- Sighing is an essential part of an inherently chaotic system: breathing.

- Researchers find that breathing before and after sighs fits the "re-setter hypothesis" for sighs.

- Sighs help people on ventilators, but too much sighing can be trouble for anyone.

Scientists studying breathing patterns think they have found the reason we sigh: To reset breathing patterns that are getting out of whack and keep our respiratory system flexible.

The study entailed rigging up eight men and 34 women with sensor-equipped shirts that record their breathing, heart rates and blood carbon dioxide levels over 20 minutes of quiet sitting.

What the researchers at the University of Leuven in Belgium were looking for were specific changes over one-minute periods encompassing sighs that could confirm or contradict the "re-setter hypothesis" for the function of sighing. And they think they found it.

"Our results show that the respiratory dynamics are different before and after a sigh," writes Elke Vlemincx and her co-authors in the latest issue of the journal Biological Psychology. "We hypothesize that a sigh acts as a general re-setter of the respiratory system."

The re-setter hypothesis is based on the idea that breathing is an inherently dynamic and rather chaotic system, with all sorts of internal and external factors changing how much oxygen we need and keeping our lungs healthy and ready for action.

This sort of system requires a balance of meaningful signals and random noise to operate correctly.

Occasional noise in a physiological system -- like the respiratory system -- is essential because it enables the body to learn how to respond flexibly to the unexpected, Vlemincx said.

"A sigh can be considered a noise factor because it has a respiratory volume out of range," said Vlemincx.

In this experiment, a sigh was defined as at least two times as large as the mean breath volume.

"A breath is defined by a specific volume (depth), the amount of air we breathe in and out, and a specific timing, the time it takes to breathe in and out," Vlemincx told Discovery News. "Both these characteristics vary: from one moment to the next we breathe slower, faster, shallower, deeper."Vlemincx explained that when breathing is in one state for too long, the lungs deteriorate. They become more stiff and less efficient in gas exchange.

Sighing is critical for keeping our breathing systems flexible, new research shows.Getty Images

So in times of stress, when breathing is less variable, a sigh can reset the respiratory system and loosen the lung's air sacs, or alveoli, which may be accompanied by a sensation of relief, Vlemincx said.

Knowing this, it would seem logical then to add some sighs to the breathing regimes of people on mechanical, ventilators. As it turns out, it has been tried.

"If you put in a few sigh breaths, people feel better," said Frank Wilhelm a clinical psychologist at the Universität Basel in Switzerland.

Wilhelm has studied the role of breathing in psychological disorders extensively.

On the other hand, too much sighing can add too much noise to the system and can also throw the system out of whack. This appears to be what happens to people experiencing panic attacks, said Wilhelm.

"Panic victims don't recover from sighing," said Wilhelm.

In fact, people experiencing panic attacks have been long observed to involve a great deal of sighing, and show all the symptoms of hyperventilation: dizziness, numbness in the extremities, etc., he said.

For that reason a training program involving biofeedback was developed to help panic disorder victims get control of their sighing. It works, said Wilhelm, and further confirms the re-setter hypothesis for sighs.

"It's like a miracle cure, when you think about it," said Wilhelm.