"significantly increased sustained attention"
"significantly lower cortisol level after training"
"significant decrease in negative affect after intervention"
Those are the summarised findings of an eight-week training program of diaphragmatic breathing, where participants were taught to breathe at 4 breaths/minute. Quite a drop from the typical adult who breathes at 12 breaths/minute.
Here's a link to the article.
And to kick start you to learn diaphragmatic breathing, try this facilitated breathing and facilitated breathing 2.
And here is Gray Cook's take on breathing.