causes different gravitational effects on the southern and northern hemisphere during the 27.3-day lunar standstill cycle.

The researchers found that menstrual rhythms were very different in women and over time in individuals. When evaluating the six women who kept records for the longest time – between the ages of 19 and 32 – they found that five of these women’s periods were intermittently associated with the moon. When data for a single woman were combined, there was a significant association of the onset of menstruation with the full moon and new moon, but not with other parts of the luminance cycle.

Of the eight women who recorded their periods for a shorter period at a young age, six showed a temporary link between the beginning of their period and the full or new moon, and when all menstruations for individual women were combined, three of these menstrual disorders occurred significantly in women 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 length of women’s periods was 29.4 days for women under 35 and 26.3 days for women over 35. Because younger women’s cycles are closer to the moon’s 29.5-day luminance cycle, the researchers firmly suspected that their periods would couple to the moon more often than older women’s. When combining the younger women, they found that the younger women’s menstruation was synchronized an average of 23.6 percent of the time with the new or full moon of the lunar luminance cycle. Older women only synchronized on average 9.5 percent of the time with the new or full moon.

The researchers also found that the start dates of the periods coincided 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 all women’s records were combined, menstruation occurred more frequently with the full moon, new moon, and perigee than would happen to be expected. The researchers say these results suggest that both moonlight and the moon’s gravity affect menstruation, although it is unclear how people perceive these fluctuations.

Pick up moon cues

There is growing evidence that the moon affects human biology. In a study published in Molecular PsychiatryThomas Wehr, psychiatrist at the National Institute of Mental Health and co-author of the current study, found that manic-depressive cycles in patients with bipolar disorder fluctuated with the gravitational cycles of the moon. In a study published alongside the latest in Advances in scienceAnother group of researchers found that sleep duration was shorter near the full moon and longer during the new moon. Because the results were similar in people without electricity and in a big city where urban light sources obscure the effects of moonlight, the researchers suggest that the moon’s gravity could explain these effects.

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

The study finds that the young women whose cycles were not at all synchronized with the lunar luminance cycle were “night owls,” suggesting that exposure to much artificial night light could override the possible effects of moonlight.

Helfrich-Förster says it is “extremely unlikely” that humans can feel these changes in order to capture the gravity of the moon. Instead, she believes humans could indirectly measure the moon’s influence on other variables. “I can only speculate,” she says. “Maybe it’s atmospheric pressure. . . or maybe it has to generate electromagnetic fields that are also influenced by the moon. “

“I can only speculate,” she says. “Maybe it’s atmospheric pressure. . . or maybe it has to generate electromagnetic fields that 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 works with Helfrich-Förster, although she found that one limitation of the study was the small sample size of only 22 women.

Virginia Vitzthum, a biological anthropologist at Indiana University who was not involved in the research, is less convinced. In an email to The scientistShe says that because the study found that synchronization was intermittent and not shared by most women, “it is not a convincing case that biologically significant synchronicity occurs.”

Two studies in the 1980s similarly found that women with cycle lengths of After about 29.5 days, menstruation began, which was linked to the phases of the moon. However, a handful of other studies – including an unverified analysis of more than 7.5 million menstrual cycles – found no correlation between menstrual and lunar cycles. Most of these studies didn’t take into account women’s age or cycle length, and mass analysis of many women’s menstrual cycles over a short period of time could miss patterns as individual women’s cycles vary throughout their lives, says Anna Wirz-Justice, chronobiologist at the Psychiatric Clinic from the University of Basel, which was not involved in the study.

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

Development of the moon synchronization

Helfrich-Förster and colleagues speculate that the synchronization of human reproduction with lunar cycles may have been stronger in ancient times, but exposure to artificial light in modern life has dampened the influence of the moon. A recent study found that ovulation occurs an average of 12.4 days before menstruation begins. When menstruation begins near the full moon, the woman’s most fertile phase would occur near the new moon. Because it may have been dangerous to go out at night without moonlight, people who were in the safety of their shelters and spent that time giving birth to babies may have had an evolutionary advantage, says Helfrich-Förster. Another study found that badgers mostly mated during the darkest phases of the moon.

Vitzthum thinks this hypothesis is “improbable”. In an email to The scientistShe says that her and other researchers’ studies of non-industrialized populations in Bolivia and Mali suggest that, prior to modern contraception, women “spent most of their adult reproductive years either pregnant or breastfeeding – which suppresses ovulation / cycling – and in general maybe only 40 years old. 50 cycles in life – which means that natural selection did not have much opportunity to promote synchronization with the moon. ”

The synchronization of reproduction with the phases of the moon is well documented in marine life such as plankton, crabs, fish and corals. “Life developed in the ocean,” says Helfrich-Förster. “When life developed on earth, the moon was much closer to earth. Probably the forces of the moon on earth were also much greater. “Ancient organisms were likely more heavily influenced by lunar cycles, and she says that if” the adaptation is still in our genes, “human reactions to the moon may be a relic from our evolutionary past.

Vitzthum does not consider this a probable hypothesis either. A few days before a woman’s period begins, the production of the hormone progesterone stops, and low levels of this hormone trigger menstrual bleeding. She says that for a woman’s menstruation to synchronize with the full moon, the progesterone-producing structure would need to receive a signal before the full moon. “In order for such a complex set of signals to develop and maintain over the course of evolution, there would have to be a reproductive benefit – I can’t imagine one.”

C. Helfrich-Förster et al., “Women temporarily synchronize their menstrual cycles with the luminance and gravimetric cycles of the moon”. Advances in science, doi: 10.1126 / sciadv.abe1358, 2021.

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