Ever wonder why you smash a HIIT set one week, then struggle through it the next? Where you are in your menstrual cycle could have something to do with it. As leading global expert on female-athlete physiology Stacy Sims, PhD, says, "it's your physiology, not your fitness.”

Learning to train to your cycle can help you manage stress, boost immunity, understand your metabolism and improve your performance. Leaving you feeling fitter, stronger and faster.

With the help of Stacy Sims, we’ve created a collection of workouts adapted to the three phases of a woman’s cycle. Take Day 1 of your period as the kick-off point to start cycle syncing and get the best out of your workout

While every body is different and cycle lengths can vary, science is still a great guide for our fitness choices:

Constantly in flux and controlled by the menstrual cycle, your immunity dips significantly during ovulation (days 13–19ish). It’s to give "a fertilized egg the best chance of survival,” says Sims. Yep. Even if you’re not pregnant.

So if you’re feeling sluggish, Sims suggests “taking time to relax and eat lots of colorful fruit and vegetables.” Drink water more frequently and swap max workouts for moderate ones.

The Early Phase is when things start to ramp up. Your immunity is at its strongest, and your results can be too. So feel free to challenge yourself, but always listen to your body.

By the End Phase your body is preparing to shed your endometrial lining, marking your period. A potential drop in your immunity might call for a ‘feet up and kettlebells down’ day or two.

Racing pulse. Heavy breathing. Sweaty palms. Remind you of your last workout or work deadline? It's not by chance. “These stress responses are actually a natural part of your every day,” says Sims. Triggered by physiological, emotional and physical stimuli (like training), our bodies are under different levels of stress at every stage of our menstrual cycle.

Luckily, our bodies are smart, and can adapt to small doses of stress. But if we are overwhelmed by constant stress, we need to find an outlet; most often, physical activity. There are points during your menstrual cycle where your body can handle higher loads of stress; and others where you need to be more aware and add small, daily, stress reducers to support the body’s systems.

Exercise is hands down great for your mood and metabolism. Ever wonder why you're smiling even in the hardest drills? Now you know. It's those feel-good hormones or 'endorphins'. These neurotransmitters can increase your pain threshold, and even boost yourself-esteem. But there's a fine line between exercise that relieves stress and exercise that causes it. Sims recommends always taking the time to recover and fuel right. A workout is only as effective as your diet. "Under-fueling is a form of stress that can negatively impact training and other aspects of your health like your thyroid and metabolism," says Sims. The right food, she explains, "rebuilds and repairs muscle tissue, replaces glycogen [carbohydrate] stores and maintains immune health.” To plan your optimal diet, consider your needs for each phase

Ever wonder why you’re craving sweet, salty, and protein-rich foods in the week before your period? It’s not a mystery, it’s your cycle. This hankering is the body’s way of letting you know it’s not getting something it needs. What’s the solution? More good calories. About 100 to 150 calories per day, in the week leading up to your period. “If you surround yourself with foods like fruit, avocado, nuts and seeds you'll be less tempted by packaged foods,” says Sims. That’s because these healthy foods trigger the same pleasure response in the brain, while providing a better source of energy.

The food cravings are part physiological and part psychological: shifts in hormone levels means concentrations of serotonin also change in the brain. More specifically, a drop in estrogen can cause the feel-good hormone serotonin to also fall, “which can make women feel flat, anxious, and depressed,” says Sims. (Yep, she’s talking PMS.)Sweet, carbohydrate-rich whole foods —like fruit, sweet potatoes, whole-fruit jam, honey and dates —to the rescue. They don’t just taste good, they also make us feel relaxed thanks to a little boost in serotonin. In the End Phase especially, protein cravings are totally normal. Why? Because our bodies are busy building endometrial lining tissue. Sims suggests “increasing your intake of leucine-rich protein, which ensures levels of serotonin remain stable in the brain.” These foods, like eggs, fish and tofu, reduce cravings for sugary, sweet foods, and lowers anxiety and depressive mood changes. Win-win.

Breezing through a run one day, and then struggling through it the next, might have something to do with where you are in your menstrual cycle, says Stacy Sims. Just like training, you can adapt your running routine to the three main phases of your cycle.

In the Early Phase, run faster, recover better and feel more energetic. During the Middle Phase Sims says: "If you are feeling bulletproof, nail that PR!" Does the End Phase have you feeling flat or are you experiencing PMS symptoms (headaches, bloating, fatigue)? Go for easy hikes, jogs and work on your form.

To learn more about the relationship between your menstrual cycle and stress, nutrition, running and immunity head to our expert tips and Cycle Sync Workout Collection here.