Current prescription pharmacological approaches often have side effects like negative impact on sleep architecture, addiction, and cognitive performance consequences. Further, veterans often report that they use alcohol and other drugs in order to fall asleep and stop nightmares, compounding the issue. Vagal nerve stimulation approaches have potential, especially in this population, to improve sleep quality and to also improve emotional/cognitive state. Further, brain effects of vagal nerve stimulation include attenuated activation of limbic structures critical to manifestation of symptoms in PTSD including the amygdala as well as reduced physiological arousal including decreased blood pressure. Thus, our objective is to determine the effect of tVNS in changing sleep architecture and subjective quality including autonomic state and subsequent (next morning) emotional and cognitive performance in patients with PTSD (with and without mild TBI) and combat expose healthy controls.The study team will use a within-subject cross-over design in a sleep lab based study to assess the impact of tVNS administered at "lights out" on several critical sleep variables including sleep onset, sleep architecture and autonomic profile, and morning emotional state, sleep quality evaluation, and cognitive performance.