Sigh
Encyclopedia
A sigh is a deep and especially audible, single exhalation
Exhalation
Exhalation is the movement of air out of the bronchial tubes, through the airways, to the external environment during breathing....

 of air out of the mouth or nose, that humans use to communicate emotion. It is voiced pharyngeal fricative, sometimes associated with a guttural glottal breath exuded in a low tone. It often arises from a negative emotion, such as dismay, dissatisfaction, boredom, or futility. A sigh can also arise from positive emotions such as relief, particularly in response to some negative situation ending or being avoided. Like a gasp, a yawn
Yawn
A yawn is a reflex of simultaneous inhalation of air and stretching of the eardrums, followed by exhalation of breath. Pandiculation is the act of yawning and stretching simultaneously....

, or a moan, a sigh is often an automatic and unintentional act. In literature, a sigh is often used to signify that the person producing it is lovelorn.

Scientific studies show that babies sigh after 50 to 100 breaths. This serves to improve the mechanical properties of lung tissue, and it also helps babies to develop a regular breathing rhythm. Behaviors equivalent to sighing have also been observed in animals such as dog
Dog
The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...

s, monkey
Monkey
A monkey is a primate, either an Old World monkey or a New World monkey. There are about 260 known living species of monkey. Many are arboreal, although there are species that live primarily on the ground, such as baboons. Monkeys are generally considered to be intelligent. Unlike apes, monkeys...

s, and horse
Horse
The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...

s.

In text messages and internet chat rooms, or in comic books, a sigh is usually represented with the word itself, 'sigh', possibly within asterisks, *sigh*.
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