Elissavet Chartampila4.5 billion years ago, an object the size of Mars collided into a young Earth, ejecting enough material to form the Moon. Ever since, the Moon has remained a loyal companion, but not an idle one – tugging at Earth’s oceans, shaping its climate and steadying its spin.
New research suggests its pull may extend even further. A study published in Science Advances found that menstrual cycles once aligned with the Moon’s phases–but this synchrony is fading, overshadowed by the light on our screen.
The research team behind this study, led by Dr. Charlotte Helfrich-Förster, senior professor at Julius-Maximilians-Universität Würzburg, analyzed records from 176 women, spanning two centuries, and three continents. They found that menstruation often began around the full or new moon. “But after 2010, synchronization with the Moon was almost completely gone,” says Helfrich-Förster.
She attributed the shift to artificial light. “From 2010 onwards, LEDs came into the market. So, all conventional light bulbs were slowly replaced. At the same time, smartphones were becoming widely used,” she explains.
It’s a quiet tug-of-war between ancient rhythms and modern illumination. The Moon’s subtle sway over our biology now competes with the harsh glow of our cities and screens.
When the Moon Meets City Lights
Across the animal kingdom, the Moon’s rhythm marks time in subtle but vital ways. Corals release their eggs under the full moon, certain fish spawn with the tides, and even some worms time their mating dances to lunar light.
Humans may also carry echoes of this ancient clock. Our bodies are guided by multiple rhythms, tuned to environmental cues like light and temperature. The circadian clock, for example, tracks the 24-hour cycle of the sun, coordinating a network of physiological changes that ultimately dictate when we sleep.
The Moon, by contrast, moves to a slower, softer beat. Its light waxes and wanes over nearly a month, mirroring the average menstrual cycle.
For most of human evolution, the only powerful source of nocturnal light was moonlight. But since 2010, we have experienced a surge in light pollution, drowning out these natural cues and their influence on human rhythms.
Light may be the most obvious way the Moon influences life on Earth, but according to Helfrich-Förster, “it’s not the only one. A secondary synchronizing cue is gravitation.” As the Moon orbits Earth in an elliptical trajectory, its distance shifts, causing slight fluctuations in its gravitational pull.
“These fluctuations are smaller than what you would experience if you climbed a tree or a ladder,” she says, but they appear to be modulating the saliency of moonlight. During winter months, when the Moon’s gravitational influence is strongest, synchrony between the lunar and menstrual cycles emerges even after 2010.
Ancient Traditions, New Evidence
The history of this scientific field dates back to the 1970s. But the notion itself predates modern science by millennia–from the Greek goddess Selene, who guided cycles of growth and birth, to African and Native American traditions linking lunar phases with conception, harvest, and renewal.
Helfrich-Förster’s own interest began not with myth but with curiosity. Decades ago, she started tracking her menstrual cycles and noticed a recurring alignment with the full or new moon. She then expanded her analysis to friends and colleagues, then a longitudinal study of 22 women, and finally this one. The correlation persists.
Establishing more than a correlation between the lunar cycle and physiological events is quite the challenge, however. We cannot manipulate the Moon’s brightness or shift its orbit to examine whether our biological rhythms can entrain to the new conditions.
So, research in this field has relied on correlational studies that provide little insight into how our bodies sense and respond to lunar cues – leading some to question their validity, attributing the synchrony to chance.
Still, the pattern persists. Even without controlling for factors like stress, body weight, diet, or artificial light exposure – all known to affect menstrual timing – traces of lunar synchrony continue to emerge in the data.
Similar patterns have appeared elsewhere, hinting that the Moon’s influence may also extend beyond reproduction. Other studies have linked lunar cycles to changes in sleep duration and quality, and even to the timing of mood shifts in people with bipolar disorder.
While the evidence remains mixed, these findings suggest that human physiology may still carry faint echoes of a lunar clock. Despite the uncertainty, for Helfrich-Förster the message is clear: “Perhaps it would be better to have a more naturalistic life,” attuned to environmental cycles (lunar, annual, daily) and wary of how technology is impacting these natural rhythms.
Peer edited by: Savannah Muron
Illustrated by: Lauren Griffith
