Working out too much can cause adrenal fatigue, which includes symptoms like body aches, lightheadedness, and extreme tiredness. (Watch out for even more signs you’re working out too much.) And when these symptoms hit, all those hours you spent at the gym could backfire—big-time.

A stress hormone called cortisol is partly to blame, according to personal trainer and injury-prevention specialist Liz Letchford, MS, ATC. Pushing your body past its limits through high-intensity, long-duration exercise can mess with that hormone, leading to fatigue, reduced performance, and weight gain around your belly.

In other words, exercise “is not just about strong muscles and losing fat,” Letchford told PopSugar. “Your entire central nervous system is regulated by a complicated system of hormones. Throw them out of whack, and you’ve got some serious issues.”

Luckily, preventing your body’s burnout is easier than you think. Letchford recommends choosing an exercise routine—either long duration/low intensity or short duration/high intensity—that matches your fitness and weight loss goals. And avoid going to the gym more than once a day, Letchford says. “Your body needs it to recover from all of the breaking down that has been happening during workouts.”