Why sighing could save your life: it is a vital defence against lung failure
A sigh is more than just a sigh - it is a lung-rescuing reflex that can be life-saving.
Now scientists have pinpointed two tiny nerve clusters in the brain stem that are responsible for turning normal breaths into sighs.
The discovery may one day help patients who cannot breathe deeply without assistance, or who suffer from disorders marked by constant sighing.
Lead researcher Professor Jack Feldman, from the University of California at Los Angeles, US, said: "Sighing appears to be regulated by the fewest number of neurons we have seen linked to a fundamental human behaviour.
"One of the holy grails in neuroscience is figuring out how the brain controls behaviour. Our finding gives us insights into mechanisms that may underlie much more complex behaviours."
Different kinds of neurons in the brain act like "buttons" that switch on different types of breath, said the scientists.
One programmes regular breaths, another sighs, and others may trigger yawns, sniffs, coughs, or even laughs and cries.
The team screened more than 19,000 gene activity patterns in the brain cells of mice and identified 200 neurons in two brain stem clusters that appeared to control sighing.
On average, people sigh every five minutes, or about 12 times an hour.
The purpose of sighing is to inflate the alveoli, tiny balloon-like sacs in the lungs where oxygen enters and carbon dioxide leaves the blood stream.
Occasionally, individual alveoli collapse, compromising the vital ability of the lung to exchange oxygen and carbon dioxide.
Prof Feldman said: "The only way to pop them open again is to sigh, which brings in twice the volume of a normal breath. If you don't sigh, your lungs will fail over time."
A therapy that could switch on sighing could benefit people who cannot breath deeply on their own, he pointed out.
Early artificial breathing devices did not regularly give patients a deep breath, leading to many deaths. Modern ventilators deliver a large inflation of air every so often to mimic sighs.
"If you don't sigh every five minutes of so, the alveoli will slowly collapse, causing lung failure," Prof Feldman said. "That's why patients in early iron lungs had such problems, because they never sighed."
In contrast, inhibiting the sigh reflex could help in the treatment of anxiety disorders and other psychiatric conditions which feature a debilitating level of sighing.
Stress and other emotional states are linked to sighing in ways that are not yet fully understood, Prof Feldman added.