In ancient times, women’s menstrual cycles were more closely synchronised with the moon, but scientists say this connection has weakened in the modern era due to widespread exposure to artificial light at night.
However, their research reveals that while artificial light may have dimmed the moon’s influence on menstruation, women’s cycles continue to follow lunar gravity during powerful seasonal alignments.
News-Medical.net reports that a menstrual cycle is roughly as long as a lunar cycle, but there is little evidence of an actual correlation between them. Now, a study published in Science Advances has examined 24 years’ worth of menstrual records for lunar-menses associations.
In many animals, mating behaviour follows repeating cycles like the lunar cycle, which may optimise reproductive success by increasing the number of potential mates available at the same time. In women, those whose menstrual cycles last about one lunar month (29.5 days) show the highest pregnancy rates.
In this group, menstruation tends to begin more often at the full moon, suggesting ovulation typically occurs near a new moon.
The same authors – Bavarian and Italian – have previously demonstrated, in a 32-year menstruation study, that cycle length declines with age.
Their analysis of all menstrual data in circular plots also revealed that women with lunar-like cycle lengths often experience months at a time when menstruation began in a bimodal pattern, clustered around either the full moon or the new moon.
Other studies, all based on data collected before 2000, also found similar patterns, with menstruation sometimes starting around the full moon or midway through the waxing phase. This suggests that moonlight may act as a zeitgeber, an external cue that weakly influences menstrual rhythms.
At the same time, human reproduction may be driven by an internal clock that tracks the lunar cycle independently of moonlight exposure.
In contrast, more recent studies relying on cellphone-tracked data failed to find such correlations. To address this, the current study extended the research into the 21st century, testing menstrual-lunar synchrony with the full and new moon phases and two additional gravitational cycles of the moon.
Lunar cycles
The moon follows three short cycles. The synodic cycle, lasting about 29.5 days, is the time between the new and full moons. It reflects the two syzygies, when the moon, Earth, and sun are in a straight line, producing changes in both moonlight and the gravitational pull on Earth.
The anomalistic month, about 27.6 days long, measures the moon’s movement from the nearest point to earth (perigee) to the farthest point (apogee) in its elliptical orbit.
The tropical month, or lunar standstill cycle, lasts around 27.3 days and is driven by the angle of the moon’s orbit relative to the earth’s equator. This causes the moon to shift north and south in the sky over the year. The anomalistic and tropical months affect gravity but have a lesser effect on moonlight intensity.
These cycles interact with seasonal events. Tides become stronger when the lunar standstills coincide with the syzygies at the winter and summer solstices. The effect is the greatest at the winter solstice, when earth is at perihelion (between 2 and 6 January each year), and the sun’s gravitational pull adds to the moon’s.
In addition, two longer gravitational rhythms, lasting ~18 years each, occur because of interactions between the moon’s and the earth’s orbits.
Study findings
This study analysed 176 records of women, each covering two to 37 years. A few began at menarche (the first menstrual period), most before 30. Of these, 111 records began after 2000, 60 before 2000, and five spanned both periods. Most of the women were from Italy and Germany, and some records were from Israel and North America.
Older records were kept on paper, while the post-2000 records were mostly tracked with cellphone apps, reflecting their widespread use. The researchers examined the timing of menstruation in relation to the full and new moons, lunar standstills, and the moon’s perigee and apogee positions.
The findings point to a possible internal circalunar clock that helps regulate and synchronise the menstrual cycle with the moon’s short lunar cycles. External factors may influence this clock, but do not appear to drive it.
Menstrual cycles lasting 26 to 36 days were more likely to align, at least intermittently, with the synodic cycle (the new-to-full moon cycle), even after age 35. Very short (<27 days) or very long (>33 days) cycles showed only “relative co-ordination”, a weaker kind of synchronisation in which cycles occasionally aligned with lunar rhythms but often ran freely on their own.
The study defined entrainment ranges as ~26-36 days for the synodic month, ~24-31 days for the anomalistic month, and ~23-30 days for the tropical month.
Similarly, this relative co-ordination was observed at the edges of the anomalistic and tropical ranges. The fraction of women whose cycles synchronised varied depending on their natural cycle length, with the strongest entrainment occurring with the synodic month.
In the pooled (old and new) data, menstrual onset tended to align with the full or new moons during the winter solstices and at two lunar standstills, points where gravitational and light-related influences are strongest. This effect was more pronounced before 2010.
After that, as night-time artificial light became more widespread, the menstrual-synodic synchrony was no longer apparent. However, synchronisation with the anomalistic and tropical cycles persisted, and in some cases, became slightly stronger.
This suggests that since 2000, gravitational cycles may have gained relatively greater influence, though not a complete replacement of light-based cues. As expected, synchrony with the synodic month peaked at the solstice, particularly the winter solstice.
For the anomalistic and tropical cycles, synchrony was confined to solstices and was strongest during the winter solstice, when earth is closest to the sun (perihelion).
In Italy, menstrual onset showed a bimodal pattern, with peaks two days before both the full and new moon. In Germany, however, a single peak was seen 1.5 days before the full moon. The authors suggest that higher light pollution in Northern Italy, where most Italian data were collected, could be a factor. However, they caution that this remains hypothetical without direct light exposure measurements.
Longer lunar cycles also produced observable effects. During minor lunar standstills, menstrual onset showed sharply peaked synchronisation with the full moon. This coincided with the eclipse sequence, called Saros series #137, which reinforced the moon’s gravitational effects.
In contrast, major lunar standstills showed weaker synchrony, biased toward the new moon.
These observations suggest that the menstrual cycle is an internal clock influenced, though not determined, by lunar cycles with partial phase alignment.
Independent evidence came from Google Trends, which showed that searches for “period pain” and related terms peaked at perihelion in multiple countries. However, the pattern was not observed in some countries, such as Italy, France and New Zealand.
Notably, the effect occurred in the southern hemisphere during its summer, despite long daylight hours, contradicting the idea that moonlight alone drives the synchrony.
Conclusions
The authors conclude that humans are affected by the moon's luminescence and gravimetric cycles, with the menstrual cycles aligning with the synodic month. Both light and gravity act as zeitgebers for women, with gravity probably indirectly influencing cycles through broader geophysical effects.
The researchers also suggest that, in ancient times, menstrual cycles were more closely synchronised with the moon, but this connection has weakened in the modern era due to widespread exposure to artificial light at night.
See more from MedicalBrief archives:
Ovarian cycle regulated by circadian rhythm – French-US study
Early periods can be tied to what girls eat – US study