I kept coming across "the vagus nerve" in relation to deep breathing and mental calmness: in the new book Yang Sheng: The Art of Chinese Self-Healing, a Harvard Health blog post, and many other recent publications. It sounded as if we had something like a secret piano key, under our skin, to press internally to calm us down. In reality, the vagus nerve is not a single inner cord, stretching from the head to the stomach; it is a squiggly, shaggy, branching nerve connecting most of the major organs between the brain and colon, like a system of roots or cables. It is the longest nerve in the body, and technically it comes as a pair of two vagus nerves, one for the right side of the body and one for the left. It's called "vagus" because it wanders, like a vagrant, among the organs. The vagus nerve has been described as "largely responsible for the mind-body connection," for its role as a mediator between thinking and feeling, and I'm tempted to think of it as something like a physical manifestation of the soul. "Stimulating the vagus nerve to the heart has a really powerful effect on slowing the heart rate," said Lucy Norcliffe-Kaufmann, associate professor of neurology at NYU-Langone. And this, specifically, is what relaxes us. The vagus nerve is basically listening to the way we breathe, and it sends the brain and the heart whatever message our breath indicates. Breathing slowly, for instance, reduces the oxygen demands of the heart muscle, and our heart rate drops. I was surprised by the idea that it's specifically the exhale that triggers the relaxation response, but Norcliffe-Kaufmann confirmed: "Vagal activity is highest, and heart rate lowest, when you're exhaling." She mentioned that the ideal, most calming way to breathe is six times a minute: five seconds in, five seconds out. She also noted that in the study that determined this rate, researchers found that this style of slow breathing is also what practitioners naturally lapse into during meditation with mantras, and during the Ave Maria prayer with rosaries. "Each time you do either the rosary prayer or a meditation mantra," Norcliffe-Kaufmann said, "it naturally synchronizes your breathing at six times per minute." Some other practices believed to improve vagal tone include laughing, singing, humming, yoga, acupuncture, and splashing the face with cold water — or having a full-body cold rinse. "If you're in a stressful situation," Norcliffe-Kaufmann said, "and when you're like, How do I respond, how do I respond? — Just consciously slow down your breathing for one minute, or even a few seconds, you can put yourself in a calmer state, and be able to better communicate."