In the early days of this stage, both estrogen and progesterone levels are low. At this point, Dr. Sims says to "go for it!" and push hard, as this will be the part of the month where you feel strongest. You could even go as far as trying to schedule your biggest challenges (a long run or a hard hike) during these days. It's a small window, though, so it may not always be possible to do this. Once estrogen begins to creep upwards again as you head towards ovulation, you may begin to feel more fatigued, she says.The few days before and during ovulation are when estrogen levels peak and then quickly decrease. In addition to needing a little extra sleep, some research suggests women may be especially prone to injury from here through the PMS phase. In particular, studies have shown that before puberty, boys and girls tend to get ACL injuries at about the same rate. Once puberty hits, however, girls are about twice as likely as boys to have those injuries. Other studies have shown that women who take hormonal birth control (and therefore don't experience the hormonal ups and downs of a normal menstural cycle) have lower rates of ACL injuries than those who don't. So, researchers think our hormones may have a role to play here. These hormonal changes may increase reaction time slightly and make you feel a bit foggier, Dr. Sims says. This doesn't mean you need to skip your favorite indoor cycling or bootcamp class during ovulation or anything. Instead, it's just something to keep in mind. "Take it easy this day," Dr. Sims says.Even if your PMS tends to be relatively mild, this is when shit can get real for your willingness to work out: Progesterone begins to increase at this point and estrogen (after the drop that occurs right after ovulation) also begins to increase again around this time. The result: you might feel like you've "lost your mojo," says Dr. Sims, because these hormones can lead to fatigue. Plus, progesterone can have catabolic effects, meaning that it encourages the breaking down of muscle proteins, Dr. Sims explains. It also increases the amount of sodium you excrete. So, your workouts may feel more difficult and recovering from them might take a little longer, too. "After a hard, intense workout, the need for protein and hydration (fluid with sodium) is increased in times of elevated progesterone," Dr. Sims says. On top of that, progesterone is known to be thermogenic, meaning it can heat up your core temperature. To combat all of that, Dr. Sims suggests drinking cold beverages before and during your gym session and adding a bit of salt to your food. She also recommends eating 25 grams of protein with a high leucine content (e.g., chicken, fish, and legumes) within a half hour of finishing your workout.During your period, estrogen and progesterone levels are at their lowest, which makes it easier for your body to access glycogen as a source of energy, rather than relying on the slower break down of fatty acids, Dr. Sims explains. So, you might find that you're hitting higher intensities faster during your workouts when on your period. However, everyone's cycle is unique in fun (or not so fun) ways. Plus, your fitness level matters a lot, too. While it's important to stay active, it's also crucial to pay attention to the full picture — including taking rest days when needed, eating a balanced diet, and getting enough sleep — all month long.