Characterization of Fatigue in Military Personnel With Traumatic Brain Injuries

Background: People who have had a traumatic brain injury (TBI) often experience fatigue. Fatigue is the feeling tired all the time. Researchers want to learn more about how TBI and fatigue are related. Objective: To better understand fatigue after TBI in active duty military and veterans. Eligibility: Active duty service members or veterans ages 25-40 who have sustained at least 1 TBI more than 6 months but less than 5 years ago Design: Participants will be screened with: * Medical history * Physical exam * Blood and urine tests Participants will have Visit 1 the same day as screening. This will include questionnaires and interviews. These will be about their fatigue, quality of life, and health. Participants will wear an activity monitor on their wrist and complete a sleep diary for 7 days at home. Participants will have Visit 2: They will stay in the clinic for 2 nights. The visit will include: * Tests of memory, attention, and thinking * Placement of intravenous (IV) line: A needle will guide a thin plastic tube into the participant s arm vein. * 2 overnight sleeps tests: Participants brain waves will be recorded while they sleep. Small electrodes will be placed on the scalp. Monitors will be placed on the skin. These will measure breathing, heart rate, and movement. Blood will be drawn overnight through the IV line. * Optional hydrocortisone stimulation test: Participants will receive the hormone through the IV line. Blood will be drawn through the IV line 5 times over 1 hour. * Optional MRI: Participants will lie in a machine. This machine is a metal cylinder that takes pictures of the brain.