The relaxation response triggered by yoga and meditation counters the stress response. When the mind-body has repeated experiences of stress, the stress response triggers faster as a survival mechanism and the stress hormones, over time, cause health problems. Stress may contribute to, or exacerbate, some of these familiar health problems:
anxiety, arthritis, constipation, depression, diabetes, headaches, heart problems, heartburn, infectious diseases such as colds and herpes, insomnia, irritable bowel syndrome, backaches, joint aches, abdominal pain, PMS, and ulcers.
This is now widely accepted knowledge as I am learning in the Mind-Body Medicine: The New Science of Optimal Health by Professor Jason M. Satterfield Ph.D (Great Courses from The Teaching Company). The stress response is well-explained in the following excerpt from:
Collectively, the hypothalamus, pituitary gland, and adrenal glands make up the HPA axis, which plays a pivotal role in triggering the stress response. The hypothalamus sends a chemical messenger (corticotropinreleasing factor, or CRF) to the nearby pituitary gland, which then releases its own chemical messenger (adrenocorticotropic hormone, or ACTH) into the bloodstream. ACTH travels to the adrenal glands, which respond by releasing a number of stress hormones into the bloodstream. At the same time, the sympathetic nervous system releases stress hormones, too. The combined effects of these hormones are widespread…Senses become sharper, muscles tighten, the heart beats faster, blood pressure rises, and breathing quickens. All of this prepares you to fight or flee in the face of danger. Simultaneously, the hypothalamus fires up the autonomic nervous system.
Yoga and meditation activate the relaxation response through the parasympathetic nervous system which counteracts the overactive sympathetic nervous system. (This has finally become mainstream and is no longer “fringe medicine”!) Stress hormones such as cortisol are reduced. Blood pressure may drop, heart beat slows down, breathing is slower, the muscles relax.
Along with life style changes, cultivating positive behaviors, and improved diets, yoga and meditation offer very cost-effective ways to significantly improve health outcomes. Cognitive Behavior Therapy is also found to be effective, but it can be very expensive.
For a study at the Benson-Henry Institute for Mind-Body Medicine at Massachusetts General Hospital (see the link above), two groups were studied: long-term practitioners of yoga, meditation, and repetitive prayers and a group with no prior experience of these techniques. The novice group was taught a 20-minutes sequence with diaphragm breathing or belly breath, body scan, mantra repetition, and mindfulness. Blood samples were taken from both groups to examine gene activity–specifically on how the body deals with free radicals.
The long-term practitioners had the most significant positive change and the novice group saw some positive change after eight weeks of practice. However, this effect is not long-term, suggesting that like physical exercise, the relaxation response needs to be triggered regularly. Yoga, meditation, prayers, need to be part of a regular routine.
Mahasri Yoga 
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