This is a multicenter, double-blind, randomized study to evaluate the efficacy and safety of a daily single oral dose of tasimelteon and matching placebo in male and female participants with 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.