Effect of the Moon
The menstrual cycle
Debated
Intermittent synchrony in long records, weaker since 2010.
The evidence
Records kept by 22 women for up to 32 years showed cycles at times in step with the Moon. A 2025 follow-up found this synchrony weaker after 2010, remaining mostly in January; the authors suggest artificial light at night, and the strong pull of Moon and Sun in January. The samples are small.
Key studies
- Women temporarily synchronize their menstrual cycles with the luminance and gravimetric cycles of the Moon Long cycles were at times in step with the Moon’s light and gravity cycles, less so with age and artificial light.
- Synchronization of women’s menstruation with the Moon has decreased but remains detectable when gravitational pull is strong Synchrony weakened after 2010 and remains mostly in January, when the combined pull of Moon and Sun is strong.