The Science of Yoga

renews, energizes and strengthens. It somehow makes us feel better.
    But beyond such basicslies a frothy hodgepodge of public claims and assurances, sales pitches and New Age promises. The topics include some of life’s most central aspirations—health, attractiveness, fitness, healing, sleep, safety, longevity, peace, willpower, control of body weight, happiness, love, knowledge, sexual satisfaction, personal growth, fulfillment, and the far boundaries of what it means to be human, not to mention enlightenment.
    This book cuts through the confusion that surrounds modern yoga and describes what science tells us. It unravels more than a century’s worth of research to discern what’s real and what’s not, what helps and what hurts—and nearly as important, why. It casts light on yoga’s hidden workings as well as the disconcerting reality of false claims and dangerous omissions. At heart, it illuminates the risks and the rewards.
    Many, it turns out, are unfamiliar.
    I came to this book as a knowledgeable amateur. During my freshman year of college, in 1970, I got hooked on yoga because it felt good and seemed to make me healthier in body and mind. My first teacher said it was important to do some—even a little—every day. That’s always been my goal, despite the usual struggle with good intentions. Yoga has become a good friend to whom I turn no matter how crazy my life gets.
    I began my research in 2006. My plan was simple. I’d track down the best science I could find and answer a lot of questions that I had accumulated over the decades, things I had wondered about but never had a chance to explore.
    My first surprise was how yoga had morphed into a confusing array of styles and brands. I knew enough to understand that the origin of it all was Hatha yoga—the variety that centers on postures, breathing, and drills meant to strengthen the body and the mind (as opposed to the yogas of ethics and religious philosophy). Today, Hatha and its offspring are the most widely practiced forms of yoga on the planet, having produced scores of variations that range from local styles in most every country to such ubiquitous global brands as Iyengar and Ashtanga.
    My enthusiasm for gyms and swimming also gave me a reasonable perspective on how yoga differs from regular exercise. In general (with exceptions we’ll study closely), it goes slow rather than fast, emphasizing static postures and fluidmotions rather than the rapid, forceful repetitions of, say, spinning or running. Its low-impact nature puts less strain on the body than traditional sports, increasing its appeal for young people as well as aging boomers. In terms of physiology, it takes a minimalist approach to burning calories, contracting muscles, and stressing the body’s cardiovascular system. Perhaps most distinctively, it places great emphasis on controlling the breath and fostering an inner awareness of body position. Advanced yoga, in turn, goes further to encourage concentration on subtle energy flows. Overall, compared to sports and other forms of Western exercise, yoga draws the attention inward.
    I began examining the yogic literature with a sense of wariness. Long ago, while working at the University of Wisconsin on a study of respiratory physiology, I came across a flat contradiction to one of modern yoga’s central tenets—that fast breathing floods the body and brain with revitalizing oxygen. In contrast, a textbook I was reading at the time said the pace of human respiration “can drop to one-half or rise to over one hundred times normal without appreciably influencing the amount of blood oxygenated.” I see now that, in 1975, I underlined that passage quite heavily.
    Unfortunately, my survey lived up to my low expectations. Some books and authors shone brightly. (See Further Reading for a list.) But on the whole, I found the literature dull with dreaminess, assertions with no references, and a surprising number of obvious untruths. I wanted tips for tracking down good science but instead got a muddle. The writing, old and new, turned out to run toward the curiously dogmatic and, at best, to contain only a smattering of science. Much of it was similar to what Richard Feynman, a founder of modern physics, disparaged as cargo-cult science—that is, material that appears scientific but lacks factual integrity.
    By contrast, my plunge into the scientific literature left me heartened. Federal officials at the National Institutes of Health