Can Your Period Be Affected By The Moon?

There are several urban legends and myths surrounding the moon's cycles and human beings. While werewolves and crazed mermaids are only for movies and TV, the concept of the moon affecting our bodies isn't seen as crazy. Some cultures believe it more than others, and when it comes to modern myths, a lot of the beliefs have to do with moods and menstrual cycles.

For instance, you've probably heard the concept that when a full moon is out, people don't know how to act. As Scientific American reported, the words "lunatic" and "lunacy" come from "lunar" and the Roman goddess of the moon, Luna. A full moon is thought to cause irregular behavior, all based on this urban legend that the moon makes you a little bit crazy. Obviously, science has been able to prove that the moon is what causes the tides. So why wouldn't it impact humans, who are made of 80% water? Okay, it's a bit more complicated than that, but the jury is still out on whether the moon affects menstrual cycles. But there are interesting reports on either side of the argument.

One study suggests the moon can potentially affect menstrual cycles

The belief that the moon has an impact on menstrual cycles is old and pretty well-known, even by non-believers. But as The Scientist reported, a lot of periods are the same length as lunar cycles. Plus, Charlotte Helfrich-Förster, a chronobiologist at the Julius-Maximilians University of Würzburg in Germany, said that some marine animals use the moon for "synchronizing their reproduction." So it isn't an all-out absurd hypothesis.

To figure this out, the American Association for the Advancement of Science (AAAS) reported that Helfrich-Förster was the lead author of a 2021 study that looked to answer the question of moons and periods. They didn't have a large testing pool — only 22 women — but they looked at their menstrual cycles over the course of five to 32 years. They found "most women's menstrual cycles" aligned with the same cycle as the moon and "certain intervals." Women 35 years and younger synched "with the full or new moon for 23.6 percent of the recorded time, on average." Women over 35 only synched 9.5 percent.

So what does that all mean? Well, the testing pool was very small. The Scientist also reported it was done with mostly women in rural areas. So even though there were some consistent results, there are millions more people with periods who live under different circumstances and conditions, which makes it hard to definitively say that the lunar cycle impacts menstrual cycles based on that study.

But there isn't enough info to firmly say the moon impacts periods

Again, due to the 2021 study's small size, even though it looked at nearly three decades for specific women and the limited geographic locations they existed in, we don't know if the moon impacts periods. Other studies have also stated they haven't, but regardless, Virginia Vitzthum, a biological anthropologist at Indiana University, told The Scientist, "[The study] is not a compelling case that biologically meaningful synchrony is occurring." 

Regardless, chronobiologist Charlotte Helfrich-Förster speculated that the moon most likely affected periods more in ancient times. We now consume so much artificial light that it's changed any internal clock that lined up with the lunar cycle. Just like leaving a TV on while you're sleeping can mess up melatonin production, it's possible that less access to moonlight no longer means periods can synch with it.

Speaking of modern times, the period-tracking app, Clue, concluded that out of 7.5 million tracked cycles from 1.5 million users they studied, menstruation doesn't sync up with the moon. Dr. Marija Vlajic Wheeler, a Clue data scientist, said some cycles are as long as lunar cycles, but "period start dates fall randomly throughout the month, regardless of the lunar phase." Even though you do have to account for user errors or inaccuracies, with a sample size so large, the odds are high that this is more accurate to modern menstruation cycles.