If periods were planets, they would have a very strong gravitational pull.
Alas, periods are periods. And while menstruation may not have the power to sway massive orbiting bodies, it still has a pull of its own, and a strong one at that, according to research led by Stanford scientists.
Using data gathered by an app, the study tracked four cycles -- daily, weekly, seasonal and menstrual -- as well as variation in mood, vital signs and behavior in 3.3 million people who menstruate from more than 100 countries.
It turns out menstrual cycles have a stronger influence over variations in behavior, mood and vital signs than the three other cycles, with a few exceptions. The data, for instance, showed that days prior to a period, those who menstruate were more likely to feel negative emotions, such as sadness, stress and sensitivity.
"The menstrual cycle is only one cycle of many that people experience, but to our knowledge, prior to our work, no one had ever been able to compare all four of these cycles -- daily, weekly, seasonal and menstrual -- at the same time to see how they impact specific aspects of human health and behavior," said Emma Pierson, PhD, a Stanford computer scientist who led the study.
With the rise of period apps, which allow menstruators to track and record information about their bodily cycles, a new type of data collection and way to analyze data has opened up.
"The menstrual cycle has been particularly hard to study on a population level because you need to know when each person's cycle starts," said Pierson. "But now we can take advantage of this new data source to better break down mood, health and behavior as it corresponds to a person's period."
The pre-menstrual slump
It may seem like an odd fit for a computer scientist to study menstruation, but Pierson's pursuit of the topic is purposeful.
"I felt that the menstrual cycle has been understudied and stigmatized, even though it's a fundamental topic in health. So I wanted to set out to investigate it and normalize it as a topic of study in a male-dominated discipline," said Pierson.
Pierson and Leskovec's study analyzed millions of self-reported, de-identified data points from a period-tracking app that captured moods, such as variations in happiness, sensitivity, social inclinations, energy and more; physical health parameters, such as resting heart rate and weight; and behavioral change, such as exercise, sexual activity and sleep duration.
Overall, the menstrual cycle had the largest influence of the four cycles tracked in the study, swaying moods, behavior and vitals signs. There were, however, some exceptions. Productivity and sleep, for instance, were more heavily influenced by the weekly cycle. (Sleep went up on the weekends, while productivity went down.)
The study found that moreover, just before the start of a period, people tended to experience a sort of mood slump. Past analyses have found this too, but this study did so in more detail and across larger, more diverse populations, Pierson said. Importantly, she said, they saw that the menstrual mood dip occurred consistently across dozens of countries.
During this pre-period dip, the data showed that menstruators tended to experience more feelings of sadness, but also distractedness, and they felt more withdrawn socially. After people's periods started, sexual behavior decreased, while social engagement, focus and energy increased. The study also found that body temperature, weight, and resting heart rate dipped at the onset of a period.
Menstruators are not more volatile
Pierson emphasizes that the paper is not suggesting that menstruation is the only thing to sway mood, behavior and physical health -- not by a long shot. The paper only considers cycles, but non-cyclic events like the loss of a loved one or even a big win from a favorite team can change mood dramatically as well.
Likewise, she cautions against using their work to suggest people who menstruate are more prone to mood swings or volatility than those who do not.
"These findings do not imply that," said Pierson. "First of all, the paper doesn't compare any of these parameters between people who menstruate and people who don't. We don't even have data on sex or gender, all we know is that these people are logging their information on a menstrual cycle app."
Where to go from here
Pierson says that the rise of period-tracking apps opens doors for other types of investigations too, such as better pregnancy prediction and studies that probe the relationship between menstrual cycles and fertility, or menstrual cycles and certain diseases.
"With this much data, I could see new efforts that track menstrual cycle patterns and potentially find associations with diseases such as endometriosis," said Pierson. "These types of health apps offer an opportunity to shed light on understudied conditions that cause a lot of suffering. That's an exciting direction to me."
Photo by Fausto Sandoval