Efficacy of a treatment is defined as the capacity to produce the desired effects. In this study, we will evaluate and compare the capacity of two active experimental treatments to reduce insomnia and nightmares associated with PTSD, and one inactive intervention, called a placebo, for people who continue to have sleep difficulties despite receiving treatment with an antidepressant medication called a selective serotonin reuptake inhibitor (SSRI, like Prozac, Paxil, Zoloft, Celexa). The two active experimental treatments are a medication, prazosin, and a brief behavioral intervention, which involves exercises and techniques to reduce nightmares and improve sleep quality. Prazosin is an approved medication by the Food and Drug Administration (FDA) against high blood pressure, but is not FDA-approved for posttraumatic insomnia and nightmares.