Most of the data on TRI comes from research on populations with PTSD. Difficulty initiating and maintaining sleep is designated as one of the heightened arousal symptoms of PTSD in the DSM. Sleep studies have suggested increased wake after sleep onset (WASO), reduced slow wave sleep (SWS) in some PTSD populations and fragmented rapid eye movement (REM) sleep when PTSD is developing, and during its more acute stages. Suvorexant is a first in class orexin antagonist and is approved by the FDA for the indication of insomnia. Orexin antagonists dampen the activity of a specific arousal enhancing system in the brain during sleep. In rodent models suvorexant has been shown to enhance, and in healthy humans, to not affect slow wave and REM activity (in contrast with traditional hypnotics which can diminish both). Reducing arousal during sleep while reducing WASO and maintaining REM and slow wave sleep is a promising profile for the treatment of TRI. We are therefore proposing a placebo controlled evaluation to assess the efficacy of suvorexant for treating TRI with and without PTSD and its tolerability in these populations. We will include polysomnography (PSG) in order to have objective sleep outcomes and probe potential mechanisms and biomarkers predicting response. The proposed study will meet the objective below and test the following hypotheses: