Alcohol is commonly used as "self-medication" by those who are suffering from combat trauma. If nightmares where they're revisiting combat stress and combat scenes keep people awake, alcohol has traditionally been a way (not necessarily a constructive way, but certainly an easily accessable, socially acceptable one) to de-stress before sleep, and try to avoid the nightmares. Unfortunately, of course, it usually doesn't work to improve things, and there can be a huge rebound effect; or, heavy drinking can cause other problems. So at most it's a stop-gap solution, despite its popularity. A fact sheet from the VA discusses alcohol use and PTSD. Click here for the link.