It is a certain direction that the breath takes that brings about sleep, and it is the direction of the breath that brings vigor or fatigue. A man may, by the help of the breath, become stronger by doing physical exercises, and another by physical labor may become exhausted and worn out. The laborers in India who have to lift heavy weights have a certain way of breathing, and by understanding this they can lift a great deal and work a great deal and yet feel little fatigue.