Last month, on the day my period started, I had a few cramps, felt a little bloated, was slightly irritable, and noticed a lovely little pimple just to the left of center on my forehead. On most days, making all that go away (except the pimple) would be as easy as going out for a run. But on that day, lacing up my sneakers was too daunting. I felt exhausted, and gearing up for a workout seemed incredibly difficult. So I skipped the run—and didn’t feel better until the next day.

Why So Tired?

I didn’t think much about why I felt too tired to exercise that day, but if you have ever experienced something similar, there may be an explanation. A study from the University of Adelaide in Australia suggests that the phenomenon is quite real and probably related to hormone levels, which vary dramatically throughout the course of the menstrual cycle.

“Exercise is most difficult from approximately three days prior to beginning the menstrual period through to ovulation,” says Leanne Redman, who reported the findings of this study. “During this time, two of the principle menstrual cycle hormones, estrogen and progesterone, are very low.”

Redman explains that these hormones may influence the body’s energy metabolism at rest and during exercise. Research suggests that when these hormone levels are low, women may end up contending with more waste products—such as lactic acid and carbon dioxide—from the metabolic consumption of carbohydrates. “This can contribute to muscle soreness and premature fatigue,” Redman notes.

Testing Their Theory

In the University of Adelaide study, a group of women went through the same exercises at different points during their menstrual cycles. “Although the women completed the same exercise test in both phases of the menstrual cycle, the exercise test conducted at the beginning of the month took longer to complete and the women reported feeling more physically and mentally fatigued at this time point,” Redman says.

Exercise Could Help (If You Weren’t So Tired)

The fact that hormone levels may lead to difficulty with exercise during the menstrual period is especially unfortunate because exercise is such a wonderful remedy for the symptoms many women encounter before and during their periods. “Exercise boosts endorphins and can improve your mood, increase your metabolism, and help reduce bloating,” says Carol L. Otis, MD, a fellow of the American College of Sports Medicine and a physician with the Women’s Sports Medicine Clinic (part of the Kerlan-Jobe Clinic) in Los Angeles.

Exercise can also help to ease cramps. “We don’t know exactly why,” Dr. Otis says, “but evidence suggests that it has quite a positive effect.” Because it helps with so many symptoms, Dr. Otis recommends trying to find a way to exercise for at least 30 minutes per day—with at least one longer session per week—even during your period. “If you can’t seem to do it all at once, try to do ten minutes of exercise here and there a few times throughout the day,” Dr. Otis advises.

What About Your Diet?

If period fatigue is in fact a result of low levels of estrogen and progesterone leading to poor metabolism of carbohydrates, then limiting your intake of complex carbohydrates like bread, pasta, rice, and potatoes during your period may help combat the fatigue. Redman suggests increasing fruit and vegetable intake as well.

Other strategies recommended by Dr. Otis to reduce bloating and breast tenderness and increase energy include getting at least 1,200 milligrams of calcium daily, reducing intake of salt and caffeine, and taking a B-complex vitamin. If your life is hectic, “Scheduling 20 minutes per day of relaxation might also help,” Dr. Otis adds.

Medication May Help

If you still have severe symptoms and fatigue before and during your period, even after incorporating lifestyle changes, there are medicines that might help. Talk to your doctor about your options.

Know Yourself

Of course, not all women will experience difficulty with exercise at exactly the same point in the menstrual cycle. “Each woman experiences the menstrual cycle in her own way,” Dr. Otis says. “Each woman should track her symptoms and learn how she responds to the phases of her menstrual cycle.” If you do experience difficulty finding the energy to exercise, you will know that there is a reason—and that you can develop a plan to deal with it.