Periods are sometimes nicknamed  a “moon cycle,” (among other things including red wedding, shark week and more. Read more about period euphemisms here.) Just like the moon goes through a natural cycle of waxing and waning throughout the month, so do human bodies, which go through four phases of the menstrual cycle each month. Periods are often depicted in pop culture with moon symbols and imagery. 

But are periods really connected to the moon’s lunar phases? And why do those associations exist in the first place? Here are the answers to all those questions and more.

Why is the moon linked to the menstrual cycle?

The most common reason why menstrual cycles have gotten their association with the moon has to do with the origins of the word itself. The terms “menstruation” and “menses” come from Latin and Greek words meaning month (mensis) and moon (mene).

The link between the moon and menstruation dates back to the days of Aristotle, the Greek philosopher who lived in the 4th century BC. Aristotle believed that menstruation took place every month during the waning of the moon (waning is when the crescent moon is getting smaller instead of bigger) and believed that any person’s cycle who didn’t line up was simply an anomaly. His explanation for this was, “This time of the month is colder and more humid because of the wasting and disappearance of the Moon.” However Aristotle didn’t exactly have access to the same knowledge of modern medicine we have now.

Although Aristotle didn’t exactly nail it with that particular philosophy, he was right about one thing: menstruation is a cyclical process, similar to moon phases. Just as the moon cycle waxes and wanes every month, so does the lining in your uterus, becoming thick with nutrients to prepare for fertilization and then shedding itself out if fertilization doesn’t occur.

The average menstrual cycle is around 28 days (though it can range from 25-30 depending on the person). Similarly, it takes the moon 27 days, 7 hours, and 43 minutes to make one full revolution around the Earth, meaning that the timing of both cycles is very similar. 

Do lunar phases actually have an impact on my period?

Over the years, a number of scientific studies have been done to attempt to locate the moon’s influence on menstruation— with conflicting results. A 1986 study found that many participants did begin their periods around the new moon which suggested there may be a link between the two. 

However, a subsequent study conducted in 2019 done by period tracking app Clue found that the synchrony between menstrual and moon cycles is a “myth.” The research analyzed data from 1.5 million Clue users and found overall that most periods do “not sync with the lunar cycle.” According to Dr. Marija Vlajic Wheeler, “Looking at the data, we saw that period start dates fall randomly throughout the month, regardless of the lunar phase.”

That being said, a new study conducted in 2021 shows there might be more to it. Although the researchers admit it is “very clear that most women appear not to be in synchrony with the moon,” they investigated the cycles of 22 women over the course of 10 years to determine exactly how the moon affected their cycles. 

They found that the moon’s “light intensity cycle seemed to be the most influential lunar cycle in terms of its effect on menses onset.” (Or, people seem to get their periods in relation to the brightness of the moon.) Researchers noted that most of the people who participated in the study reside in rural areas where they receive more exposure to moonlight, which suggests their bodies might be able to perceive changes in the moon’s brightness.

That said, not everyone is convinced. Because the period’s arrival due to the brightness of the moon was not a trait shared across all subjects, “it is not a compelling case that biologically meaningful synchrony is occurring,” says Virginia Vitzthum, a biological anthropologist at Indiana University.

So what is the truth with lunar and menstrual cycles?

Ultimately, there is little to suggest the moon has a significant impact on the timing of people’s periods on earth. We know that not everyone gets their period at the same time and Earth only has one moon, so if the moon actually did impact when people get their period, it stands to reason that more people would get their period at the same time. 

Although it’s comforting to believe that there are outside natural forces governing the function of our bodies, there is little evidence to support it. If you’d like to learn more about the phenomenon of period syncing, click here.