The anxiety, the discomfort or prolonged and repetitive negative stress are associated in the brain with a potential danger and activate the sympathetic part of ANS in a mindless way. If this settles in duration, the stress can become chronic and can lead to many diseases as the “burn-out”, the depression, musculoskeletal disorders (MSDs), etc. Similarly, excess of parasympathetic activity is not desirable neither, the ideal being to stay in an equilibrium zone.
The “negative” emotions who settle in duration have a measurable physiological effect, as the ANS acts on the body inherently, from the brain’s perception. For example, fear or anxiety accelerate the heart even in the absence of physical effort. This occurs because the heart is the center of the autonomic nervous system, it accelerates when the sympathetic part is activated and it slows during phases of recovery and rest. This is visible on the signal measured from an electrocardiogram (ECG) through the intervals between two heart beats, called Heart Rate Variability (HRV) or R-R interval, as shown below: