People may think they sigh just for the heck of it, but UCLA and Stanford researchers have pinpointed two specific clusters of neurons in the brain stem that appear to turn normal breaths into sighs—and that process may happen for a vital reason, a press release notes. Using mice for a study published online in Nature, the researchers found one set of about 200 neurons that release two different neuropeptides that help brain cells talk to each other (and those same peptides are especially active in the part of the human brain that influences breathing). Another set of 200 neurons excited by the peptides made the animals' breathing rate jump from 40 sighs an hour to 400 sighs an hour. If one peptide set was blocked, the mice sighed only half as much; both sets blocked meant sighing ceased. (Popular Science notes the animals' regular breathing remained unchanged.) Not good, as scientists say sighing is a "life-sustaining reflex."
Increased oxygen brought in by sighing is needed to reinflate the lungs' alveoli sacs, which occasionally collapse. "The only way to pop [the alveoli] open again is to sigh, which brings in twice the volume of a normal breath," study co-author Jack Feldman says in the release. "If you don't sigh, your lungs will fail over time." This is probably why humans sigh about 12 times an hour, animals even more, Popular Science notes. But researchers still aren't sure how the emotional component of sighing—Nature points out that people tend to sigh when they're sad, relieved, or tired—plays into the process, the Guardian reports. "It may be that neurons in the brain areas that process emotion are triggering the release of the sigh neuropeptides, but we don't know that," Feldman says. Being able to control the sigh rate in the future could potentially allow scientists to help people with certain disorders: For example, those with anxiety disorders who sigh a lot may benefit from decreased sigh rates, while people with breathing issues may get relief from more sighing, NPR notes. (Music may also help patients breathe.)