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RENEE MONTAGNE, HOST:

And, as you might have noticed, we're talking about stress this week. NPR conducted a poll with the Robert Wood Johnson Foundation and the Harvard School of Public Health on the subject. And we found, no surprise, that people often describe stress as a bad thing. But as NPR's Richard Harris reports, if you ask a biologist about stress you'll hear that not only can it be good - it's essential.

RICHARD HARRIS, BYLINE: Plug the word stress into the National Library of Medicine's search engine and you'll come up with more than half a million scientific publications that use the word. Referring to everything from allergies to the yeast genetics. Is the world simply drowning and stress? Not exactly.

BRUCE MCEWEN: Stress is an overused word.

HARRIS: Bruce McEwen at the Rockefeller University has built his career around understanding what stress really means in the biological sense. The truth is living creatures can't live without it. For example, stress hormones help animals react to unexpected situations and those hormones flow to the brain to help lab subjects, like rats, remember those notable moments.

MCEWEN: So that if it turns out to be dangerous and if the animal actually turns out to survive danger, then it will be aware of this as a potentially dangerous place.

HARRIS: So in that sense, stress is good?

MCEWEN: In that sense, stress is good.

HARRIS: People also experience this good kind of stress. It's your body's reaction as you get pumped up to speak in public or compete in an athletic event.

MCEWEN: We do something that takes a little bit of a risk and has a good outcome, we feel exhilarated - that's good stress.

HARRIS: Of course, that's not what most were thinking of when our poll-takers called them and ask them if they were experiencing stress in their daily lives. They told us about stress resulting from tension at work or family friction. And there the biological mechanisms that help us in the short run can grind on for weeks or months. McEwen says, often people can find ways to tolerate that stress.

MCEWEN: And then we have something which has been called toxic stress.

HARRIS: That's the stress system gone awry. Huda Akil, prominent neuroscientist at the University of Michigan, says that kind of stress can literally make you sick.

HUDA AKIL: When stress is sustained or repeating or extreme then all of this gets disrupted and eventually you do it long enough and it starts impacting other systems, including immune responses - it can affect the heart, it affects brain cells. It depends on how long we're talking about.

HARRIS: Our poll finds that to cope with excessive stress many people end up eating more or sleeping more while others end up sleeping less and eating less. Sometimes these reactions can help reduce stress, other times not so much. David London a professor of neuroscience at Johns Hopkins University has been thinking about the source of all this stress.

DAVID LONDON: I think a lot of people imagine that our modern lifestyle with constantly chirping cell phones and e-mail and all of these demands has led to more stress and the truth is it really doesn't seem to be that way.

HARRIS: Think how stressful it must've been for our forebears who lived in constant worry that their children would die of infectious diseases.

LONDON: So it's not just like stress is mounting up in our modern age, it's just the flavor of it is changing.

HARRIS: And that flavor isn't changing as much as you might think. Our poll finds that the major causes of stress in Americans are still those age-old troubles - illness, disease and the death of a loved one. Now, 14 percent of the people we polled said, they didn't feel stressed at all. But that doesn't mean a few people have found a secret eden right here on earth. Huda Akil says, some people are simply more tolerant of stress than others. She sees this clearly in her animal studies.

AKIL: Some animals like risky situations, actually go seek them and other animals cower away from them and it's clearly genetic. We can breed for it. And it shows up very, very early in life. And it is in humans - what we define often as temperament.

HARRIS: That temperament is shaped by both biology and our experiences as young children. It can have a lifelong impact on how we perceive stress and cope with it. Akil says, the good news is this mix of psychology and biochemistry is malleable.

AKIL: A lot of stress can be very destructive but a little bit of stress, it's kind of like working your muscles, your emotional muscles, and you build them up and you learn how to cope and so this is not something that irretrievably wired to be bad or good. It can be retuned and fine tuned and it's never too late.

HARRIS: Stress is, at root, a mechanism for adapting to changing circumstances. So when a challenging situation inevitably appears the trick is to get the stress reaction to work to your advantage. Richard Harris, NPR News.

MONTAGNE: This is NPR News.

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