This trial tests if heart rate variability biofeedback can help people with mild traumatic brain injury improve their heart rate patterns and recover better from stress. The study will compare those who receive the biofeedback training to those who do not, to see if the training leads to better physical, emotional, and cognitive health. Heart rate variability (HRV) biofeedback has been explored as a method to improve emotion regulation and social functioning in individuals with severe traumatic brain injury.