Does the moon affect your period? What the research shows

For most people, the Moon does not set the timing of their period. The largest analysis so far, 7.5 million cycles tracked in the Clue app, found that periods start on random days of the lunar month, with no cluster at the new moon or the full moon. Underneath that clear answer sits a more interesting one: long-term menstrual diaries suggest that an individual cycle can fall into step with the Moon for a while, especially at younger ages and with little artificial light at night.

This article covers where the idea comes from, what the big datasets and the long-term diaries found, the spiritual reading of white and red moon cycles, and how to check your own cycle.

Why does the menstrual cycle seem to follow the Moon?

Start with language. The word menstruation comes from the Latin mensis, meaning month, and the English words month and moon share an ancient root tied to the Moon’s phases as a way of counting time. Old English even had a word for a period that translates as month-blood. For most of history, a month simply was the time from one new moon to the next, so it is no surprise that the body’s monthly rhythm and the sky’s monthly rhythm ended up described in the same words.

Then there are the numbers. The synodic month, the time from one full moon to the next, averages 29.53 days. As you will see below, the average menstrual cycle is remarkably close to that. Two rhythms of almost exactly a month are hard not to connect.

And then there is memory. If your period arrives on the night of a bright full moon, you notice. When it arrives on an ordinary half-moon evening, you do not. By chance alone, roughly half of all periods start within three days of either a new moon or a full moon, because those two windows together cover nearly half the lunar month. That leaves plenty of hits to remember, the same trick of memory that keeps so many moon stories alive, from sleep to mood.

How long is a menstrual cycle, really?

The 28-day cycle from school textbooks is mostly a convenient round number. A 2019 study in npj Digital Medicine analyzed 612,613 ovulatory cycles from 124,648 users of the Natural Cycles app. The average cycle lasted 29.3 days, and only 13 percent of cycles were exactly 28 days long. Average cycle length also fell by about 0.18 days for every year of age between 25 and 45. About nine in ten cycles fell within the conventional normal range of 21 to 35 days, which shows how much ordinary variation there is.

So the average really does sit close to the lunar month. But an average says little about you. If your cycle lasts 26 days, each period starts about three and a half days earlier in the lunar cycle than the one before. After two cycles, a start that once fell on the full moon has slid back to around first quarter, and after four cycles it lands near the new moon. Only a cycle of almost exactly 29.5 days stays on one phase for long, and even then it wanders, because real cycles vary by a few days from month to month.

What do the big app datasets show?

If periods were tied to the Moon, you would expect them to bunch up around one phase. That is exactly what Clue, the period-tracking app, checked. Its data science team compared the start dates of 7.5 million cycles from 1.5 million users with the phases of the Moon, leaving out users who had logged hormonal birth control. As Clue data scientist Marija Vlajic Wheeler put it, period start dates fall randomly throughout the month, regardless of the lunar phase. No peak at the new moon, no peak at the full moon.

Smaller studies point the same way. A 2021 study of 529 women aged 25 to 39 in Japan, using six recent cycles logged in the Luna Luna app, found no overall link between the onset of menstruation and the lunar phase.

One very large analysis did detect something, and it is worth reading carefully. A 2024 study in Fertility and Sterility looked at 311,064 cycles from 35,940 women aged 18 to 40 in Europe and North America, recorded from 2019 to 2021. Periods started slightly more often on Thursdays and Fridays, a small weekly rhythm. A lunar rhythm was also statistically significant, but it was less pronounced than the weekly one, and its peak shifted with the seasons. With enough data, even tiny patterns become measurable, and here the day of the week was a clearer signal than the Moon.

Can your cycle sync with the Moon for a while?

The most intriguing work comes from a different angle. Instead of asking whether everyone’s cycle follows the Moon at the same time, chronobiologist Charlotte Helfrich-Förster of the University of Würzburg in Germany asked whether one woman’s cycle might follow the Moon now and then over many years. In a 2021 study in Science Advances, written with Thomas Wehr of the US National Institute of Mental Health and colleagues, her team analyzed the diaries of 22 women who had recorded the start of their periods for an average of 15 years, and in one case for 32 years.

