by Bill Katovsky.

Research scientists love tormenting lab rats. The furry little rodents are starved, shocked, bullied, and even water-boarded. Their torture is encouraged under the rational aegis of science—to find out how stress affects the brain. Because rats provide a fairly reliable indicator of human behavior, scientists use them to examine how stress affects overall health, including blood pressure, immune system, and depression.

In 2009, scientists at the University of Minho in Portugal discovered that chronically stressed rats acted rather un-ratlike. They’d continually press a bar for food pellets even when they had no intention of eating. The rats were stuck in a habit-forming groove of futile, non-productive behavior. It’s as if their stressed brains were unable to make intelligent decisions like, “Hey, no food, so why don’t I do something else with my time?”

Speaking with the New York Times, Robert Sapolsky, a neurobiologist at Stanford University School of Medicine, called the Portuguese study “a great model for understanding why we end up in a rut, and then dig ourselves deeper and deeper into that rut. We’re lousy at recognizing when our normal coping mechanisms aren’t working. Our response is usually to do it five times more, instead of thinking, maybe it’s time to try something new.”

Stress had an important evolutionary role in keeping our ancestors alive. Survival in the forest or on the savanna demanded quick action when danger lurked. Stress hormones like cortisol and adrenaline would suddenly flood into the bloodstream, causing the heart to beat faster, which increased blood flow to the muscles. But after the danger passed, the “flight or fight” hormones would settle down and the body would return to its normal physiological state.

But in today’s modern world, stress receptors often get stuck open in a locked position. Since the body can’t function all the time like this, stress hormone production is ultimately affected. Natural defense mechanisms weaken. The overloaded brain shuts down critical areas such as the hippocampus and prefrontal cortex, which affect learning, memory, and rational thought. A stressed-out person will end up engaging in harmful, counter-productive behavior, like having three beers after work, or eating junk food when not hungry.

Given identical stressful conditions, such as losing a job or breaking up, some people are better able to cope, while others will emotionally fall apart—and remain depressed for a long time. In a 2009 Newsweek cover story titled, “Who Says Stress Is Bad For You?,” science reporter Mary Carmichael cited several studies that pointed to genetic differences in determining the individual outcome to stressful situations. But which specific genes are responsible? No one knows. “The science is still young,” she writes.

Yet there’s good news for the stressed-out population. The Portuguese scientists found that stress-caused behavior is indeed reversible. Once removed from a stressful environment, the rats resumed acting like normal rats. No more pressing the food bar when there wasn’t any food. Their brain circuitry had somehow rewired itself.

“The brain can grow new cells and reshape itself,” says Carmichael. Furthermore, “meditation appears to encourage this process. Monks who have trained for years in meditation have greater brain activity in regions linked to learning and happiness.” The monks grew new brain cells.

Carmichael brought up another classic rat study: “Something that should lower stress can actually cause stress if it’s done in the wrong spirit. Scientists put two rats in a cage, each of them locked inside a running wheel. The first rat could exercise whenever it liked. The second rat was forced to run whenever its counterpart did. Exercise, like meditation, usually tamps down stress and encourages neuron growth. The second rat, however, lost brain cells. It was doing something that should have been good for its brain, but it lacked one crucial factor: control. It could not determine its own ‘workout’ schedule, so it didn’t perceive it as exercise. Instead, it experienced it as a literal rat race.”

So even too much of a good thing like exercise can turn harmful if it’s controlling you rather than vice versa. It’s a primary reason why many athletes get injured or sick if they train or race too hard and don’t take time off. The stress switch can’t indefinitely remain open.

This article is excerpted from Bill Katovsky’s latest book, “Return to Fitness:Getting Back in Shape After Injury, Illness, or Prolonged Inactivity.