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The four-way study
The volunteers wore trackers for heart rate, heart rate variability, and sleep. One group meditated for five minutes a day, one did box breathing, one did big inhales with passive exhales, and one did the sigh. Every group benefited, and the sigh group benefited the most. A single sigh turned out to slow heart rate right away.
Why the sigh works
A brain area called the parafacial nucleus exists to generate this pattern. Stress, claustrophobia, or low oxygen raises carbon dioxide, and the brain triggers a gasp reflex. The second short inhale reopens the lungs' tiny air sacs, which are fluid-lined and stick together like empty balloons, and the exhale then offloads the most carbon dioxide.
The vagus nerve
The long exhale does the calming through the vagus nerve. Eighty-five percent of its traffic runs up from the body, but a path runs down to the heart and slows it with every exhale. That slowing is heart rate variability, so deliberate long exhales through the day raise it.
How do scientists know sighs are built in?
In 2016, labs led by Jack Feldman at UCLA and Mark Krasnow at Stanford published a paper in Science identifying two small groups of neurons in the breathing center of mice that drive sighing. Cutting off those cells left the animals breathing normally but sighing far less. Sighs occur spontaneously every few minutes in healthy mammals. They happen even during sleep.
Why does the heart slow on every exhale?
Heart rate rises slightly during inhalation and falls during exhalation, a pattern called respiratory sinus arrhythmia. Inhaling briefly reduces vagal output to the heart's pacemaker cells, and exhaling restores it. The vagus nerve slows those cells by releasing acetylcholine onto them. The swing in heart rate is larger in young, healthy people and shrinks with age.
Why do air sacs in the lungs collapse?
Each air sac is lined with a thin film of fluid, and the surface tension of that film pulls the walls together. Cells in the sac walls release surfactant, a soapy mixture of fats and proteins that cuts the tension. Premature babies often lack enough surfactant, which causes respiratory distress syndrome. Artificial surfactant therapy became standard for premature infants in the early 1990s.
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