Menstrual & Lunar Cycle Synchrony, Circadian Rhythms | Charlotte Förster | 259

Oct 19, 2025

Charlotte Förster, a senior professor and chronobiologist, shares her fascinating research on how biological clocks, including lunar rhythms, impact human menstrual cycles. She discusses historical data indicating menstrual synchrony with lunar phases before 2010 and how this has changed due to blue light from modern devices. The conversation explores the effects of light pollution on biological rhythms and potential implications for fertility. Additionally, Förster reveals her extensive studies on fruit fly circadian mechanisms and their relevance to human health.

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INSIGHT

Multiple Biological Clocks Govern Behavior

Biological clocks include circadian, tidal, lunar, and annual rhythms across species.
Lunar clocks particularly time reproduction in many marine organisms and influence behavior seasonally.

ANECDOTE

Full Moon Social Nights In Argentina

A colleague studied sleep in three Argentinian populations with differing artificial light.
People in low-light communities stayed up ~1 hour later before full moon to chat and celebrate.

INSIGHT

Menstrual Cycles Often Match Lunar Period

Many women's menstrual cycles (~29 days) weakly synchronized to full or new moon in older studies.
Synchrony appeared mainly in women whose cycle length approximated the lunar period.

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How artificial light impacts female menstrual cycles and their relationship to lunar cycles of the moon.

Summary: Dr. Förster talks about how biological clocks, including circadian, tidal, lunar, and annual cycles, regulate behaviors in various species, with a focus on lunar cycle effects on human menstrual cycles. They explore historical and modern data suggesting that menstrual cycles may synchronize with lunar phases, a phenomenon potentially disrupted by modern artificial lighting, particularly blue light from LEDs post-2010. The conversation also covers circadian rhythm mechanisms in fruit flies and humans, highlighting the role of light and neuropeptides in maintaining biological synchrony.

About guest: Charlotte Förster, PhD is a senior professor at the University of Würzburg specializing in chronobiology, particularly circadian rhythms in fruit flies, and has recently explored lunar cycle influences on human menstrual cycles.

Discussion Points:

Biological Clocks: Various clocks (circadian, tidal, lunar, annual) regulate behaviors; circadian clocks manage 24-hour cycles, while lunar clocks influence reproduction in marine species like corals and Christmas Island crabs.
Lunar Cycle & Menstruation: Historical data (pre-2010) showed many women’s menstrual cycles synchronized with lunar phases (full or new moon), but this decreased post-2010, possibly due to blue light from LEDs disrupting biological rhythms.
Blue Light Impact: Blue light from modern devices mimics daylight, potentially desynchronizing circadian and lunar clocks, with melanopsin in the eyes playing a key role in light sensitivity.
Winter Synchronization: Menstrual cycle synchrony with lunar phases is stronger in winter, particularly January, possibly due to brighter moonlight or gravitational effects when Earth is closest to the Sun.
Circadian Mechanisms: In fruit flies, 240 neurons manage circadian rhythms via clock genes with a 24-hour feedback loop, conserved in humans, where neuropeptides regulate slower, sustained rhythms.
Health Implications: Disrupted circadian rhythms can desynchronize body clocks, impacting digestion, immunity, and increasing risks of cardiovascular issues, obesity, and cancer.
Lifestyle Tips: To maintain synchrony, maximize daytime light exposure, minimize nighttime blue light, time meals appropriately, and exercise during the day, not late at night.

Reference paper:

Study: Synchronization of women’s menstruation with the Moon has

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