The idea that menstrual cycles fall in step with the Moon is old, widespread and unusually well studied, and the answer is messier than either side would like.

An analysis of 7.5 million cycles from a tracking app found no synchrony at all. A 2025 study of long-term records found weak coupling that was clearly present before about 2010 and largely gone afterwards, and suggested artificial light at night as the reason. Two cycles of similar length will drift in and out of step by chance, which is what makes this so hard to settle and so easy to feel.