The Autonomic Nervous System, unlike the Central Nervous System, regulates the body and mind more or less outside of conscious awareness. It has two parts: Sympathetic and Parasympathetic. The Sympathetic part regulates arousal or energy. For example, it moves Heart Rate up and down. The Parasympathetic System works to control energy via Heart Rate Variability–the interval between heart beats. This interval changes in under a second. If your pulse is 60 beats per minute, it does not beat once every second. Rather, it may beat 0.8 seconds, then 1.2 seconds, then 0.7 seconds, and so on. The greater the change in heart beat intervals the more control you have of your energy.
The Sympathetic System works to prepare one for fight or flight reactions. So, it reduces functions not critical for immediate action such as digestion. But it increases heart rate, dilates pupils, and opens the airways to the lungs. These actions prepare one for fighting or fleeing a dangerous situation. In contrast, The Parasympathetic System increases digestive functions, and constricts pupils and airways. You can see how the Parasympathetic System works to control arousal. Ideally arousal is proportional to the degree of actual activity called for by any situation. Proper proportionality produces optimal mental and physical action. When one over or under reacts to situations, mental and physical illnesses may ensue.