The vagus nerve and breathing: what is known

What the vagus nerve is

There are two of them, one on each side. Cleveland Clinic describes them as the main nerves of the parasympathetic system, the part of your nervous system that runs things you don't decide, like digestion and heart rate. They leave the base of the brain, travel down through the neck and chest, and reach the large intestine. It's the longest of the twelve pairs of cranial nerves, and its name comes from the Latin word for wandering.

One fact changes how you picture it. According to a 2018 review by Breit and colleagues, 80 to 90 percent of its fibers carry information from the body up to the brain. Only 10 to 20 percent carry commands from the brain down to the body. It is, above all, a nerve that reports.

Established: it's the heart's brake

The same review sums up its role in the chest: it's the main parasympathetic supply to the heart, and it slows your heart rate. In the gut, it regulates muscle contraction and glandular secretion.

There's also a medical use. Vagus nerve stimulation is done with an implanted device that sends electrical pulses. According to Cleveland Clinic, it's FDA-approved for epilepsy and for depression that doesn't respond to standard treatment. That's a decision for a medical team, and it has nothing to do with breathing exercises. Even there the evidence has caveats: Breit and colleagues note that the depression studies were open-label and that no randomized, placebo-controlled trial reliably shows the effect.

Established: your pulse follows your breath

Your pulse rises a little when you breathe in and falls when you breathe out. It's called respiratory sinus arrhythmia and, despite the name, it's a normal pattern. A 2017 review of slow breathing describes it this way: the intervals between beats shorten on the inhale and lengthen on the exhale. The most common explanation is that autonomic signals to the heart are held back during the inhale and released during the exhale.

Its exact origin is still argued over. The review says the debate centers on whether it's generated mainly by the baroreflex or by a breathing center in the brain, and that the accepted view is a network of central, peripheral and mechanical elements.

What does hold up is this: when you breathe slowly, the swing grows. A 2022 meta-analysis of 223 studies found higher vagally mediated heart rate variability during slow breathing, right after a single session, and after multi-session programs.

Vagal tone, and why it's hard to measure

Nobody measures your vagus nerve while you breathe in a chair. What gets measured is your pulse, and from that, researchers estimate how much the vagus is acting on your heart. That estimate is called vagal tone.

In 2007, Grossman and Taylor laid out the reasons to read it carefully. The way you breathe can alter the measure all by itself. The relationship is strong within one person and modest when comparing people. Body movement and sympathetic activity change it too. And under some circumstances, the measure and the nerve go separate ways.

The practical upshot: when you breathe slowly and the number goes up, you can't tell from that number alone how much the nerve changed and how much the measurement did.

What's still a hypothesis

• That slow breathing stimulates the vagus, and that's why it calms. Gerritsen and Band proposed it in 2018 as a model: slow breaths with long exhales would stimulate the nerve. They write themselves that the mechanisms are proposed, not proven, and that the research is sparse.
• That signals traveling up to the brain explain the effect on mood. Lehrer and Gevirtz frame it as speculation and point to other possible explanations, such as distraction or the sense of being able to do something.
• That a long exhale activates the vagus more. In two small 2024 experiments, with 26 and 16 people, exhaling for twice as long as the inhale produced no more heart rate variability than equal phases.
• That more variability means more well-being. In a 2023 trial of 400 people, four weeks of breathing at about 5.5 a minute did no better than placebo breathing at 12 on stress or anxiety.
• Evolutionary stories about the vagus. Grossman and Taylor argue that polyvagal theory's evolutionary speculations don't accurately describe how vagal control of the heart evolved.

What to do with all this

You can practice without settling the debate. Breathing slowly widens the swing in your pulse, and many people use it to calm down. You don't have to believe you're training a nerve for a few quiet minutes to feel good.

To try it, Coherent breathing 5-5 and Resonant breathing 5.5-5.5 are the paces used in the studies cited here. 4-6 breathing and Long exhale 4-8 stretch out the breath on the way out. Choose by comfort, not by a promise.

If you get dizzy or tingly, stop and breathe normally. If you have fainting spells, palpitations or a very slow pulse, or if you live with epilepsy or depression, that's a conversation for your care provider. No breathing practice replaces treatment.