The answer was neither a simple yes nor a simple no. Cycles longer than 27 days lined up with the full or new moon intermittently, not permanently. For women aged 35 and younger, periods were in sync with the full or new moon for an average of 23.6 percent of the recorded time. For women over 35 it was 9.5 percent. As the women got older and their cycles shortened, the synchrony faded. The women who stayed up late with plenty of artificial light showed no obvious link with the Moon at all. The team also found traces of two gravitational lunar cycles: the 27.55-day cycle of the Moon’s changing distance and the 27.32-day tropical month.

In 2025 the same group published a larger follow-up in Science Advances, using 176 long-term records, mostly from women in Italy and Germany, with others from elsewhere in Europe, Israel and North America. Records from before about 2010, when LED lighting and smartphones became widespread, showed significant synchrony with the Moon. After 2010, the coupling appeared mostly in January, which the authors linked to the combined pull of the Moon and the Sun when Earth is closest to the Sun.

How convincing is this? Claude Gronfier, a circadian biologist at Inserm in Lyon, France, told the Science Media Centre that the core link between the lunar and menstrual cycles was very convincing and not down to chance. He was more cautious about the rest. The groups are small, the light the women were exposed to was never measured, and in his words there is no consensus on a role for lunar gravity in human biology, nor any known mechanism. The fair reading is the one the scientists themselves offer: an intriguing lead, not a settled case.

The Moon does not act as one clock that every cycle follows. At most, an individual cycle may fall into step for a while, until age or artificial light pulls it away.

White moon and red moon cycles: the spiritual reading

Alongside the research there is a popular spiritual way of reading the cycle, and it deserves a fair explanation. In this framework, a white moon cycle means you bleed around the new moon and ovulate around the full moon, a pattern associated with fertility, nurturing and turning inward. A red moon cycle means you bleed around the full moon and ovulate around the new moon, associated with creativity, healing and sharing wisdom. Cycles in between are seen as moving from one to the other.

Many people use this language to pay closer attention to their bodies, and that attention has real value. It helps to be clear about what it is, though: a way of giving meaning to your cycle, not a description of something the Moon does to you. The numbers above show that most cycles walk through every phase of the Moon over a year. A white moon cycle becomes a red one and back again, not because anything changed inside you, but because 29.3 and 29.5 are not the same number.

How to track your own cycle against the Moon

You do not need a study to see what your own cycle does, only a little patience:

1. Note the first day of each period. Use an app, a paper diary or the printable moon calendar. Three cycles give you a first picture, six give you a clearer one.
2. Look up the moon phase for each date. The moon phase finder works for any date, not only birthdays, and shows how much of the Moon was lit.
3. Work out your cycle length. Count the days from one start date to the next. If it is not close to 29 or 30 days, you will see the start day move through the phases, month after month.
4. Look at your evenings. In the Würzburg diaries, late nights under bright light seemed to weaken any link with the Moon. It is an interesting thing to notice, nothing more.
5. Keep it light. A cycle that follows the Moon is normal, and so is one that does not. There is nothing to win or lose.

One exception to the lightness: if your cycle suddenly becomes much more irregular or painful, or your period stops, see a doctor. That has nothing to do with the Moon and deserves ordinary medical attention.

Moon phases in 2027 to set beside your cycle

If you want to lay your cycle next to the sky in 2027, these are the anchor points. There are twelve full moons in 2027, one each month, and thirteen new moons. The year opens with the new moon of January 7, 2027 at 20:25 UTC, which is already January 8 in India and Australia, followed by the Wolf Moon of January 22 at 12:17 UTC, the supermoon and brightest full moon of the year. Given the 2025 finding about January, that month is a natural place to start a diary.

August 2027 has two new moons: the new moon of August 2, with a total solar eclipse across North Africa and the Middle East, and the new moon of August 31, a black moon in the Americas, Europe, Africa and India (in Sydney and Auckland it falls on September 1).

Every new moon is listed in the new moon calendar for 2027, and every full moon in the full moon calendar. Put your own notes beside them, and within a few months you will know more about your cycle than any myth can tell you. If writing helps, try the full moon journal prompts.