I think sighing, rather than yawning, makes more sense with the brain cooling theory. When we are distressed, we release cortisol, which is a pro-oxidizing fre

I think sighing, rather than yawning, makes more sense with the brain cooling theory. When we are distressed, we release cortisol, which is a pro-oxidizing free radical that raises body temperature and blood pressure. Signing involves a fast and prolonged exhalation of air. We tend to sigh when we are distressed, so it possibly serves as a way to cool ourselves down similar to an exhaust fan on a desktop chassis.