The answer is the sympathetic nervous system. This regulates the reactions of the body to an acknowledged threat and is accountable for the "fight or flight" reaction. Over time, this over-stimulation of the SNS can go to clear-cut marks of imbalance like: anxiety, shallow breathing, augmented heart rate, poor quality sleep, restlessness, night sweats, increased agitation/irritability, fatigue, and nervousness just to name a few.