**1. Introduction**

Brain‐based hypersensitivity to environmental stimulation underlies pathological states that have been defined as "disorders of arousal" [1]. "Autonomic tuning" is the term that was historically used to describe the process of normally balanced sympathetic and parasympa‐ thetic branches of the autonomic nervous system (ANS), in contrast to the disorders of arousal which are characterized by ANS dysfunction, affective lability, anxiety, stress, and emotional disorders:

It is a matter of everyday experience that a person's reaction to a given situation depends very much upon his own mental physical, and emotional state. One might be said to be "set" to respond in a given manner … the autonomic response to a given stimulus may at one time be predominantly sympathetic and may at another time be pre‐dominantly parasympathetic.([2], pp. 90–91; quoted in [3], p. 179)

ANS dysregulation impacts on both physical (increasing cardiovascular risk) and mental (compromising psychological well‐being) health at multiple levels. Loss of regulation of normal autonomic control of cardiac adjustment to environmental stressors leads to negative impacts on physiological function affecting arterial blood pressure, heart rate and rhythm, and vagal afference. Allostatic load is a term that has been used for decades to describe "the wear and tear on the body" which grows over time when the individual is exposed to repeated or chronic stress [4]. Allostatic load is the physiological consequence of chronic exposure to fluctuating or heightened neural or neuroendocrine response that results from repeated or chronic stress. Thus, it is that chronic autonomic imbalance finally leads to allostasis of affective, cognitive, and behavioral level of function. The effect of heart rate variability (HRV) biofeedback (HRVB) is to manipulate peripheral autonomic state feedback to the central nervous system circuits regulating emotional, cognitive, and sensorimotor activity. The study of HRV and effects of HRVB provide important insights into the mechanisms of autonomic arousal in normal, successful adaptation and pathological states such as PTSD.
