It has an effect, period

A huge, multi-country analysis of menstrual cycles has revealed their strong influence on individual mood, behaviour and vital signs. 

A research team led by Emma Pierson from Stanford University and Microsoft Research New England, US, found that the menstrual cycle had a greater effect on fluctuating mood, behaviours and vital signs compared to daily, weekly and seasonal cycles.

The team’s study, published today in the journal Nature, is built on a substantial dataset of 241 million observations from 3.3 million women across 109 countries, including Australia and New Zealand. The data were sourced from a women’s health mobile app named Clue, launched in 2013 by European technology company Biowink GmbH. Data recording was consensual and anonymous.

“Cycles are fundamental to human health and behaviour,” says Pierson. “We were seeking to describe cycles in women’s mood, behaviour, and vital signs – particularly the menstrual cycle, which has been understudied and stigmatized in past work – but other cycles as well, like the daily, weekly, and seasonal cycles.

The study breaks each dimension of mood, behaviour, and vital signs into four simultaneous cycles – daily, weekly, seasonal, and menstrual. 

“We think is the first time this has been done,” says Pierson. “A main finding is that the menstrual cycle is a primary cycle: it is larger than the daily, weekly, and seasonal cycles for 11 of the 15 dimensions of mood, behavior, and vital signs that we study.

“We also study how menstrual patterns vary across countries, and find that the premenstrual mood dip occurs consistently across countries, and other significant menstrual patterns are consistent as well. This is important because previous studies have often been small, confined to one country, and potentially culturally specific.”

Pierson stresses that the key finding – that the menstrual cycle is often larger than the daily, weekly, and seasonal cycles – doesn’t mean that women are more volatile than men.

“We make no comparison across genders or sexes; previous research has not found sex differences in volatility, and men have hormone cycles too,” she says. “Rather, it just means that the menstrual cycle is at least as important as the less stigmatised daily, weekly, and seasonal cycles, and we should normalise it and destigmatise talking about and studying it.

“It should be as easy to talk about as the sleep cycle. We should also prioritise collecting data on it – for example in health records and health apps – which is not always done. We should also make sure clinicians consider it when treating patients – which they do not always do.”

Results showed that some behaviours – such as sleep and exercise – remain constant over the menstrual cycles, but mood, weight and sexual behaviour varied. 

For the “happy-sad mood” indicator, for instance, the affect of menstrual cycle was 1.4 times higher than for daily cycle, 3.3 times higher than weekly cycle and 2.3 times higher than seasonal cycle.

“[This indicates] that the menstrual cycle has a more significant impact on happy-sad mood than the daily, weekly, and seasonal cycles in our dataset,” explains Pierson. “It doesn’t mean the menstrual cycle is the only or most important thing that affects mood  – since our analysis doesn’t even consider non-cyclic effects, like losing a big football game.

“But it does mean that the menstrual cycle is important relative to the other three cycles.”

It also has an even greater effect on mood than singular events that affect large portions of the population, such as Christmas, when happiness generally increases (1.7x higher), or the 2016 US Presidential election, the day after which happiness was shown to decrease (0.7x).

“That’s a funny finding, isn’t it?” quips Pierson. “I think it’s likely driven by the fact that the users of the app are mostly not US citizens, and [are] mostly younger women – so, not big Donald Trump fans for multiple reasons.”

In spite of its massive dataset, the study had some limitations, among them that it required data input through a smartphone, which has a bias towards people of a higher socio-economic status. 

Pierson acknowledges both the problem and the research team’s tactic to overcome it.

“In our main analysis, we filter for women who use the app regularly over a long period – they log in 12 unique months,” she says. “We also apply various quality control filters to some of the features we study – like body weight or temperature – to make sure that people are logging reliably.”

Pierson says that from the beginning, her main goal in pursuing this line of research was to send the message that the menstrual cycle was a normal thing it was okay to talk about. 

“I hope this paper will contribute to that message,” she says. “Over the years, I have heard from many women sharing their own stories because of this work. I have also seen men learning more about the menstrual cycle and realising it’s really cool. 

“That has been the most rewarding part of the project for me.”