Women’s menstrual cycles temporarily synchronize with lunar cycles

The blog “Ladyplanet. Natürlich Frau sein” is pretty sure: “Our cycle is related to that of the moon. The most obvious connection is the length of the two cycles,” he says. The newspaper Berliner Tagesspiegel comes to the opposite conclusion: “The length of women’s menstrual cycles is an average value; for some it lasts longer, for others it is shorter. Even one and the same woman can have cycles of different lengths. If they were truly linked to the lunar cycle, all women would have their fertile days at the same time, ”reads the journal’s knowledge section.

So what is true? A team led by Würzburg chronobiologist Charlotte Förster has now used scientific methods to examine the link between lunar and female menstrual cycles. The result: Scientists hypothesize that in ancient times, human reproductive behavior and the female menstrual cycle were synchronous with the moon, but our modern lifestyles and artificial light have largely changed this synchrony. Förster holds the Chair of Neurobiology and Genetics at the University of Würzburg (JMU). The results of his study are now published online in the journal Scientific advances.

Correlation between moon phases, pregnancy and birth rate

“We know of many animal species in which reproductive behavior is synchronized with the lunar cycle to increase reproductive success,” explains Charlotte Förster. Given that women’s menstrual cycles are similar in length to the lunar cycle at around 29.5 days, a connection seems likely. This is also supported by a number of other findings: For example, several older studies show that women whose cycles are synchronized with those of the moon have the greatest likelihood of getting pregnant. Two large longitudinal studies show a significant correlation between the birth rate and the lunar phase with a slight increase in the birth rate at the full moon and a corresponding decrease at the new moon. Recent evidence also suggests that births are more likely to occur at night during a full moon and during the day when there is a new moon.

To clarify the influence of the moon on human reproduction, Förster and his colleagues in Munich, Buenos Aires and the United States studied the course of the menstrual cycles of 22 women who had kept a menstrual diary – in some cases over a period of time. 32 years old. “To our knowledge, this approach to analyzing this kind of long-term data has never been used before,” says Förster. Instead, previous studies had analyzed large numbers of women in their entirety, combining results from different women, age groups, years and seasons.

The moon revolves around the Earth in several cycles

The team correlated the recordings of each of the 22 women with the lunar cycle. Whereas the “lunar cycle” is in fact an unacceptable simplification. “Scientifically speaking, the moon exhibits three distinct cycles that periodically change its luminance and the severity with which it hits the Earth,” says Förster. On the one hand, there is the change between a full moon and a new moon which occurs on average every 29.53 days with slight variations. Second, the moon does not circle the earth in a fixed orbit. Instead, its position varies relative to the equator. Sometimes it’s further north, sometimes further south. This cycle lasts 27.32 days. The third cycle is a bit longer with an average of 27.55 days. It results from the fact that the moon accompanies the Earth in an elliptical orbit and is therefore sometimes closer, sometimes further away.

All of these cycles affect the intensity of moonlight and gravity, which can be seen in tides, for example. In addition, they interact with each other and can lead to special constellations at longer intervals, producing special phenomena, like a solar eclipse, which is part of a regular cycle where the darkening of the sun is repeated approximately every 18 years.

Moonlight is the most powerful clock generator

“The three lunar cycles influence the onset of menstruation in women”: this is the conclusion that scientists draw after assessing the records of study participants. Nighttime moonlight appears to be the most powerful clock synchronizer, but the moon’s gravitational forces also contribute to this effect.

Of course: Not all women follow the change of light and dark in the night sky and if they do, usually only during certain periods of time. On average, in women under 35, menstruation occurs synchronously with the full moon or new moon in just under a quarter of the recorded time. For women over 35, this is the case on average in just a tenth of the time. The synchronism of the lunar and menstrual cycle does not only decrease with age – it also appears to decrease as women are exposed to artificial light sources at night. Typical “night owls”, who go to bed late and leave the lights on longer, show no obvious synchronization with the moon.

A sense of gravity

According to scientists, the fact that synchronization occurs only sporadically and that the course of women’s menstrual cycles varies suggests that the moon’s light-dark cycle alone is not a powerful synchronization factor for menstruation. They have the first evidence that gravity also influences monthly cycles. “In the second half of 1961, 1979, 1997 and 2015, the menstrual cycles of seven out of nine women were synchronous with the change of full moon and new moon,” explains Charlotte Förster. This 18-year interval is exactly the rate in which the three lunar cycles combine to produce very special constellations. This conjunction may have enhanced the strength of the moon as a clock generator.

The observation that gravity sets a rhythm for humans could explain why certain cycles, such as menstruation but also falling asleep and duration of sleep, are temporarily linked to either the full moon or the new moon: in both phases, the influence of the moon’s gravity on Earth is similar. The effects of gravity could also explain a study’s observation that students’ sleep and sleep duration are synchronized with the lunar cycle, even though they live in Seattle, a city so bright at night as it is clear. of moonlight is barely noticeable.

For Förster and his colleagues, all of these observations suggest that the human organism may respond not only to rapid changes in gravity, as perceived by the equilibrium system, but also to slow, periodically recurring gravitational changes. However, scientists are aware of the limited importance of their study due to the relatively small number of women studied. Its hopes are therefore based on the use of a technology that is both simple and modern: a mobile application. This will allow us to study the relationship between the menstrual and lunar cycles and the influence of artificial light on a large number of women around the world.