My friends have always joked that I can fall asleep anywhere. They’re not … wrong. Nothing signals luxury to me like a stolen cat nap, and there’s nothing I won’t do to try and get some extra sleep. But until life after college, I had explained away any behaviors associated with sleep as just that—a love, and deep appreciation for, snoozing.
It wasn’t until much later in life, after still being exhausted following a good night’s rest, that I realized there was something strange going on. My own research and talking to experts helped me figure out that my constant and perplexing exhaustion wasn’t due to lack of sleep; it was my body’s reaction to—and coping strategy for—spikes in stress levels. And knowing that was the key to fixing it.
Google “stress and sleep” and you’ll find myriad articles about exhaustion. Restlessness. Insomnia. But falling asleep due to stress? Not much. Most resources on the topic surmise that it may be due to an underlying sleep disorder, or that your stress is leading to depression (which leads to fatigue), or that it somehow has to do with having to process memories and experiences.
But, as Dr. Curtis Reisinger of Northwell Health explains, mental stress really does physically tire out our bodies. And it has everything to do with just how much physical energy it takes to keep feelings like anger, frustration, and sadness to ourselves.
Dr. Reisinger, a clinical psychologist who specializes in stress moderation, explains the phenomenon like this: You find yourself presented with a stressful situation. Say, a demanding boss. And, sure, you’d love to tell your boss to shove it. But society dictates you can’t—or you’d risk being fired. So what do you do? You keep quiet and you get the job done. In inhibiting normal “stressed” behavior—crying, yelling, etc.—you expend energy. Your muscles tighten or you clench your teeth as you internalize your frustration. You’re now fatigued. Cue the nap.
“I think it’s probably more common in today’s society than in past generations and past centuries,” Dr. Reisinger says. We’re a little more influenced by societal norms than we tend to think, including which emotions we’re allowed to express. “I see it as a sort of relatively modern syndrome that’s probably not going away anytime soon.”
Some would say this response is atypical, that in times of stress you’re on high alert, but Dr. Reisinger explains that when it comes down to it, there’s a huge range of human responses to stress—and this is one that’s commonly missed. “If you come in with your boss and your boss yells at you, and you start getting really exhausted and fatigued, then it’s probably a lot of self-critical behavior, a lot of energy being used to control your response. That’s where the exhaustion comes from,” Dr. Reisinger says.
Dr. Reisinger emphasizes that if you’re keeling over from fatigue in the middle of a workday, something else is going on. It’s not just stress. Head to your doctor to rule out things like narcolepsy, diabetes, low iron, and other energy-impeding problems. But if you find yourself yawning whenever you get a stack of papers dropped on your desk or your credit card statement arrives in your inbox, you may actually be better served by a therapy session than another hour of sleep. Because if you’re like me, who is very good at internalizing issues and going about your day—reacting by not reacting—it would seem we’re in this boat together, friend. We’re tired. Now at least we now know why.
This article originally appeared on InStyle U.S.