One of the main symptoms of post traumatic stress disorder is hyperarousal. According to Dorland's Medical Dictionary, hyperarousal is "a state of increased psychological and physiological tension marked by such effects as reduced pain tolerance, anxiety, exaggeration of startle responses, insomnia, fatigue and accentuation of personality traits."  It has also been described as a chronic state of fight or flight.
The fight or flight response was necessary when mankind was faced with physical threats, such as those of wild animals, on a daily basis. Veterans returning from combat have lived through a constant barrage of physical threats and their fight or flight responses are often in high gear. For most of us, the daily threats are more emotional or psychological in nature, even so, when faced with such a threat, your fight or flight response may kick in. When this happens, your heart beats rapidly, your breath becomes shallow, your muscles tense and you are instantly alert. The longer your fight or flight response stays active, the more draining - both physically and emotionally.
Some of the symptoms of hyperarousal include:
Constant feeling of being in danger
Hyperarousal can impact your ability to function on a daily basis and interfere with your relationship with family and friends. Marsha described her husband's experiences after being a witness to a shooting. "Any loud noise would set him into a state of hyperarousal, and this could last anywhere from a few minutes to 24 hours. It could be fireworks, a car backfiring, a neighbor hammering, dropping a pot in the kitchen, any loud and unexpected noise. Joe would go into a state of panic, checking around the house for intruders, wanting to keep the kids indoors where they would be safe, jumping at any noise and getting angry if anyone in the house did something he felt was unsafe, such as answering the door or inviting a friend over. During a hyperarousal event, he usually didn't sleep for at least 24 hours, making him even more irritable and he might have nightmares for several days. We would all walk around on eggshells for a few days, whispering and trying to not make any noise." 
Treating hyperarousal usually falls within general treatment for anxiety. Medications and desensitization therapy can help. Cognitive behavioral therapy teaches ways for you to rethink your response to stimuli.
Therapy focusing on stress reduction, such as relaxation techniques (meditation, progressive relaxation, visual relaxation, journaling your thoughts) have also been found to be helpful. Deep breathing and strenuous exercise during a hyperarousal event may help to reduce the feelings of loss of control. For some, taking control of health care, such as quitting smoking, eating right and attending therapy and doctor's appointments may help. This may be because hyperarousal may occur because of feelings of not having control over the situation, symptoms, such as being hypervigilant are a way of gaining control over your environment. When you take control of your health care, you may feel as if you are "taking control of your life" and the feelings of hyperarousal may diminish.
"Event-related Potential Patterns Associated with Hyperarousal in Gulf War Illness Syndrome Groups," 2012, June, Gail D. Tillman et al, NeuroToxicology
"Hyperarousal," 2007, Dorland's Medical Dictionary
"Physiological Correlates of Childhood Abuse: Chronic Hyperarousal in PTSD, Depression and Irritable Bowel Syndrome," 2000, Kathleen A. Kendall-Tackett, Family Research Laboratory, University of New Hampshire, Published in Child Abuse & Neglect