For centuries, women have shared about the mysterious connection between their monthly cycles and the phases of the moon. A new study shows that the link is real, and believe it or not, smartphones may have disrupted it, studyfinds.org reports.
An analysis of 176 women's menstrual cycles spanning nearly a century shows that women's reproductive cycles were synchronized with the phases of the moon until around 2010. That's when LED lights flooded the market and smartphones became ubiquitous, dousing modern life with artificial blue light around the clock.
“Women's menstrual cycles recorded before the introduction of LEDs in 2010 and the widespread use of smartphones were significantly synchronized with the moon, while those recorded after 2010 were linked to the moon primarily in January,“ the researchers report in their paper “Scientific Advances“.
Led by Charlotte Helfrich-Förster of the University of “Julius-Maximilian“ in Würzburg, researchers have tracked menstrual data from women across generations, comparing records kept in paper calendars from the mid-20th century with data from smartphone apps from recent years. The contrast is dramatic.
Before 2010, women’s periods clustered around the full moon and new moon in a pattern too consistent to be random, although this synchronization was always temporary, lasting only months or a few years before shifting. After 2010, this synchronization largely disappeared (except in January, when the combined gravitational pull of the sun, moon, and Earth reaches its annual peak).
Exposure to artificial light, especially from LED screens and light bulbs, can disrupt the body’s ability to detect the natural cycles of moonlight. Unlike older incandescent bulbs, LEDs emit high levels of blue light, which disrupts circadian rhythms and can mask subtle environmental cues that once guided reproductive timing.
The study reveals something even more intriguing: Menstrual cycles appear to respond not just to moonlight but also to the moon’s gravitational pull. Women’s periods are synchronized with three distinct lunar cycles (the familiar 29.5-day pattern of lunar phases, plus two gravitational cycles lasting about 27 days each).
This gravitational influence explains why the synchronization continues into January, when Earth reaches its closest point to the sun. During this period, the combined gravitational forces of the sun and moon create the strongest tidal effects of the year, powerful enough to overcome interference from artificial light.
The researchers examined records from 176 women over 24 years, creating one of the largest long-term studies of menstrual patterns ever conducted. Most of the participants were European women who tracked their cycles for an average of six years, with some records spanning nearly four decades until menopause.
The smartphone revolution has changed the way people perceive light and darkness. Before 2010, most people encountered artificial light primarily from incandescent bulbs, which emit a warm, yellowish glow similar to that of a fire. LED screens and light bulbs produce blue light that closely resembles daylight, confusing the body's internal clock.
Satellite measurements show that global light pollution has increased dramatically since 2010, consistent with the timeline when lunar-menstrual synchronization weakened. Countries with higher light pollution, such as northern Italy, show a smaller menstrual-lunar correlation than areas with darker night skies.
Google search data supports this link. Searches for “menstrual pain“ have been steadily increasing in January in multiple countries, indicating that women around the world experience stronger menstrual effects during the month when the moon's gravitational forces peak.
Studies have shown that human reproductive cycles function like a “circadian clock“, similar to the circadian clock that governs daily rhythms, but tuned to monthly lunar patterns.
Like other biological clocks, this lunar clock has a limited range. Menstrual cycles can only synchronize with lunar phases when a woman's natural cycle length falls within certain windows: roughly 26 to 36 days for lunar phases, with narrower ranges for gravitational cycles. Even before 2010, this synchronization was intermittent, lasting only months or a few years before shifting out of phase.
As women age and their cycles typically shorten, they fall outside these ranges of synchronization, which explains why lunar connections weaken over time. Modern lifestyle factors that shorten cycles (including exposure to artificial light) make menstrual synchronization even less likely.
If artificial light can disrupt fundamental reproductive rhythms that have evolved over millennia, it raises questions about whether other biological processes might also be affected.
The connection between women's cycles and the moon has not disappeared completely, but it is now much weaker than in the past. Today, it only appears under specific conditions, such as in January or the seasonal solstices, when gravitational forces are strongest.