[kbutler]

Except sighing isn't generally a natural response to stress, but a response to boredom.

So maybe it's consciously emulating a natural behavior matching a lower-stress state, to help shift to that state.

From personal experience, the deep inhalation preceding a sigh (extended exhalation) is also helpful.

[nobodyandproud]

A sigh of frustration is pretty common. Biting one’s tongue and taking a breathe.