Period syncing is the idea that being in close proximity to someone else who menstruates can affect your biology and prompt your period to start sooner or later than what's normal for you.
In 1971, Harvard doctor Martha McClintock conducted a period syncing study that started this theory. She studied a group of 135 women living in a college dorm. Her research found that over the school year, period synchronisation increased among roommates and close friends, but not among random pairings of women who hadn't spent time together.
McClintock believed this synchronisation was the result of the women spending time together, eating together and experiencing stress together. She even hypothesized a theory called the "alpha uterus". McClintock believed an "alpha uterus" had a strong hormonal pull that could cause other cycles around it to menstruate in unison. This theory is known as the McClintock Effect, which is the idea that our pheromones communicate with each other due to closeness, triggering cycle syncing.
Since then, multiple studies have been conducted showing varying results, leading to much scepticism among experts towards this theory - however, according to a 1999 survey, 70% of participants said they had personal experience with this phenomenon.
While perhaps more research is needed on this topic to know for certain if we do cycle sync, it's something we can experiment with ourselves in our own lives.
Have you noticed that you cycle sync with close friends, housemates or work colleagues?
Study referenced from ro.co