Menses onsets can intermittingly couple to full or new moon. In 9 of 13 women who recorded their menses for at least 10 years (grey bars), menses onset occurred intermittingly around full moon (yellow bars) or new moon (blue bars). In some women menses onset switched between the full and new moon, but overall menses onset at full moon was more frequent. This is also evident in the circular plot (left), in which all recorded menses onsets were aligned. Menses onsets were bimodally distributed with most menses occurring few days before full moon (yellow arrow head). In these women, ovulation had happened during the darkest nights in the moon cycle (blue arrow head). A second less pronounced maximum was present before new moon. The bimodal distribution could be simulated by a von Mises model (orange curve in the circular plot) and was significantly different from a uniform distribution (data derived from Helfrich-Förster et al., 2021, Sci. Adv.)