Modern Light May Be Dimming the Moon’s Influence on Women’s Menstrual Cycles

A century of speculation has linked menstrual cycles to the Moon. Now, scientists say that connection may be fading in our brightly lit, digital world.

A new study from the University of Würzburg finds that women’s menstrual cycles used to align more often with the Moon’s phases, but that synchrony has weakened in the age of smartphones and artificial lighting.

Scientists revisit whether the menstrual cycle aligns with the Moon

The notion that women’s menstrual cycles might follow the Moon’s rhythm has long fascinated scientists – a pattern seen across many marine and terrestrial species that synchronize reproduction with lunar phases to enhance reproductive success.

Since the human menstrual cycle averages ~29.3 days, remarkably close to the lunar month that sits at ~29.5 days, researchers have long wondered if a link might exist.

Older studies from the 1970s and 1980s reported that menstruation tended to cluster around full or new moons. More recent work, however, failed to confirm any clear pattern, which has left the field skeptical.

In 2024, a team looked to see whether cycle dates coincided with the lunar calendar, and although a weak link was observed, the researchers noted that this connection is more likely to be driven by lifestyle factors.

These inconsistencies, together with suspicion toward “lunar effects” in human biology, left the question unresolved.

Lead author Dr. Charlotte Helfrich-Förster, a professor at the University of Würzburg, and colleagues sought to revisit the issue using both historical and contemporary menstrual records.

The team also aimed to understand whether factors such as artificial lighting at night or lifestyle changes have disrupted any original coupling between the cycle and the moon.

Long-term records reveal a fading lunar link after 2010

The team analyzed 176 individual menstrual records from women who were not using hormonal contraception, with each record spanning between 2 and 37 years. They compared the timing of menstrual onset against three lunar-cycle measures:

• The synodic month (~29.5 days, combining lunar phase and gravitational cycling).
• The anomalistic month (~27.55 days, changes in the Moon’s distance from Earth, affecting gravity).
• The tropical month (~27.32 days, the Moon’s declination cycle, also influencing gravitational effects).

Data were separated into two eras: pre-2010 (before widespread light-emitting diode (LED) lighting and smartphone screen use) and post-2010.

Using circular statistics and visual “mensograms,” the team found that, before 2010, many women’s cycles periodically aligned with the full or new Moon. After 2010, roughly coinciding with widespread LED and smartphone use, this population-level synchrony largely disappeared, although weak coupling persisted in January, when the Moon–Sun–Earth gravitational pull is strongest.

“All women were only temporarily synchronized with the Moon. For some women, this occurred only once during the recording period and only for a few months, while for others it occurred for several years,” Förster told Technology Networks.

Menstrual timing remained correlated with purely gravitational lunar cycles even when luminance effects seemed absent. Synchronization was especially detectable during the winter and summer solstices.

“The most interesting and surprising cases were those in which the coupling with the lunar cycle occurred several times during the recording period. If this were the case, the coupling always happened at the same phase, either shortly before the full Moon or shortly before the new Moon,” said Förster.

“This repeated coupling and uncoupling cannot be explained by mere coincidence,” she added.

To explore external confirmation, the authors analyzed Google Trends data for searches such as “period pain.” They reported modest peaks around early January in both hemispheres, which they interpreted as indirect evidence that gravitational forces could influence menstruation patterns or related symptoms.

A possible “circalunar clock” connects biology and the Moon

Förster and the team suggest much like the 24-hour circadian clock that governs sleep, humans may also possess a circalunar clock – an internal rhythm that loosely tracks the Moon’s 29.5-day cycle. In chronobiology, entrainment means an internal rhythm syncing up with an external time cue, such as light from the Moon or Sun.

“We found that women with a cycle length close to 29.5 days showed the highest likelihood to entrain to the lunar cycle, while women with shorter periods (less than 27 days) were less likely to entrain, and those with periods of less than 25 days never entrained. In my opinion, this is the strongest argument in favor of a circalunar clock.”

The study’s results suggest that artificial light may be the main disruptor of this ancient coupling.

“Much has changed over the years, but since 2010, powerful LEDs have entered the market and replaced traditional incandescent bulbs and fluorescent tubes in indoor and outdoor applications,” said Forster.

Unlike older incandescent or fluorescent bulbs, LEDs emit higher-intensity blue light, which suppresses melatonin and alters hormonal rhythms.

“Light at night obscures the natural moonlight,” she added. “Exposure to bright artificial light has been shown to shorten the menstrual cycle, further reducing the likelihood of synchronization with the lunar cycle.”

Artificial light may have disrupted menstrual–Moon synchrony

If valid, the findings suggest a remnant biological link between human reproduction and the Moon, yet the evidence remains correlative and alternative explanations must be considered.

Many records were self-reported, non-uniformly distributed across decades and heavily from European individuals. Variations in record accuracy, lifestyle and fertility status could all mimic periodic patterns. Statistical over-interpretation of weak signals is also possible since small deviations can appear meaningful in large datasets.

The supposed “gravitational coupling” relies on temporal coincidences rather than demonstrated biological mechanisms. Humans are not known to perceive such subtle gravitational fluctuations directly.

“I have no proof of causality. We have only done a correlative study,” said Förster.

Other factors, such as seasonal behavior, temperature, stress or holiday patterns, could also explain the winter peaks in menstruation or Google searches. January-related trends might reflect lifestyle or cultural effects rather than cosmic forces.

Menstrual cycles also naturally fluctuate with age and health, and the data did not include hormonal or environmental measurements that could disentangle these influences.

However, the study highlights how artificial light may alter physiological rhythms. If bright nocturnal environments shorten menstrual cycles, as previous work suggests, this could partly explain the modern loss of lunar synchrony without invoking gravity.

Future research could employ controlled longitudinal designs, combining precise hormonal assays, light-exposure tracking and geographic diversity.

Whether lunar gravity subtly shapes human physiology or whether coincidental seasonal patterns mislead us, the study underscores how deeply environmental rhythms – celestial or artificial – intertwine with biological timekeeping.

 

Reference: Helfrich-Förster C, Domenie ED, Mitesser O, et al. Synchronization of women’s menstruation with the Moon has decreased but remains detectable when gravitational pull is strong. Sci Adv. 2025;11(39):eadw4096. doi: 10.1126/sciadv.adw4096

 

About the interviewee:

Dr. Charlotte Helfrich-Förster is a zoologist, neurobiologist and a professor at the University of Würzburg. Förster’s main research interests are to decipher the circadian clock on the molecular and neuronal network level, to understand how it is synchronized to the environmental cycles on earth (mainly to the Zeitgebers light and temperature) and how it controls behaviour.