For centuries, people speculated about a link between the menstrual cycle and the phases of the moon. A new study in Science Advances shows that the connection held until about 2010, the year smartphones and LED bulbs became part of everyday life.
Researchers at Julius-Maximilians University of Würzburg analyzed records from 176 women across nearly a century. They compared handwritten logs from the mid-20th century with digital app entries from the 2000s. Data collected before 2010 shows women’s cycles tied to the lunar calendar, most often beginning with full or new moons. The alignment held long enough, and across enough women, that researchers saw it as more than a coincidence.
After 2010, the pattern faded. The researchers found that menstrual cycles linked with the moon largely disappeared, except for January. That month brings the strongest annual gravitational pull from the sun and moon, a force powerful enough to cut through artificial light.
The Moon and Menstrual Cycles Used to Align. Then Smartphones Happened.
The findings suggest that cycles respond to both moonlight and lunar gravity. Women’s records showed synchronization with three different lunar rhythms, including the 29.5-day phase cycle and two shorter gravitational cycles of about 27 days. The combined effect appears to have guided reproductive timing for decades.
The glow of screens has transformed the night environment. LEDs and smartphones emit blue light that unsettles circadian rhythms while washing out natural moonlight. Satellite data reveal a dramatic climb in light pollution since 2010, and countries with the brightest skies show weaker menstrual links to lunar cycles. Online search traffic points the same way, with January spikes in period pain searches aligning with the annual gravitational peak.
The length of a woman’s cycle also seems to play a role. Natural cycles ranging from 26 to 36 days are most likely to align with lunar rhythms. As women age and their cycles shorten, alignment becomes less common. Lifestyle shifts that influence cycle length, including late-night screen exposure, may further reduce the chances of syncing with the moon.
The study shows that menstrual cycles once mirrored the sky in ways that are now harder to detect. The connection hasn’t vanished completely but appears during certain conditions, such as January, when lunar and solar forces are strongest. In a world bathed in artificial light, the body’s ancient clock is still ticking but often drowned out by a glow of our own making.
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