The fastest way to change how you feel is not to breathe in more. It is to breathe out longer. Most people, told to "breathe deeply," pull in a large breath and hold the strain there at the top. The breath that actually settles a restless afternoon is the one that leaves slowly — longer going out than it came in.
Why the exhale.
The inhale belongs to the part of the nervous system that readies you to act. The exhale belongs to the part that lets you rest. Lengthen the exhale past the inhale and you tilt the balance, quietly, toward rest. There is nothing mystical in this. It is mechanical, and it works within a few rounds.
Three short practices.
The first: in for four, out for six. Nothing else. Two minutes, seated or standing, eyes open if you like. Use it before a difficult conversation.
The second: in for four, out for eight, with a soft pause at the bottom before the next breath arrives. Six rounds. Use it when the afternoon has run away from you.
The third: no counting at all. Simply make each exhale a little longer than feels necessary, and let the inhale look after itself. Use it in the last minutes before sleep.
Keep them short. A breath practice you do for two minutes daily will do more than one you mean to do for twenty and never start.