This is a double-blind, randomized, two-period crossover study to evaluate the efficacy and safety of a single oral dose of tasimelteon and matching placebo in male and female subjects with DSWPD and the CRY1Δ11 variant.

Inclusion Criteria:

• Ability and acceptance to provide written informed consent.
• A confirmed clinical diagnosis of Delayed Sleep-Wake Phase Disorder (DSWPD).
• Carrier of CRY1Δ11 variant.
• Men or women between 18 - 75 years, inclusive.
• Body Mass Index (BMI) of ≥ 18 and ≤ 40 kg/m^2.

Exclusion Criteria:

• 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.