By reducing perceived stress and anxiety, yoga appears to modulate stress response systems. This, in turn, decreases physiological arousal — for example, reducing the heart rate, lowering blood pressure, and easing respiration. There is also evidence that yoga practices help increase heart rate variability, an indicator of the body's ability to respond to stress more flexibly.

A small but intriguing study further characterizes the effect of yoga on the stress response. In 2008, researchers at the University of Utah presented preliminary results from a study of varied participants' responses to pain. They note that people who have a poorly regulated response to stress are also more sensitive to pain. Their subjects were 12 experienced yoga practitioners, 14 people with fibromyalgia (a condition many researchers consider a stress-related illness that is characterized by hypersensitivity to pain), and 16 healthy volunteers.

When the three groups were subjected to more or less painful thumbnail pressure, the participants with fibromyalgia — as expected — perceived pain at lower pressure levels compared with the other subjects. Functional MRIs showed they also had the greatest activity in areas of the brain associated with the pain response. In contrast, the yoga practitioners had the highest pain tolerance and lowest pain-related brain activity during the MRI. The study underscores the value of techniques, such as yoga, that can help a person regulate their stress and, therefore, pain responses.

References:
1. Brown RP, et al. "Sudarshan Kriya Yogic Breathing in the Treatment of Stress, Anxiety, and Depression: Part I — Neurophysiologic Model," Journal of Alternative and Complementary Medicine (Feb. 2005): Vol. 11, No. 1, pp. 189–201.

2. Brown RP, et al. "Sudarshan Kriya Yogic Breathing in the Treatment of Stress, Anxiety, and Depression: Part II — Clinical Applications and Guidelines," Journal of Alternative and Complementary Medicine (Aug. 2005): Vol. 11, No. 4, pp. 711–17.

3. Janakiramaiah N, et al. "Antidepressant Efficacy of Sudarshan Kriya Yoga (SKY) in Melancholia: A Randomized Comparison with Electroconvulsive Therapy (ECT) and Imipramine," Journal of Affective Disorders (Jan.–March 2000): Vol. 57, No. 1–3, pp. 255–59.

4. Khalsa SB. "Yoga as a Therapeutic Intervention: A Bibliometric Analysis of Published Research Studies," Indian Journal of Physiology and Pharmacology (July 2004): Vol. 48, No. 3, pp. 269–85.

5. Kirkwood G, et al. "Yoga for Anxiety: A Systematic Review of the Research," British Journal of Sports Medicine (Dec. 2005): Vol. 39, No. 12, pp. 884–91.

6. Pilkington K, et al. "Yoga for Depression: The Research Evidence," Journal of Affective Disorders (Dec. 2005): Vol. 89, No. 1–3, pp. 13–24.