Heart disease is the number one killer in America. One way to improve your heart health is to improve your vagal tone. Well conditioned athletes have higher vagal tone because aerobic breathing creates healthy vagal tone, which results in a lower resting heart rate. Healthy cardiac function is directly linked to stimulating the vagus nerve. (source)
Stimulating the vagus nerve leads to release of acetylcholine, the first neurotransmitter discovered. Acetylcholine is used in the autonomic, parasympathetic, and sympathetic nervous systems. Inside the brain acetylcholine functions as a neuromodulator—a chemical that alters the way other brain structures process information rather than a chemical used to transmit information from point to point. Acetylcholine plays an important role in arousal, attention, and motivation.