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 disa

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