Many combat veterans (c-Vs), service members (SMs) and civilians with mild traumatic brain injury (mTBI, also termed concussion) suffer from persistent post-concussion symptoms (PPCS) that degrade physical and cognitive well-being and have been linked with early neurodegeneration. PPCS include somatosensory (e.g., headache, disturbed vision), vestibular (e.g., dizzy, clumsy), cognitive (e.g., executive function, memory) and affective symptoms (e.g., sleep disturbance, irritability, depression). A key pathological process that provokes and perpetuates PPCS is autonomic nervous system (ANS) dysfunction such as unremitting sympathetic nervous system activation that also exacerbates other pathology such as inflammation. ANS activity can be readily quantified by measuring heart rate variability (HRV), or the variation in heart rate over time, which serves as a valid index of both physiological and psychological health