A sigh is a kind of paralinguistic respiration in the form of a deep and especially audible, single exhalation of air out of the mouth or nose.
Sighing may have purely physiological triggers (see: Respiratory variability preceding and following sighs: A resetter hypothesis), or they may arise from negative emotion, such as dismay, dissatisfaction, boredom, or futility.
According to a 2014 study in the Psychophysiology journal :
“How dimensions of emotion affect respiratory regulation assessed by respiratory variability and sighing is unknown.”
There are also theories that such sighs have functions as signals to convey emotional states to others :
“[they are] often unintentional expressions of an activity, plan or desire that has to be discarded, creating a pause before it can be replaced by a novel initiative.” source:
“[…] in contrast to largely involuntary respiratory acts like yawning or sneezing, sighing is wholly manipulable, which suggests that its occurrence in interaction may be purposeful.” source: