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A good day for Katherine Schreiber begins with an hour of yoga. Without a morning stretch, “I just don’t feel okay in my skin,” she says. A health and fitness writer who lives in Manhattan, Schreiber leaves a yoga mat stretched out on the floor of her apartment at all times, “just in case.” After yoga Schreiber will usually head to the gym for a quick hour or so of mixed exercise, “cardio and core,” before heading to work. 

Once several years ago, according to Schreiber, the soft tissue inside one of her spinal disks escaped its enclosure. The resulting herniated disk compressed a nerve running along the nearest vertebrae, creating throbbing pain in her lower back and intermittent, shooting pains down her legs. “The doctor said I needed surgery,” Schreiber remembers, “but there was no way I could take the time off from exercising.” Schreiber, who has structured her life around exercising, is addicted to her workouts. Her apartment is minimally furnished and instead boasts free weights, a TRX machine, a body bar, and spin bike, “and I have an elliptical coming,” she says. Schreiber relies on her routine to feel okay, and the thought of missing a workout causes extreme anxiety. As a consequence, she holds memberships to two separate gyms in Manhattan, so she can workout at nearly any time, regardless of where in town she finds herself.  

Told she needed surgery, Schreiber instead hit the gym. “I had a herniated disk for over a year and I went to the gym everyday,” she says. “Hunched over” an elliptical machine, Schreiber worked out in agony. “I couldn’t even hold my back upright,” she remembers. “It was excruciating.”

As it is for most living creatures, movement is a fundamental part of daily life. It is, or should be, as natural as eating or sleeping. In the developed world, though it clearly is not. We’re constantly told that too many of us are inert. But now a select few are too active. As a sedentary society struggling to reckon with the confining world we have built, our relationship to movement lurches between extremes. Eighty percent of us do not exercise regularly—and a small but notable portion of the rest may exercise too much. The research is limited but the best estimate—from a — suggests that three percent of regular exercisers are exercise addicted. If that rate holds for the U.S., nearly two million Americans could have an unhealthy dependence on their daily workout. These millions are apt to exercise to the exclusion of friends and family and the loss of work and social relationships. Sometimes they will cause themselves intense and possibly irreversible physical harm. And they cannot stop. Small reviews of dedicated athletes paint a more troubling picture: as many as one out of every five amateur runners could be exercise addicted. For marathoners and triathletes that number is one in two. 

Schreiber is tall, and blond, with a wide, friendly smile, and the built of an athlete. She has, she believes, struggled to control her dependence on exercise for more than a decade. In that time she has experienced several of the “wake-up call” moments familiar to addicts with more common addictions. She recorded one salient post for Addiction.com. The wake-up came one day when a stress fracture of her left foot, “got so painful that I could no longer run nor get on an elliptical without agony pulsing from my toe to my knee,” she wrote. That injury proved “a blessing in disguise” forcing her to step off the treadmill of her regular routine and downgrade to less activity. Nevertheless, she concedes in the post, she continued to exercise everyday, her fractured foot encased in a plastic surgical boot.

If given the choice a rat will run. A lot. When housed in a cage with a wheel a rat will run at night, “more and more and more,” says Ben Greenwood, a neuroscientist and exercise physiologist at the University of Colorado-Boulder. In rodents, as in humans, exercise burns up stress hormones—cortisol in humans, corticosterone in rats—and triggers the release of pleasant and potentially addicting chemicals, like endogenous morphine. A few weeks of exercise will lead to increased activity in the neurons of a rat brain’s pleasure and reward networks, according to , and an almost simultaneous downgrade in the sensitivity of neurons implicated in anxiety and depression. Not surprisingly, “rats like to run,” says Greenwood. “And if you take their wheels away they develop anxiety.” 

Exercise dependence has been called a “positive addiction.” But that’s a stretch. The severe addict is compelled to exercise two, three, sometimes more than four hours a day, everyday. Often a traumatic event—the fracturing of an arm, or passing out in a spin class, both actual examples—can act as a wake up call. But like any addiction, cessation can lead to withdrawal, symptoms which include nausea, insomnia, anxiety, anger, and irritability. Tolerance to the exercise dose, meanwhile, forces the addict into increasingly longer or harder workouts. Some will go into debt pursuing their activity. Others will be told by a doctor that, without rest, they will end up in a wheelchair or worse. 

