An estimated 85 percent of women experience at least one symptom of PMS per month, the American College of Obstetricians and Gynecologists estimates.

While PMS and its related hormonal changes are often talked about and are even the subject of endless jokes on TV sitcoms, the truth is that the hormones in a woman’s body can influence her moods and actions throughout the month.

“We know hormone levels fluctuate throughout the month,” says Diana Schwarzbein, MD, an endocrinologist and author of Menopause Power. “There are going to be corresponding fluctuations in other hormones that are definitely going to affect emotions, processes of thought, and physical well-being.”

Here’s what your reproductive system is doing during throughout your menstrual cycle and how hormonal changes may make you feel and act.

Feeling Good: The Follicular Phase and Ovulation

The follicular phase of your menstrual cycle begins on the day you start your period and lasts for about 10 to 14 days. During this time, the hormone estradiol begins to rise.

Follicle-stimulating hormone, or FSH, is secreted, stimulating the production of follicles in the ovaries that contain eggs.

This is more likely to be your “happy” time of the month. “It may just be in contrast to the second part of the month,” says Tracey Banks, MD, an ob-gyn with Adriatica Women’s Health in McKinney, Texas. “Those two weeks are usually good compared to how you feel toward the end [of the cycle].”

Sheets agrees, saying she’s much more likely to notice her bad moods than her good ones. “The times of the month when I’m energetic, happy, and inspired, I don’t spend much time dwelling on them,” she says.

However, there may be a biological basis for the “happiness” of the first half of a woman’s cycle. The good feelings may stem from a more sensitive brain.

Women in the follicular phase of their cycle might display greater brain activity at the thought of possibly winning money than women who were in other stages of the menstrual cycle, an experimental study published in PNAS found.

The estradiol rising in the body can help to tamp down the effects of the stress hormones adrenaline and cortisol, Dr. Schwarzbein says, and that could also play a part in preserving happy moods.

Feeling Hot, Hot, Hot: The Ovulatory Phase

During a woman’s ovulatory phase, a substance called luteinizing hormone increases.

This hormone prompts the release of an egg from the ovaries into the fallopian tubes for fertilization. Estradiol is present in significant quantities around the time of ovulation, and it can interact with other hormones to increase your libido.

“Estradiol makes insulin more effective,” Dr. Schwarzbein says. “Then the insulin tells the body to release more testosterone, and testosterone is one of the hormones that regulate sex drive.” Some experts surmise that this may be nature’s way of encouraging women to have sex during their most fertile time.

Recent studies have concluded that women are indeed more likely to display sexual behavior just before ovulating and may have a greater tolerance to pain too. You might also be more likely to buy clothes, makeup, and other items to help yourself feel more attractive, according to a recent study published in the Journal of Consumer Psychology.

The Other Shoe Drops: The Luteal Phase

After ovulation, the empty follicle that once contained the egg begins to secrete the hormone progesterone to thicken the lining of the uterus and prepare it for the possible implantation of an embryo.

As progesterone levels rise, you may begin to feel moodier. This happens because progesterone helps the body make cortisol, a hormone that tends to be higher in people who are stressed. If cortisol levels are already elevated because of outside factors, like a busy workweek, the progesterone can cause an excess of cortisol in the body. “If I’m already doing something to give myself high cortisol levels, by the time I get to the second half of my cycle, I’m going to be irritable,” Schwarzbein says.

The “yucky” feelings that come in the days before your period might have you looking for creature comforts to feel better. The same Journal of Consumer Psychology study found that women are more likely to eat high-calorie foods during the luteal phase of their cycle. “We do find that women crave certain foods during this time,” Dr. Banks says. “However, everyone is a little different, and not everyone has the same symptoms.”

Although the unpleasant symptoms of the luteal phase can be hard to deal with, Schwarzbein says you can do a great deal to shut them down by developing healthier lifestyle habits. Eating a poor diet, drinking lots of alcohol, and skimping on sleep can all disrupt the body’s hormone levels, making premenstrual symptoms much harder to deal with. “If someone is having PMS, there’s something wrong with her lifestyle habits more so than a hormone problem,” Schwarzbein says. If you’re practicing good habits and still have period-related moodiness, contact your doctor, as you could have a hormone imbalance that needs correcting.

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