Breathe in through the nose and out through the nose. Repeat.

About 20 million people follow this routine in yoga classes throughout the nation. Disciplines of yoga differ significantly, but all styles are built on the basic concept of the breath. Yogis generally agree that the meditative breath calms the body and focuses it on the inner, more spiritual self. But what actually occurs physiologically to encourage this spiritual feeling? What science backs up yoga’s claims to bring those who practice it closer to their inner selves?

Theoretically, yoga stimulates the connection between mind and body, and dealing with the concerns of the soul through bodily movements. Wading through some of the false claims that the commercial yoga complex has propagated (including the myths that yoga quickens your metabolism or that yoga breathing injects more oxygen into the blood) can be touch.

But there are definite scientific truths behind the healthy feelings that yogis seem to exude. Here are some physiological facts that help to explain why yoga feels the way it does:

It slows down your metabolism

Slow breathing relaxes your body and encourages digestion and healing, according to William Broad, the author of “The Science of Yoga: The Risks and Rewards.” He cites a 2008 Indian study which found that women’s metabolisms lowered 16 percent when practicing yoga and men’s lowered percent.

It produces happy neurotransmitters

Practicing yoga can produce more GABA, serotonin, and dopamine neurotransmitters, which work to control and regulate your mood. These particular neurotransmitters serve as both anti-depressant and anti-anxiety hormones and therefore reduce negativity.

It affects your autonomic nervous system

The autonomic nervous system controls vital functions such as respiration, pulse, digestion, and brain activity. It’s made up of two parts: the sympathetic nervous system (SNS) and the parasympathetic nervous system (PSNS). Your sympathetic nervous system kicks in during times of high stress as your “fight-or-flight” reaction. Yoga, however, helps reduce the amount of cortisol that your body releases during that reaction and therefore helps your body recover from stress faster.

In addition, yoga stimulates the PSNS, which works against the SNS to calm the body after stress. When the PSNS is triggered, blood returns to the intestines, reproductive organs, and endocrine system. You therefore have a lower blood pressure as you “rest and digest.” The slow breathing, meditation, and guided imagery involved in yoga help with total relaxation.

It can change your brainwaves

When practicing yoga, your brainwaves can switch from the beta brainwave state to the alpha brainwave state. The beta brainwave state is associated with active or anxious thinking and concentration, whereas the alpha brainwave state is one of wakeful relaxation. Instead of focusing on physical actions, the brain of a yoga practitioner concentrates on relaxation and rejuvenation.

These physiological changes induced by yoga can affect health in multiple ways. Stress reduction can help prevent migraines, ulcers, irritable bowel syndrome, heart attacks, diabetes, and osteoporosis. Yoga’s practiced repetition also can help develop discipline of the mind; therapists frequently recommend this type of exercise of children with ADHD.

Most importantly, yoga can make you forget the immediate needs of your body and find tranquility in the self.

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Assistant Ed: Karissa Ostheimer/Ed: Sara Crolick