Do you feel like you're riding a premenstrual roller coaster every month that leaves you feeling hungry, tired, angry, sad, and unsexy? Leading researcher and women's hormone expert Dr. Sarah E. Hill explains why we feel so universally icky before our periods -- and what to do about it. The problem isn't that women are hormonal; the problem is that the second half of the menstrual cycle -- the luteal phase, when the hormone progesterone rises and estrogen decreases -- has been systematically ignored by science and medicine. Progesterone is at the heart of every feeling we associate with PMS: it affects our daily sleep and calorie needs (hello, cravings!); whom we're attracted to; our sex drive; and -- as every woman can attest -- our mood. Because the luteal phase is understudied, every bit of health, diet, and relationship advice you've followed is based on that first, estrogen-glow half of the month or, worse, was designed for men. The rules that work for us in the first two weeks of the cycle don't always fit in the second, causing most of us to spend half the month following advice that is completely at odds with the way our bodies work at this time. It's no wonder we feel awful! The Period Brain is a science-tested roadmap to understanding PMS and PMDD. It's time we demand a better month, every month -- and usher in a new era in women's health. Period.