Do Periods Sync When Women Live Together?What Studies Show

It’s one of the most repeated period beliefs: live with other women long enough and your periods start arriving together. Researchers have tested it for decades, and a newer line of studies has asked about the moon too.

The dorm study that found no syncing

Researchers tracked the periods of 186 Chinese women living in dorms for over a year.¹

• Women living in groups did not sync their cycles.¹
• The authors also reanalyzed the first study that reported syncing and found its result was at the level of chance.¹

They also showed why syncing seems convincing: cycles vary in length, so two women’s start dates naturally drift closer, then apart again.¹ Catch the “closer” months and it looks like syncing.

A roommate study that claimed some

In India, 62 medical students sharing twin rooms were followed for 13 months.²

• The average difference between roommates’ start dates went from about 7.6 days to about 6.1 days.²
• The authors classed 17 pairs as in sync and 8 pairs as out of sync.²

That’s a shift of about a day and a half, in a small group, with regular cycles. Given how much cycles vary, it’s a weak case for syncing.

What about the moon?

German researchers analyzed long-term period records from individual women.³

• Cycles longer than 27 days were intermittently in step with the moon’s light or pull.³
• With age and exposure to artificial light at night, cycles got shorter and lost that link.³

A follow-up compared records from before and after 2010, when LED lighting and smartphones spread.⁴ Cycles recorded before 2010 lined up with the moon more; after 2010, mostly only in January, when the authors say the pull between the moon, sun and Earth is especially high.⁴ They describe the moon’s effect on human reproduction as still controversial.⁴

What this means for you

• Your period lining up with a friend’s is very likely chance, not a signal between bodies.
• Lining up with the moon may happen now and then, but it isn’t something to plan around.
• What matters for your health is your own pattern: cycle length, how heavy, how painful, and any change. See what a normal menstrual cycle is and why your period is late or irregular.

Try: “My cycle has started to vary a lot from month to month. I’ve tracked my start dates for three months. Is this range normal for my age?”

Period, Handled covers this in Chapter 4, “Your Period Has Numbers,” in a section called “Your Period Scorecard.” The Scorecard tracks cycle length from first day to first day, so you can see your own pattern instead of guessing from a roommate’s. See the book.