The Period Brain: The New Science of Why We PMS and How to Fix It: A Manifesto for Women
By Sarah E. Hill
HarperCollins Publishers
If you are a person who menstruates, you’ve probably been asked by someone if you’re “just on your period” when you’ve expressed strong emotion. If you’re like me, that comment did not help the situation.
Despite the misogynistic undertones of the question, it does point to something big (however unintentionally). Periods are a biological reality, and their effects extend far beyond the week of. In fact, 75-90 percent of women report feeling phsycially and psychologically less well in the two weeks preceding their periods – which is half the time.
In “The Period Brain: The new science of why we PMS and how to fix it,” Dr. Sarah E. Hill addresses a gaping hole in research and women’s health: the way that science and research has ignored the cyclical nature of women’s health.
You probably learned in science class or on Tiktok that women were excluded from medical research for a long time. Many medications and technologies such as seatbelts were designed and tested with men in mind, much to the disservice of women.
What you may not realize is that now that women are included in research, the gold standard procedure in 2025 is still to test women only during days three to seven of their cycle, when sex hormones are the lowest. This approach has the benefit of ensuring that all women in the study are relatively similar to each other hormonally. But according to Hill, it ignores the entire second half of the menstrual cycle (yes, when PMS happens), with detrimental effects.
“Having very low levels of sex hormones isn’t something that’s typical for women. We are hormonal creatures. Researchers estimate that we spend less than 20 percent (!) of our cycle in a hormonal state similar to the one we’re usually tested in,” she writes. “Because of this, knowing how we respond to a specific drug or therapy when hormone levels are low won’t necessarily provide any insight into how our bodies will respond the other twenty-two-plus days of the cycle when our hormone levels are high.”
For those who menstrate, menstural cycles have two distinct phases with their own distinct roles. The follicular phase starts on the first day of your period. The luteal phase is dominated by progesterone. The former is responsible for sex and attraction and latter for implantation and pregnancy Our mood, attraction, immune function, body temperature and even breathing change in response. Women feel “disordered” when they’re in the luteal phase of their menstrual cycles due to the changes in their bodies, but that, Hill argues, is largely because we haven’t been given an explanation for why we feel so different at different points in our cycles and don’t give our bodies what we need to thrive.
It’s important to note, as Hill does in the intro of this book and her previous one about hormonal birth control, that the research she presents focuses on cisgender, heterosexual women. This is not because she believes this is the only or most important experience of being a woman, she explains, but because most of the research we have focuses on this population.
Even when we look at only cis-het women, so much of women’s health feels like a conspiracy theory. What do you mean they just decided that the second half of the cycle was just a variable to be ignored? What do you mean 80 percent of new prescription drugs are regularly pulled from U.S. markets because of unanticipated side effects on women? (This is discussed in the book, and it’s worse than you think.) As a woman who has menstruated for the last 14 years, I find this distressing. It’s easy to fall down the TikTok rabbit hole of people talking about it without knowing if the information you’re getting is reliable or not.
Hill is an award-winning research psychologist who sits on the scientific advisory boards for companies like Flo and 28 Wellness, and her writing is conversational. It strikes a balance between being informative and accessible in a way that’s difficult to do with science communication. “The Period Brain” is a straightforward articulation of what a cycle looks like and how to deal with it that I wish was taught in Health class. Hopefully, it starts a broader conversation.
