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Correlation Between Menstrual Cycle and Lunar Phases Identified

The moon surrounded by darkness.
Credit: Mike Petrucci / Unsplash.
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For centuries, the menstrual cycle has been associated with the lunar calendar in many cultures and spiritual practices. Though often dismissed as a notion unsupported by science, a study published in Science Advances from researchers at the University of Lyon has revealed a small yet significant correlation between the length of the menstrual cycle and lunar phases.

Understanding the ovarian cycle’s rhythm

The average human menstrual cycle lasts for 29.3 days, but what causes it’s regular cycle remains relatively unknown. Some research has suggested that specific phases of the menstrual cycle may be controlled by endogenous factors such as the circadian rhythm. The disruption of these factors has also been shown to result in an irregular cycle. However, is it possible external influences may also be at play?


The circadian rhythm

The circadian rhythm refers to several physiological and behavioral changes, in the body, that are regulated by a 24-hour internal body clock – such as the sleep-wake cycle. The Circadian rhythm is mostly affected by light and is controlled by a small area in the middle of the brain.


The lunar’s 29.5-day cycle’s perceived connection with female fertility dates back to Ancient Greece. “The theory that the duration of the menstrual cycle and human reproduction is closely related to the length of the lunar cycle has been frequently postulated,” the authors said One previous study found women with menstrual cycles longer than 27 days were intermittently synchronized with the lunar cycle.

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“With regard to the menstrual cycle, the existence of a relationship, even a weak one, between the processes of the menstrual cycle and the lunar cycle would indicate the probable existence of an internal timing system and a synchronizing effect of the lunar cycle,” the authors said.

The menstrual cycle is driven by an internal clock

The researchers analyzed data from 2,303 European women and 721 North American women, looking at almost 27,000 menstrual cycles, with time series and circular statistics. They looked for menstrual cycle stability and instability, as well as evidence of jumps from one phase to another.

 

The data highlighted the presence of phase jumps, where when a cycle becomes out of sync, it self-corrects by “jumping” forward to the next stable state – a phenomenon seen in circadian clocks. The authors noted, “A few shorter cycles would compensate for a series of a few longer cycles,” making it “a strong argument for a clock-like drive of the ovulatory cycle.”

 

The authors also looked to see whether the cycle dates coincided with the lunar calendar. Although only a weak link was observed, the moon's influence varied by continent. European women’s cycles tend to begin at the waxing crescent lunar phase, whereas those in North America were more likely to begin their cycle during the full moon.

 

The lunar influence may be a remnant of the evolutionary past, as seen in organisms that live at the edge of the ocean. However, the authors do note that this connection is more likely to be driven by lifestyle factors, such as the sleep-wake cycle, rather than the moon.

Potential for personalized medicine

Further studies are needed to confirm the existence of a biological clock controlling the menstrual cycle.

 

“Medical treatment of ovulation disorders could use the chronobiological approaches that have proven successful in the treatment of cancer, sleep and circadian disorders and depression,” the authors said.

 

“As one of the potential approaches, chronotherapy and/or bright light therapy could be tested in clinical trials to test their effect on the menstrual cycle and fertility,” the authors concluded.


Reference: Ecochard R, Stanford JB, Fehring RJ, Schneider M, Najmabadi S, Gronfier C. Evidence that the woman’s ovarian cycle is driven by an internal circamonthly timing system. Sci Adv. 2024;10(15):eadg9646. doi: 10.1126/sciadv.adg9646