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In the spring of 1970, the Indian yogi Swami Rama wanted to convince Western scientists of the power of yoga, so he submitted himself for study. He arrived at the lab of the Menninger Clinic in Topeka, Kansas, to meet with Dr. Elmer Green, who pioneered research around biofeedback, or the process of becoming increasingly aware of bodily functions that are usually thought of as involuntary. Rigged up with sensors, on his first day at the clinic, he reportedly changed the temperature of his hand by widening and contracting the arteries in his wrist, leading to a difference of ten degrees Fahrenheit between the left side of his palm — which looked rosy red — and the right, which looked ashen gray, as Dr. Timothy McCall notes in his Yoga As Medicine.

Since Rama visited Topeka, yoga has become mainstream (despite his and other gurus’ penchant for sex scandals). As you can see from the number of yoga mats slung over people’s shoulders as they make their way across American cities, yoga is huge right now. According to Yoga Journal’s national survey, as of 2016, there are more than 36 million yoga practitioners nationwide, up from 20 million in 2012, and 15 million in 2003. They spent an estimated $16 billion on yoga classes and gear in the last year. Yoga has gone from New Age weirdness to thoroughly mainstream — a 2015 marketing paper found that from 1980 to 2009, the way newspapers and nonfiction books cover yoga shifted from spirituality to fitness. “Today, the term ‘power’ refers more to a physical workout than to spiritual empowerment,” the authors write.

With that, yoga has gotten more empirical scrutiny. While there’s a need for larger scale, more thoroughly designed research, studies indicate that a yoga practice helps in treating depression and anxiety (in multiple meta-analyses), managing stress, and improving the well-being of cancer survivors. Research indicates that the practice helps young musicians find flow states, women over 55 experience transcendence of the ordinary, and ashram residents reach “a radical shift in consciousness of the type most people experience only when they are using psychoactive drugs.” Therein lies the exquisite difficulty of studying it: Asana, or the series of poses that you probably think of when you hear “yoga,” is a physical exercise, yes, but also a psychological and perceptual one, like its sibling, meditation. Almost all the researchers I talked to warned me that no one completely knows “how yoga works”: The expansive fruits of long-term practice are intensely subjective, and it’s rather difficult to design a study that gains access to another person’s phenomenology.

Because of its long and intercontinental history, even finding a definition for “yoga” is difficult. Amy Matthews, educational director of the Breathing Project and co-author of Yoga Anatomy, says that yoga is any “movement practice” where your body, breath, and attention are all focused in the same place. A runner who enters into a meditative state, with breaths synced to footfalls, is doing yoga. In a standard asana practice, there’s a quality of having the body, the breath, and the attention all in conversation with each other, rather than your attention completely running the show. It’s not mind over matter; it’s mind talking to matter. Take, for example, the classic sequence of poses called a sun salutation, where the practitioner goes from standing to toe touch to lunge to plank to backbend and up again, as demonstrated by Kermit below.

The standard advice is to pair one movement with one breath, and the art is in applying that. “How do you lengthen or shorten your breath so it matches your movement? The idea is that by paying attention, noticing what my body is capable of doing, adjusting my breath to suit the movement, and changing the movement to suit the body, and also listening to what my body is capable of or feeling like doing,” Matthews tells Science of Us, whether it’s getting deep into chaturanga’s push-up, or interpreting the pose in a gentler form. “I can listen to what my body that day needs, and I can shape the way I do the sun salutation to that need,” she says. “If my breath can stay steady and smooth, that’s a clue that I’m not asking more from my body than what it can give.”

There’s anecdotal evidence that yoga practices — breathing techniques, visualizations, and the like — allow people to do near-superhuman feats. Recall how Swami Rama reportedly controlled the flow of blood into his hand. To McCall, Rama was able to do this by affecting the smooth muscles lining his arteries. This usually lies outside of conscious control, but the yogi had developed the ability to better access his autonomic nervous system, the domain of the heartbeat, digestion, and breathing. Similarly, Tibetan monks have been found by Harvard researchers in 1982 and 2013 to be able to raise their body temperatures to as high as 100 degrees Fahrenheit. “This is not something most of us can do,” McCall tells Science of Us. “But we can do the same thing, in a lesser way.” What those outlandish results show, McCall says, is that yoga, unlike conventional Western exercises, focuses on the breath, and “that turns out to be the doorway to the autonomic nervous system.” You can’t access your digestion or your heartbeat, but you can interact with your breath, and this is key.

Consider the side plank, one of my most and least favorite poses. It’s simple enough: With your feet stacked on either their left or right edge, hold your body up with a single hand to the ground. But after one, five, or ten seconds, it starts getting hard: Your hips dip with the weight, your heart thuds in your chest, you might start involuntarily shaking — how fun. But if you’re breathing slowly and deeply while all of this is going on, McCall says, you’re teaching yourself to not overreact to stress. It’s a way of “maintaining modulation of freakout mode,” he says, a training in emotional self-regulation.

If you’re doing the same poses over and over again, day after day, year after year, you’re going to get pretty intimate with how your body expresses itself in those forms, and along the way cultivate what researchers call proprioception, or the awareness of where your body is in space, and interoception, or the sensations not just of the air on your skin, but your bones, tendons, and body tissues as you mindfully contort your body, as well as your emotional state. As your yoga teachers have exhorted you to do, you’re gaining a finer-grained sense of where your skeleton is within your body, and how all the flesh layers on top of that. As Harvard Medical School assistant professor Sat Bir Singh Khalsa told me, these increases in internal awareness can change entire lifestyles. “Somebody’s who’s practiced yoga for eight weeks and then smokes a cigarette, will say, ‘My god, I never noticed how bad these things were, I can’t stand this, this feels awful,’” he says. With training, the body’s sensations become more perceptible to you, so you feel the toxicity of things at a higher intensity. For this reason, he says, yoga can be super powerful in controlling lifestyle diseases. “People change their diets, change their behaviors to ones that make them feel better, because now, for the first time in their lives, they’re actually feeling more.”

Perhaps the most mysterious piece, and the one that empiricism has the least access to, is what Khalsa calls a “unitive” experience, the changes in consciousness that may come over months and years of practices. These may very well fall under what Abraham Maslow, one of the founders of positive psychology, called “peak experiences,” moments of great joy and satisfaction where the world feels more unified and honest, a tilt toward self-actualization. The ashram residents who had such experiences with their practices reported that it was a lot like being on hallucinogens — as in LSD or psilocybin mushrooms — but without the feeling of being out of control. (Relatedly, psilocybin looks like a promising way to get people to quit smoking.)

“You have experiences that you wouldn’t believe if someone would have told you,” McCall says of an ongoing practice, be it a deep, harmonious practice session, or a flood of tears emerging in shavasana, the final resting pose that concludes practice. “The floodgates open and the tears flow, and often there can be an epiphany that comes with that. Beyond letting go, there’s some sort of realization,” he says. “I don’t know for sure what the mechanism is, but as speculation, we have this notion that memory is only housed in the brain, but that may not be true — it may be that there are bodily memories.” From the perspective of yoga, McCall says, you can’t cut the brain away from the body, and talk about physiology and psychology as if they never meet.

Mind and body are two features of the same united thing, he says. “Yes, it can be useful, thinking of the brain as being distinct — that’s a legitimate way of thinking about things, but it’s a metaphor, not the truth,” he says. “They’re two parts of the same thing,” better known as a person. And with yoga, quite uniquely, you’re engaging all of it.