The Scientific Basis of Yoga Therapy

essential oils



Stress and the Autonomic Nervous System

Although yoga is much more than a stress-reduction method, stress adversely affects a wide range of health conditions, and yoga is arguably the most comprehensive approach to fighting stress ever invented. Stress isn't just a factor in conditions commonly labeled "stress-related," such as migraines, ulcers, and irritable bowel syndrome, but it appears to contribute to such major killers as heart attacks, diabetes, and osteoporosis.

Even diseases such as cancer—for which there is surprisingly little evidence that stress is a causative factor—are extremely stressful once a person has been diagnosed and begins treatment. Yoga can improve not only the quality of life after diagnosis, but it appears to diminish the side effects of surgery, radiation, chemotherapy, and other treatments, and may increase the odds of survival.

To appreciate the role of stress in disease and of relaxation in prevention and recovery, it's important to understand the function of the autonomic nervous system (ANS), which controls the function of the heart, liver, intestines, and other internal organs. The ANS has two branches that work in conjunction: the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). In general, when activity is high in the SNS, it is lower in the PNS, and vice versa.

The SNS, in conjunction with such stress hormones as adrenaline and cortisol, initiate a series of changes in the body, including raising blood pressure, heart rate, and blood sugar levels. These changes help a person deal with a crisis situation. They mean more energy and more blood and oxygen flowing to the large muscles of the trunk, arms, and legs, allowing the person to run from danger or do battle (the so-called "fight-or-flight" response).

The PNS, in contrast, tends to slow the heart and lower the blood pressure, allowing recovery after a stressful event. Blood flow that was diverted away from the intestines and reproductive organs, whose function isn't essential in an emergency, returns. In contrast to fight or flight, these more restorative functions can be thought of as "rest and digest." They are also sometimes dubbed the relaxation response.

Many yoga practices, including quiet asana, slow breathing, meditation, and guided imagery, increase activation of the PNS and lead to mental relaxation. Yoga techniques are more than just relaxation, however. Practices like vigorous sun salutations, kaphalabhati breathing, and breath retentions actually activate the SNS. One of yoga's secrets, documented in research from the Swami Vivekananda Yoga Research Foundation near Bangalore, is that more active practices followed by relaxing ones lead to deeper relaxation than relaxing practices alone.


I believe some of yoga's most profound effects on health have to do with its ability to alter long-standing dysfunctional behavior. People often have unhealthy habits of thought and deed that undermine their health—habits they may recognize but haven't been able to change. In addition to the direct health benefits of asana, Pranayama, meditation, and other yoga practices, it's not uncommon for regular practitioners to start eating better, to cut back on caffeine or alcohol, to quit jobs with unreasonable demands, or to spend more time in nature. Once people become more sensitive to the effects of different actions on their bodies and minds (whether it is practicing alternate nostril breathing or eating huge, fatty meals), they increasingly want to do what makes them feel better.


Dr. Timothy McCall is a board-certified internist, Yoga Journal's medical editor, and the author of the forthcoming book Yoga as Medicine (Bantam Dell, summer 2007). He can be found on the Web at