As a runner, some days I’m able to happily gallop across a field (okay, the street) or on the treadmill as though it’s the easiest thing in the world and my legs are made of springs. Other days, though? My legs feel 100 times their normal weight—almost impossible to get moving—and I find myself stopping to walk way too frequently for my liking. And so the run, rather than being an easy glide around the park, becomes a torturous task that feels akin to climbing Mount Everest.

I know that my seemingly polar running days aren’t a foreign occurrence, though. Other runners agree that this happens to them as well, but still—none of us have a clue why this variation in running stamina happens. And so I set upon a quest to finally figure out why there are good and bad run days.

The factors that influence your run

You know how some mornings you can practically leap out of bed and others require hitting the snooze button several times? Your body just functions differently every day—and a lot of variables contribute to this. “It all depends on your sleep, diet, and exercise recovery,” says Krista Stryker, fitness expert, 12-Minute Athlete creator, and author. “Some days you’re just going to naturally feel stronger and more energized than others.”

Sleep is your number one recovery tool, she says, so if you don’t sleep well, it’ll definitely affect your workout. Also important? What you eat. “Did you eat enough before your run? Food is fuel and if your body doesn’t have enough to run off of, your workout’s going to suffer,” adds Stryker. “Ideally you’ll have eaten a meal of complex carbohydrates and protein within two hours of your workout.” For a smaller boost, she recommends a banana with a tablespoon of almond butter or a protein shake.

Also, overtraining can have an impact on the endurance of your runs. “Overtraining can have a number of negative effects on your body,” she says. “If your performance has been going down and you’re not sure why, you might need to take an extra day to rest or take a close look at your recovery tactics.”

Matt Nolan, an instructor at Barry’s Bootcamp, echoes that this is a major influence on how well you’re running on certain days. “The most likely reason you’re [having a hard run] is that your body is not yet fully recovered from the day before,” he explains. “The pounding you put on your body is a lot, and most of us need at least 48 hours to fully adapt and be ready for the next hard workout.” His suggestion? Alternate your running days between easy runs and harder runs to help with the issue over the long haul.

How your mindset plays into your run

Though you probably are wanting your body to just do the run and get it over with, there are likely mental blocks happening that are preventing you from running like you normally do. Sometimes it’s best to just go along with it. “It’s all about accepting and not resisting what your body needs in the moment,” says Jordana Jacobs, PhD, a New York-based clinical psychologist. “If you’re tired, let yourself walk for a bit, after which point you may naturally feel like running again. The key is to listen to your body rather than solely push yourself with your mind.”

If you’re able to ignore these signals of fatigue and frustration and push through the pain, it may even have a negative effect—as in, you’ll be less motivated to run as often if it feels so torturous. “If you respond to what the body needs during a workout, it’s much more probable that you’ll achieve the balance of pushing yourself and letting yourself rest that’s necessary in order to maintain a healthy exercise regiment over time,” says Dr. Jacobs.

After all, if you’re just feeling extra tired or less motivated one day, at least appreciate the fact that you’r’e moving your body.

How you can still get a good workout

Struggling yet still determined to salvage that run? Don’t worry—intervals have got you. “You can step up your run by alternating between sprints and walking, which is a fantastic workout,” says Stryker. “As a bonus, you can actually workout for much less time. An example would be to sprint for 30 seconds, rest for 30 seconds, and repeat this 10 times.”

If you’re sticking to a jog and feel the need to keep breaking, no worries. Stryker just advises to keep your walking breaks to a minute or less. “The key is to keep your heart rate up and maintaining the set time of your run so that it doesn’t creep over your expected window.” Regardless, there’s no need to be ashamed if you’re not reaching your own PR every day of the week.

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