Women’s menstrual cycles are temporarily synchronized with lunar cycles

The blog “Ladyplanet. Natürlich Frau sein” is completely certain: “Our cycle is linked to that of the moon. The clearest connection is the length of the two cycles,” it 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 are really on the lunar cycle , all women would have their fertile days at the same time, ”reads the knowledge section of the newspaper.

So, what is true? A team led by Würzburg chronologist biologist Charlotte Förster has now used scientific methods to investigate the relationship between menstrual and female menstrual cycles. The result: Scientists assume that human reproductive behavior and the female menstrual cycle in ancient times were synchronous with the moon, but that 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 her study are now published online in the journal Scientific progress.

Correlation between lunar phases, pregnancy and birth rate

“We know many species of animals in which reproductive behavior is synchronized with the lunar cycle to increase reproductive success,” says Charlotte Förster. Since the menstrual cycle of women is the same as the lunar cycle with about 29.5 days, it seems likely. This is also supported by a number of other findings: For example, several older studies show that women whose cycles correspond to the moon are most likely to become pregnant. Two large longitudinal studies show a significant correlation between birth rate and lunar phase with a slight increase in birth rate at full moon and a corresponding decrease at new moon. Recent evidence also suggests that births occur more at night during a full moon and during the day when there is a new moon.

To illuminate the influence of the moon on human reproduction, Förster and her colleagues from Munich, Buenos Aires and the USA studied the course of the menstrual cycles of 22 women who kept menstrual diaries – in some cases over ‘ a period of 32 years. “To our knowledge, this approach to analyzing this type of long-term data has not been used before,” says Förster. Instead, previous studies analyzed a large number of women as a whole and combined the results of different women, age groups, years, and seasons.

The moon orbits the earth in different cycles

The team correlates the records of each of the 22 women with the lunar cycle. While “lunar cycle” is actually an unacceptable simplification. “Scientifically speaking, the moon exhibits three different cycles that change the brightness and gravity with which it affects the earth from time to time,” says Förster. On the one hand, there is the change between full moon and new moon that occurs on average every 29.53 days with slight variations. Second, the moon does not go in a fixed orbit around the earth. Instead, its position varies relative to the equator. Sometimes it’s more in the north, sometimes more in the south. This cycle lasts 27.32 days. The third cycle is a bit longer with an average of 27.55 days. This is due to the fact that the moon guides the earth on an elliptical orbit and consequently is sometimes closer, sometimes further away.

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

Moonlight is the strongest clock generator

“All three lunar cycles affect the onset of menstruation in women”: This is the conclusion reached by the scientists after evaluating the records of the study participants. The moonlight at night seems to be the strongest clock synchronizer, but the gravitational pull of the moon also contributes to the effect.

Of course: Not all women follow the change of light and dark in the night sky, and if they do, usually only for certain periods of time. On average, menstruation in women younger than 35 years occurs synchronously with the full moon or new moon in just under a quarter of the recorded time. For women over 35, this is on average barely one tenth of the case. The synchronism of the lunar and menstrual cycle not only decreases with increasing age: it also seems to decrease to the extent that women are exposed to artificial light sources at night. Typical “night owls” that go to sleep late and keep the lights on longer show no clear synchronization with the moon.

A Gravity Feeling

According to scientists, the fact that synchronization occurs only sporadically and that the menstrual cycles of women vary indicates that the moon’s light-dark cycle alone is not a strong synchronizing factor of menstruation. They first proved that gravity also affects the monthly cycles. “In the second half of 1961, 1979, 1997 and 2015, the menstrual cycle of seven out of nine women was synchronous with the change of full moon and new moon,” says Charlotte Förster. This interval of 18 years corresponds exactly to the rhythm in which the three lunar cycles combine to produce very special constellations. This conjunction may have improved the moon’s power as a clock generator.

The observation that gravity determines a rhythm for humans may explain why certain cycles, such as menstruation, but also sleep and duration of sleep, are temporarily linked to the full moon or the new moon: in both phases, the influence of the moon’s gravity on earth is similar. Effects of gravity may also explain the observation of a study that both sleep onset and sleep duration of college students correspond to the lunar cycle – even though they live in Seattle, a city so bright at night that the moonlight is barely noticeable.

For Förster and her colleagues, all of these observations suggest that the human organism can not only respond to rapid gravitational changes as observed by the equilibrium system, but also to slow, periodic repetitive gravitational changes. However, the scientists are aware of the limited importance of their study due to the relatively small number of women studied. Her hope is therefore based on the use of technology that is both simple and modern: a mobile app. This will make it possible to study the relationship between menstrual and lunar cycles and the influence of artificial light on a large number of women around the world.