An interesting study from Psychophysiology attempting to understand why we sigh by studying in what contexts these wistful expressions are most likely to occur. It seems, we are most likely to sigh when relieved.
Why do you sigh? Sigh rate during induced stress and relief.
Psychophysiology. 2009 May 21. [Epub ahead of print]
Vlemincx E, van Diest I, de Peuter S, Bresseleers J, Bogaerts K, Fannes S, Li W, van den Bergh O.
Whereas sighing appears to function as a physiological resetter, the psychological function of sighing is largely unknown. Sighing has been suggested to occur both during stress and negative emotions, such as panic and pain, and during positive emotions, such as relaxation and relief. In three experiments, sigh rate was investigated during short imposed states of stress and relief. Stress was induced by exposure to a loud noise stressor or by anticipation of it. Relief was induced by the end of the stressor or the anticipation that no stressor would follow. Breathing parameters were recorded continuously by means of the LifeShirt System. Results consistently showed that more sighing occurred during conditions of relief compared to conditions of stress.