The Moon, the Sun, and Menstruation

The lunar cycle lasts about 29 days, which is similar to the average menstrual cycle. Coincidence? For centuries, people have drawn connections between the moon’s phases and menstruation. Surprisingly, there’s more to this link than you might think. However, the strangest part is that when it comes to what might actually influence menstrual cycles, we might have been looking at the wrong thing in the sky all along.

Let’s start with a quick refresher on what a menstrual cycle is. The menstrual cycle prepares the body for potential pregnancy, and in humans, it spans about a month. The cycle begins on day one of your period when the uterine lining starts to shed, known as the menses phase. This phase typically lasts three to seven days and overlaps with the follicular phase, which continues from the start of your period until one of your ovaries releases a new egg, usually about ten to fourteen days after your last period began. This marks the start of the luteal phase, where the egg travels down the fallopian tube to the uterus. If the egg isn’t fertilized, a change in hormone levels triggers your next period, and the cycle starts all over again.

Given that the menstrual cycle is roughly a month long, it’s no surprise that people have linked it to another mostly-monthly cycle in nature: the lunar month, which spans about 29.5 days from one full moon to the next. The idea that these two cycles are connected is ancient; in fact, our word for both the moon and menstruation comes from the Indo-European root word “mē,” meaning “to measure.” Aristotle, the Greek philosopher, believed menstruation occurred during the moon’s waning phase, attributing it to colder and more humid conditions during that time. And this connection isn’t just limited to Europe. Similar beliefs exist among indigenous groups in Tanzania, the Central African Republic, Mexico, the US, and Canada, among others. But, can they all be wrong? Almost definitely.

Research into whether the moon influences menstruation has produced underwhelming results. For one, menstrual cycle length varies significantly from person to person, with the normal range spanning 24 to 38 days. Cycle length can also vary within an individual; a 2006 study found that about 42% of people’s cycles varied by more than 7 days in a single year. So, even if not everyone is synced with the moon, could anyone be? To answer this, we’d need to examine people with four-ish week cycles and see if their periods align with any lunar phases. While some older studies suggested tentative correlations, more recent, larger studies have found no significant connection. For example, a 2016 analysis by the period-tracking app Clue, which looked at data from 1.5 million users and 7.5 million menstrual cycles, found no correlation between lunar phases and period start dates. The evidence is clear: human menstrual cycles don’t align with any particular lunar phase and can occur at any point in the month.

However, some animals do have reproductive cycles that sync with the moon. Studies have shown that sea urchins’ gonads enlarge under a full moon and shrink under a new moon, which matters because we eat those gonads—commonly known as “uni” in sushi. Sea urchin fishers have long known to harvest urchins during a full moon when they are “full.” Similarly, some corals spawn en masse a few nights after a full moon, likely timing their spawning based on the amount of moonlight, thanks to specialized photosensitive molecules in their tissue. This synchronized spawning increases the odds of successful fertilization. Marine zooplankton also time their mating with the moon’s phases, reproducing during both the new and full moons.

You may have noticed a trend: these are all marine animals. Researchers believe these animals might stay synced with the lunar calendar by using changing tides or reacting to moonlight during a full moon. Given that humans have lived by the ocean for a long time, it’s not far-fetched to think that they would have paid attention to these natural cycles, especially when they impact food sources. This might explain part of the perceived connection between the moon and menstruation, especially among people who relied heavily on marine life for sustenance.

While there’s little evidence to support the idea that the moon influences human menstruation, there’s growing evidence that sunlight might play a role. A study in a Russian city showed that sunlight exposure could affect the length of a person’s period. More sunlight exposure before ovulation could lead to shorter periods, with menstrual cycles being about a day shorter during the summer. This correlation was specific to sunlight and not influenced by temperature or other weather factors. Other studies have suggested that increased exposure to artificial light could also shorten menstrual cycles. Artificial light seems to suppress melatonin production, which is thought to regulate some period-related hormones. Essentially, more light means less melatonin, which could mean shorter periods.

So, while there may be a connection between what’s happening in the sky and menstrual cycles, it’s not the one people have been fixated on for centuries. Even though our ancestors were wrong in linking the moon’s phases to menstruation, there was more logic behind that connection than you might have thought. Hopefully, this sheds a little moonlight on the topic.