Photo: Wellcome Library

Have you ever read something a million times only to one day, for no apparent reason, think “Wait, what is that?” This happened to me the other day for “the vagus nerve.”

I kept coming across it in relation to deep breathing and mental calmness: “Breathing deeply,” Katie Brindle writes in her new book Yang Sheng: The Art of Chinese Self-Healing, “immediately relaxes the body because it stimulates the vagus nerve, which runs from the neck to the abdomen and is in charge of turning off the ‘fight or flight’ reflex.” Also: “Stimulating the vagus nerve,” per a recent Harvard Health blog post, “activates your relaxation response, reducing your heart rate and blood pressure.” And: Deep breathing “turns on the vagus nerve enough that it acts as a brake on the stress response,” as an integrative medicine researcher told the Cut last year.

I liked this idea that we have something like a secret piano key, under our skin, to press internally to calm us down. Or like a musical string to pluck. At this point I was envisioning the vagus nerve as a single inner cord, stretching from the head to the stomach. In reality, the vagus nerve 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. Also: “When people say ‘trust your gut,’” as one Psychology Today writer put it several years ago, “they really mean ‘trust your vagus nerve.’”

I became increasingly enchanted with this nerve, even as it felt like I understood it less and less. How does this all work? How does activating a nerve calm us down? Is this why I get so needlessly upset about things?

“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 (the myocardium), and our heart rate drops.

The vagus nerve is essentially the queen of the parasympathetic nervous system — a.k.a. the “rest and digest,” or the “chill out” one — so the more we do things that “stimulate” or activate it, like deep breathing, the more we banish the effects of the sympathetic nervous system — a.k.a. the “fight or flight,” or the “do something!” stress-releasing adrenaline/cortisol one.

Put another way, “Your body senses your breathing and adapts its heart rate in response,” Norcliffe-Kaufmann told me. When we breathe in, she explained, the sensory nodes on our lungs (“lung stretch receptors”) send information up through the vagus nerve and into the brain, and when we breathe out, the brain sends information back down through the vagus nerve to slow down or speed up the heart. So when we breathe slowly, the heart slows, and we relax. Conversely, when we breathe quickly, our heart speeds up, and we feel amped, or anxious.

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.” (“That’s fascinating,” I said. “It is!” she said.)

It made me wonder if there are ways of measuring the quality of the vagus nerve, or “vagal tone,” as Norcliffe-Kaufmann described it. This is basically how healthy, strong, and functional the nerve is. One way, she said, is to measure heart rate variability (HRV) — it’s a sort of “surrogate” for measuring actual vagal tone (barring open chest surgery). Heart rate variability is the amount that the heart rate fluctuates between a breath in (when it naturally speeds up) and a breath out (when it naturally slows down). That is, heart rate rises on the inhale and falls on the exhale, and the difference between those two rates essentially measures vagal tone. Athletes are known to have higher vagal tone, for example, whereas people who experience extended periods of bed rest — and astronauts in no-gravity situations — are known to have lower vagal tone. (How quickly your heart rate slows after exercising is also a good marker of vagal tone.) Vagus nerve stimulation has also been proposed as a way to treat addiction (some heavy drinkers, for instance, have low vagal tone).

Certain devices measure HRV — and I’ve personally tried a chest strap and a wristband, but I got stumped on what to do with the data — although Norcliffe-Kaufmann is skeptical about their reliability. “Those technologies are coming,” she said, “but it’s more important to focus on breathing and feeling calm and balanced, rather than on a number.” Some other practices believed to improve vagal tone (beyond deep, slow breathing) include laughing, singing, humming, yoga, acupuncture, and splashing the face with cold water — or having a full-body cold rinse. (Stimulation of the vagus nerve, both manually and with electricity, has also been used to control seizures in epilepsy patients, reduce inflammation, and treat clinical depression.)

Writing this story, and after talking with Norcliffe-Kaufmann, I found myself breathing more slowly and feeling calmer. Not necessarily happy, but steady. Slow breathing is boring, but it’s almost sad how effective it is. I’d usually rather spend hundreds of dollars to get a gadget to track myself than do this free and more-effective thing.

“If you’re in a stressful situation,” Norcliffe-Kaufmann said, “and you’re like, How do I respond, how do I respond? — if you consciously slow down your breathing just for one minute, or even a few seconds, you can put yourself in a calmer state, to be able to better communicate.”