This is a big question! And of course, the answer differs somewhat from
therapy to therapy. But one way that many mind-body therapies  impact
the body is by reducing stress . So it is helpful to understand what
stress is and the role it plays in health and wellbeing.
To start, try this activity. Imagine the last time you felt incredible stress  or anxiety . Perhaps you had an important meeting, and you missed your plane. Maybe you had a complicated presentation, and you despise public speaking. Or perhaps you underwent important medical testing, and were waiting for the phone to ring with results. Notice how you feel.
Now stop. Close your eyes. Breathe deeply  for a few minutes. Gently relax your shoulders, your neck, and your facial muscles. Now how do you feel? (If you want to continue relaxing, try the activities in the left column.)
people report that right after imagining a stressful event, their pulse
rate was high, their breathing was fast and shallow, and their jaw,
neck, and other muscles were tight. Some report that they felt their
After breathing deeply for only a few seconds, most people report that their pulse and breathing rate decreased and their muscles relaxed.
activity probably provoked what is called a "fight or flight" response
because your central nervous system perceived a threat.
When you perceive a threat, your sympathetic nervous system releases stress hormones to arouse key organs, resulting in an increase in your heart and respiratory rate, greater muscle tension, coldness and sweatiness, a decrease in intestinal activity, and an increase in the size of your pupils. This is the fight or flight response. You may have observed some of this yourself.
In contrast, taking deep breaths and focusing on your breathing induces a relaxation response. This response engages the parasympathetic nervous system, which lowers the heart and respiratory rate, blood pressure, and muscle tension.
you did the activity, you experienced stress . Stress is a physical
reaction. Confusingly, people also sometimes use the term stress when
they are talking about events that cause the physical reaction. We call
these stressors instead. (Stressors can be negative events, such as
divorce or death, but they can also be joyful changes, like getting a
new job or going on vacation!)
Stress is what happens in your body when you respond to a stressor (an event). It is your response to the stressor that determines how your body reacts. You can deliberately change your response, as you saw when you breathed deeply and slowed your heart rate.
you are not able to change your response to the stressors that are so
much a part of modern American life, you may find yourself in a
continual fight or flight reaction, which over time, can lead to
serious health consequences, such as high blood pressure, digestive
disorders , or diabetes .
Mind-body therapies and practices can help prevent this. But note that the relationship between stress and illness is not a simple one. There is no simple, direct connection between the number and kind of stressors, how you react and how your physical health is impacted.
Some people misinterpret the mind-body connection and end up blaming themselves for being stressed and sick. This assumes a level of control over their health that isn't realistic. Instead of worrying or blaming, do what you can to take care of yourself, including stress management, but recognize that you don't have complete control.
Mind-body therapies help you change your response to stressors. Some of the ways they can do this include:
Stress not only impacts bodily functions, it can impact performance at work in a number of ways:
interesting study known as the Yerkes/Dodson law shows that when
workers are under stress, their performance initially rises. After a
relatively short period however, their performance declines steeply,
but the workers are unaware that they are failing. So while a certain
amount of stress on the job might spark some initial success, ongoing
stress hinders (rather than helps) your efforts.
Unfortunately, if the job itself is the stressor, we end up taking a lot of these reactions home with us. And we all know that common reactions to stress, such as irritability, impatience, indecision, or physical symptoms, make for difficult relationships and family life.
The principles that make mind-body therapies and practices effective in improving physical health, also apply to other aspects of our daily life. These therapies can improve your health and your overall wellbeing.
According to a recent study published in the Proceedings of the National Academy of Sciences, there is a direct link between stress and aging.
The study compared the chromosomes of 39 women, ages 20-50, who had been caring for children with serious chronic illnesses (and thus had high levels of stress) with woman caring for healthy children (lower stress).
Women with the highest levels of stress had changes in
their chromosomes (specifically, the telomeres) equivalent to at least
one decade of additional aging compared with women with lower stress.
But it wasn't only the years of caregiving that related to the change, it was the perception of high stress. Women who had the perception of high stress levels fared the worst. Given this, could mind-body practices that reduce stress also reduce aging?
American Psychological Association topic: Article on Stress 
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Kobasa, S., Maddi, S. and Puccetti, M. (1982). Personality and Exercise as Buffers in the Stress-Illness Relationship. Journal of Behavioral Medicine, 5, 391-404.
Lawson, K. (2002). Psychoneuroimmunology in Blackwell Complementary and Alternative Medicine: Fast Facts for Medical Practice. Herring, M. & Roberts, M. (ed). Malden, Mass.: Blackwell Publishing.
Pelletier, K. (1993). Between Mind and Body: Stress, Emotions, and Health in Mind Body Medicine. Goleman, D., Gurin, J. (ed). New York:Consumer Reports Books.