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A long exhale signals safety
Why a long exhale calms
About 80 percent of the vagus nerve fibers take in information from the body and report back to the brain. Some of those fibers sit in the lungs and can sense a big slow exhale. The brain gets the news that things are fine, then uses the vagus nerves to relax the body further by slowing the heart. People who try it can feel the heart rate slowing during long, slow exhales.
Theresa Larkin, an anatomist at the University of Wollongong, suggests keeping the breath out a little longer than the breath in.
What the research found
A large meta-analysis compared dozens of studies on ways to lower cortisol and found relaxation techniques, including breathing exercises, among the ones that worked best. Mindfulness and meditation did about as well. Small stresses add up across a day, and small calming moments add up too: a few slow breaths while the kettle boils or before a meeting starts lower the baseline stress level.
How do lungs tell the brain about breathing?
Stretch receptors in the lungs and airways sense how far the lungs have inflated, and they send that signal to the brainstem along vagus nerve fibers. The reflex built on this signal is called the Hering-Breuer reflex, named for two Austrian physiologists who described it in 1868. It helps regulate the depth and timing of each breath. In adults its role at rest is modest, but the same fibers continue to report on every breath.
Why does the heart slow during an exhale?
Heart rate rises slightly on each inhale and falls on each exhale, a rhythm known as respiratory sinus arrhythmia. During the exhale, the vagus nerve releases the chemical acetylcholine onto the heart's natural pacemaker, which slows its firing. German physiologist Carl Ludwig first described the pattern in dogs in 1847. Healthy hearts show it at every age, though it is largest in young people.
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