Study Links Post-2010 Decline in Menstrual–Lunar Synchrony to LED Era

The team reports fading alignment in the smartphone era, emphasizing correlation rather than causation.

Overview

Published in Science Advances on Sept. 24, the analysis examined more than 11,500 records from 176 women spanning roughly 1950 to 2024.
Before 2010, menstrual onsets showed significant population-level alignment with full and new moons and with two ~27-day gravitational lunar cycles.
After 2010, the broad synchrony largely disappeared, with a remaining signal mainly in January when combined lunar–solar gravitational effects peak.
Researchers hypothesize a human circalunar clock sensitive to moonlight and gravity, proposing that blue‑rich artificial light may disrupt these cues and shorten cycles.
Authors note key limits, including sparse individual light‑exposure data and a primarily European sample, and outside experts caution that other environmental or demographic factors could also contribute.