Evaluating the Effects of Tasimelteon vs. Placebo in Delayed Sleep-Wake Phase Disorder (DSWPD)

Inclusion Criteria: * Ability and acceptance to provide written informed consent. * A confirmed clinical diagnosis of Delayed Sleep-Wake Phase Disorder (DSWPD). * Men or women between 18 - 75 years, inclusive. * Body Mass Index (BMI) of ≥ 18 and ≤ 35 kg/m\^2. Exclusion Criteria: * Exacerbation of an existing psychiatric condition that requires change in treatment or intervention in the past 3 months. * Major surgery, trauma, illness, general anesthesia, or immobility for 3 or more days within the last 30 days. * Pregnancy, recent pregnancy (within 6 weeks), or women who are breastfeeding. * A positive test for substances of abuse. * Current tobacco user.