Nadi shodhana, “channel purification,” is the signature breath of Hatha yoga: air drawn in one nostril and released through the other, the hand alternating the gates. The Haṭha Yoga Pradīpikā, the fifteenth-century manual that anchors the tradition, places it at the very beginning of pranayama training, prescribed to clear the subtle channels (nāḍīs) so that breath, and with it attention, can move freely.
The practice
Sit comfortably upright. With the right hand, fold the index and middle fingers toward the palm (or rest them at the brow). The thumb will close the right nostril, the ring finger the left.
Close the right nostril and inhale slowly through the left. Close the left and exhale through the right. Inhale through the right. Close it and exhale through the left. That is one round: in-left, out-right, in-right, out-left.
Begin with five to ten rounds, unhurried, the breath silent and smooth. A gentle ratio to grow toward: exhale slightly longer than inhale. Traditional practice adds retentions between the turns, but the unheld form is the right first year.
The two frames
The traditional frame. The left and right channels (iḍā and piṅgalā, lunar and solar, cooling and heating) run beside the spine. The alternating breath balances them, preparing the central channel. Whatever one’s metaphysics, the phenomenology is easy to verify: the practice produces a distinct, symmetrical settledness that plain slow breathing approaches differently.
The laboratory frame. The evidence base is a collection of small controlled trials, several from Shirley Telles’s group, suggesting modest reductions in blood pressure and improvements in attention after regular practice, consistent with the general slow-breathing findings reviewed in The Physiology of a Slow Breath. The honest label: promising, under-studied, low-risk.
Cautions
Keep the breath easeful. No forcing air through a congested nostril (skip the practice during heavy congestion). Omit retentions in pregnancy and with cardiovascular conditions. Lightheadedness is the signal to stop and breathe normally.