“They know they need to cut down, but they can’t,” says Heather Hasenblas, an exercise psychologist who studies exercise addiction at the University of Florida-Gainesville. Even those who do cut back, “reduce how much they exercise but they keep going,” she says. Those unable to switch activities after an injury are often in constant pain, “but it is worth it to them because the alternative is intolerable.”

Schreiber’s addiction began gradually, as most exercise addictions seem to. “When I started exercising regularly it was very easy to get away with over-doing it,” Schreiber told me. “It was a behavior that seemed healthy.” The phenomenon may work a bit like this: we all have stressors in our lives, large or small, that we must cope with. You like to be healthy, to do things that are good for you. So when you get anxious or upset, you don’t grab a beer, instead you lace up, clip in, pull down, or whatever else makes you feel good and productive. Rather quickly the work pays off: circulating stress hormone levels drop soon after exercise has begun and stay low, especially at night. If you are anything like a rat, you will also experience a lowered stress response in the future. For a lot of people the story ends there. 

“Many people can exercise two or three hours a day safely,” says Hasenblas. But others, when they arrive home and towel off, find that their stressors waited for them. Anxiety, though dulled, returns, and the next time they head out, they stay out longer. The stress melts away and the endorphins kick in and the cycle repeats itself. Other coping mechanisms with less immediate physiological feedback, like devoting time to friends or family or community, tend to fall away. 

“For most people who are addicted it’s about escaping the problems in their life,” says Mark Griffith, a behavioral addiction researcher at Nottingham Trent University in the United Kingdom. “The paradox is that mood modification is an absolutely fundamental core to addictive behavior. People use the behavior either to get buzzed up, to get high, aroused, excited—or to do the exact opposite, to tranquilize, to escape, to numb, to relax.” It is self medication. “And people use chemicals in the same way to produce a reliable, consistent change in mood. Exercise is one of those activities that when you are absolutely in the moment you can’t think about anything else.” That’s one of the things that makes exercise so great, and potentially addictive.  

Exercise dependence may be unique among addictions (save workaholism) in that a physical and emotional dependence builds within the confines of a socially-acceptable behavior. “Nobody’s ever like—oh you smoked three packs? Awesome!” says Schrieber. This can make it easy to begin an addiction, and hard to tell when a line has been crossed. Inevitably, exercise provides all the rewards necessary for addiction: physical, psychological, social, and, if you are a professional, financial. “Each of these is going to contribute to your addiction,” says Griffith.

To Hausenblas the difference between safe and unhealthy exercise hinges on “the underlying motivation.” Is it to escape—or to achieve?  To punish, or heal?

The handful of studies that have examined exercise addiction in professional or elite athletes suggest that they may not be immune to addiction. Far from it. In a small study of ultrarunners, Griffith only found three percent that met his criteria for exercise dependence. However, when he looked at a larger group of students training for careers in sports, the number of addicted jumped to nearly seven percent. Reported rates of addiction in competitive amateur runners have varied from 22 percent to nearly 40 percent, depending on the study population and the testing protocol used. A survey of more than two hundred triathletes selected randomly from international competitions classified half as highly exercise dependent. 

To Schreiber the hinge between healthy preoccupation and unhealthy obsession is flexibility. Does a committed exerciser “maintain their schedule when they have a stress fracture or tear a muscle?” she asks. “Can they take a break? If they do, can they sleep?” Schreiber writes about addiction professionally and may understand her disease better than most. (She recently co-wrote, with Heather Hausenbla, the first popular manual on the disease). Yet she still finds herself absolutely compelled to exercise. “Relapses are involved,” she says, and admits that she hasn’t taken a day off from exercise in years.

As most of us try to increase our time spent moving within or between stationary jobs, incapacitating commutes, and sedentary social activities, we all must struggle to find the right balance between no movement and too much.