January 2021
Vol 7, Issue 5

About The Cover

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ONLINE COVER The question of whether lunar cycles affect human cycles, including menstruation and sleep, has remained controversial and largely unanswered. Helfrich-Förster et al. analyzed long-term menstrual cycle records kept by 22 women for up to 32 years, finding that women with cycles lasting longer than 27 days intermittently synchronized with cycles that affect the intensity of moonlight and the moon's gravitational pull. This synchrony was lost as women aged and when they were exposed to artificial light at night. The researchers hypothesized that human reproductive behavior may have been synchronous with the moon during ancient times, but that this changed as modern lifestyles emerged and humans increasingly gained exposure to artificial light at night. In another study, Casiraghi et al. explored the moon's effect on human sleep and nighttime wakefulness by measuring wake/sleep cycles in three indigenous Argentinian Toba/Qom communities with varying access to electricity. Across all communities, sleep patterns were clearly modulated by the moon's cycle. Participants went to sleep the latest and slept the least 3 to 5 days before the night of a full moon. The researchers also analyzed sleep recordings from University of Washington undergraduate students, unexpectedly finding moon-dependent sleep patterns in this large urban environment similar to those of the Toba/Qom communities. The findings suggest that human sleep is synchronized with the moon's phases regardless of ethnic or cultural differences—and even in locations where light pollution outshines moonlight. [CREDIT: JOEL SHARPE/GETTY IMAGES]