The purpose of this study is to obtain data investigating the safety and efficacy of eszopiclone for the treatment of post-traumatic stress disorder (PTSD)-related sleep disturbance and the impact of improved sleep with eszopiclone treatment on neuroendocrine correlates of PTSD. The investigators hypothesize that eszopiclone will be significantly more effective than placebo and well tolerated for PTSD-related sleep disturbance, improvement in sleep will be associated with improvement in overall PTSD symptoms, and patients with PTSD-related sleep disturbances will have abnormal levels of stress hormones.