Women’s menstrual cycles synchronize with moon cycles, study shows

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In a new study, researchers used scientific methods to examine the connection between lunar and women’s menstrual cycles.

They suggest that in ancient times, human reproductive behavior and the female menstrual cycle were synchronous with the moon, but that our modern lifestyles and artificial light have largely changed this synchrony.

The research was conducted by a team at the University of Würzburg and elsewhere.

Many animal species in which the reproductive behavior is synchronized with the lunar cycle to increase reproductive success.

Since the menstrual cycle of women is similar in length to the lunar cycle with its approximately 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 in sync with that of the moon have the highest probability of becoming pregnant.

Two large longitudinal studies demonstrate a big correlation between birth rate and lunar phase with a slight increase in birth rate at the full moon and a corresponding decrease at 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, the team examined the course of the menstrual cycles of 22 women who had kept menstrual diaries—in some cases over a period of 32 years.

The team correlated the records of each of the 22 women with the lunar cycle.

Scientifically speaking, the moon exhibits three distinct cycles that periodically change its luminance and the gravity with which it impacts Earth.

On the one hand, there is the change between the full moon and new moon which takes place on average every 29.53 days with slight variations.

Secondly, the moon does not go round Earth in a fixed orbit. Instead, its position varies relative to the equator. Sometimes it is more to the north, sometimes more to the south.

This cycle lasts 27.32 days. The third cycle is a little longer with an average of 27.55 days.

It results from the fact that the moon accompanies Earth on an elliptical orbit and is accordingly sometimes closer, sometimes further away.

All of 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, producing 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.

The researchers found that all three lunar cycles influence the onset of menstruation in women.

The nightly moonlight seems to be the strongest clock synchronizer, but the gravitational forces of the moon also contribute to the effect.

On average, in women under 35 years of age, 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 barely one-tenth of the time.

The synchronism of the lunar and menstrual cycle does not only decrease with increasing age: It also seems to decrease to the extent that 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.

According to the scientists, the fact that synchronization occurs only sporadically and that the courses of women’s menstrual cycles vary suggests that the moon’s light-dark cycle alone is not a strong synchronizing factor of menstruation.

The observation that gravity sets a rhythm for humans could explain why certain cycles, such as menstruation but also sleep onset and sleep duration, 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.

Effects of gravity could also explain a study’s observation that both sleep onset and sleep duration of college students are in sync with the lunar cycle—even though they live in Seattle, a city that is so bright at night that moonlight is barely perceptible.

All these observations suggest that the human organism can respond not only to rapid changes in gravity, as perceived by the equilibrium system, but also to slow, periodically recurring gravitational changes.

One author of the study is Würzburg chronobiologist Charlotte Förster.

The study is published in the journal Science Advances.

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