Your Brain On: Your Menstrual Cycle

Guys have it easy. Their bodies are pretty much perpetually locked and loaded when it comes to their reproductive capabilities. Women, on the other hand, have to endure a monthly ebb and flow of hormones in order to make conception possible.

Apart from preparing your body for pregnancy, those hormonal fluctuations have some serious repercussions for your brain. From the way you dress to the size of your brain, your menstrual cycle will mess with you in multiple ways, research shows. Here, experts explain exactly what’s going on in your head throughout the (roughly) 28 days of your cycle.

Day 1 to Day 5 (Your Period)

Hormonally, the first day of bleeding is a relatively quiet time, says Louann Brizendine, M.D., a neurobiologist at the University of California, San Francisco, and author of The Female Brain. Your brain and body’s levels of the three major hormones that control your cycle-estrogen, progesterone, and testosterone-are about as low and balanced as they’ll be, Brizendine explains.

But research shows that, starting soon, an uptick in hormone-like compounds called prostaglandins may leave you cramp-y or nauseated as your uterus releases its unfertilized egg and any extra blood or tissue that accumulated as your body prepared for pregnancy. You’ll also start to produce more estrogen and testosterone as your period progresses, although levels of both are still pretty low, Brizendine says. That uptick in estrogen should stimulate the release of feel-good chemicals like endorphins, which wipe out any remnants of the crankiness or foggy thinking you may have experienced during PMS, studies show. (More on that later.)

Day 5 to Day 14

Your estrogen levels in particular will skyrocket between now and just before you start ovulating in order to prepare your uterus for fertilization. “Estrogen acts like fertilizer for brain cells,” Brizendine says. That leads to a roughly 25 percent growth of synaptic connections in your hippocampus, which controls functions like your short-term memory and decision-making, she explains.

Rising estrogen levels also sharpen your verbal fluency, Brizendine adds. “I always tell my female students to plan to take their oral exams the day before ovulation, when there estrogen levels are peaking.” At the same time, ramped up testosterone production increase a woman’s sex drive. Combined with the swell of brain activity from estrogen, that leaves you feeling more social, flirtatious, mentally sharp, and energetic, Brizendine says. Whether or not you realize it, you’re also more likely to wear extra makeup and “Notice me!” colors like red, studies have found. Why? “You’re advertising that you’re fertile, even if you’re not conscious of it,” Brizendine explains. Testosterone also boosts your sense of competition, which means you’re more likely to feel threatened by other women, research shows.

Even your sense of smell is affected. Peaking estrogen levels make your nose more attuned to pheromones, or chemicals the body releases that can trigger sexual desire, studies show. “There’s a famous dirty t-shirt experiment,” Brizendine says. “During the week before ovulation, women were attracted to men’s sweat on their dirty t-shirts. It’s amazing how estrogen can change our behavior.”

Day 14 to Day 25

Around the time you start ovulating, there’s evidence that you view men’s faces differently. Reward regions of your brain light up, shows research from Indiana University’s Kinsey Institute. At the same time, the frontal cortex, which typical manages your self-control, may power down, the researchers found. As a result, you might find yourself attracted to more masculine looking faces, and you’re more likely to engage in risky sexual behavior, the study authors say.

Also, thanks to the high amounts of estrogen you were producing for the last week, grey matter in your brain has expanded, particularly in your hippocampus, shows research from Germany. That growth peaks at the start of ovulation and may improve your memory and your ability to process information, the German study authors say. But once you’ve begun ovulating, your estrogen and testosterone stores plummet, which studies show can leave you feeling a little cranky. At the same time, levels of another hormone, progesterone, begin to swell, Brizendine says. “Progesterone acts on the same brain receptors as Valium, so it’s very calming,” she explains. “This hormone is preparing your uterus and uterine lining for implantation in case there’s fertilization of the egg.” If you were pregnant, the fertilized egg would trigger a continued release of progesterone, she adds.

Unfortunately, as your ovulation ends and your progesterone levels continue to rise, your hippocampus and the other parts of your brain become less active, meaning your short-term memory, verbal fluency, and general sociability may all fall, she explains. The German research shows your brain’s grey matter stores also shrink.

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Day 25 to 28

When your body realizes it’s not pregnant and prepares to jettison its unfertilized egg, your progesterone and estrogen levels both plummet. The lack of those hormones causes an uptick in your brain’s levels of stress chemicals like cortisol, which further contribute to your bad vibes during PMS, studies have found. Headaches, poor sleep, and a general absence of energy and enthusiasm are all pretty common, Brizendine says.

Things could be worse. Your brain goes through a surge of activity in its medial orbitofrontal cortex during the premenstrual phase of your cycle, according to a study in the Proceedings of the National Academy of Sciences. The study authors say this brain activity may help women maintain control of their moods despite their big hormonal fluctuations. For those who experience severe PMS mood swings, your hormone levels might be especially high or variable, the authors say. But look on the bright side: In just a few days, this whole roller-coaster hormone cycle will start all over again, Brizendine says. Your estrogen levels will pick up again as your period commences, and the negative PMS symptoms will evaporate, she adds.

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