The sun’s just coming up and my alarm goes off. I roll over and look at my sneakers—I was planning on going for a run before work—but here’s the thing, I’m so freaking tired. Eyelids still heavy with sleep, I start counting the hours of sleep I got last night. Five and a half? As I let my head settle back into the comfortable groove of my pillow, I’m curious: on such groggy mornings, is it better to crawl out of bed and drag my ass to the gym, or sleep in and get some much-needed rest?

Apparently, it’s not such a simple question to answer. “Exercise, sleep and nutrition form the triangle of health, and all are related,” says Dr. Phyllis Zee, a professor of neurology and director of Northwestern University’s Sleep Health Centers. Her research has found that a good night’s sleep (logging at least seven hours of slumber) results in more productive exercise sessions later that day. Those with less sleep, not surprisingly, had reduced motivation to exercise. Likewise, “exercise can actually improve the quality of sleep,” she says, leading to deep sleep that is more “restorative and effective for performance.”

Because of this interconnected relationship, robbing yourself of either sleep or fitness seems dangerous to your overall health. But it’s clear that if you’re not properly rested, your workout won’t be as productive. What’s worse, you can even put on weight simply by sleeping less. A study published in the Journal of Applied Physiology found that chronic sleep loss results in insulin resistance and decreased glucose tolerance, making you gain more weight than someone who’s fully rested. Lack of sleep is also associated with cardiac disease and increased risk of sudden death, so there’s that too.