The Science of Yoga: The Risks and the Rewards by William J Broad

control of the life force. Prana means “vital force” and yama means “to restrain or control.”
    Paul gave many examples of how yogis manipulate their breathing to discharge less. For instance, he described a common practice in which yogis take fewer breaths. Such retention, he wrote, “has a remarkable effect” on reducing how much carbon dioxide a person exhales.
    His figures showed big drops over an extraordinary range of breathing rates, starting with the fastest. A relaxed person takes roughly fifteen breaths per minute. Paul’s findings showed that a yogi who took ninety-six breaths a minute would expel seventy-nine cubic inches of carbon dioxide. At twenty-four breaths, the level fell by more than half to twenty-four cubic inches. At six breaths—far below the pace of normal breathing—the flow dropped again by more than half to ten cubic inches. Three breaths per minute cut the level still further to five cubic inches. And one breath per minute took the measurement down to two cubic inches. In other words, slow breathing produced huge drops in the exhalation of carbon dioxide.
    Over a dozen pages, Paul told how many other yogic practices worked similarly to lower the outflow. For instance, he said the repetition of “om,” the holy syllable of yoga, “materially diminished” the carbon loss. Another tactic was to simply rebreathe the same air—a move Paul called “one of the easiest methods” for entering a euphoric trance.
    The culmination of Paul’s analysis centered on a common feature of yogic life that worked inconspicuously to concentrate stale air for rebreathing. It was to live in a gupha, or cave, a kind of subterranean retreat with little light or ventilation, made for long periods of contemplative bliss. Paul noted that a yogi’s assistant would block the small doorway of the gupha with clay—not unlike the mud that was used to seal the Punjab yogi’s crypt. That produced “a confined atmosphere,” Paul noted, where the yogi expired less carbon dioxide than would be the case “in the free ventilated air.”
    In a bold displayof naturalism, Paul laid out a metaphor that avoided any hint of the miraculous and replaced it with a common feature of the natural world. The life suspensions of the yogis, he proposed, were simply cases of human hibernation, and the gupha of the saints were analogous to the burrows of hibernating animals.
    Paul drove his point home by observing that hibernation was a common strategy that many different animals used to conserve energy. So the holy men resembled bats, hedgehogs, marmots, hamsters, and dormice, he argued. So too, like hibernating animals, yogis lined their caves with such insulating materials as grass, cotton, and sheepswool. “The guphá is as indispensably necessary to the Yogi,” Paul wrote, as a burrow is to “the hibernating animals.”
    It was human hibernation, he concluded, that let the Punjab yogi survive his long interment. “If we compare the habits of the hybernating [ sic ] animals with those of the Yogis,” Paul wrote, “we find that they are identically the same.”
    Today we know the Bengali surgeon was exactly right in terms of respiratory physiology, even if the slumber of hibernating creatures bears little resemblance to the ecstasy of accomplished yogis. Scientists have found that mammals preparing to hibernate do in fact seal off their burrows and experience the kinds of shifts in respiratory gases that Paul ascribed to yogis in their gupha. The closure of a den, notes David A. Wharton, a zoologist and author of Life at the Limits , “promotes a build-up of carbon dioxide which helps depress the metabolism of the animal,” slowing its biorhythms in preparation for deep sleep.
    Paul’s analysis was brilliant in originality and import, his work all the more impressive given his humble status and the limited tools available to scientists of his day. He began uncovering what turned out to be one of yoga’s main