This will be a multicenter, open-label study. The study has two phases: the screening phase and the evaluation phase. The screening phase is comprised of a screening visit where a patient's general health and initial eligibility will be evaluated. The evaluation phase is comprised of a baseline visit and a 52 week segment. Patients that meet all entry criteria for the study at baseline visit will enter the treatment segment where patients will be asked to take 20 mg tasimelteon daily approximately 60 minutes prior to their target bedtime for 52 weeks in an open-label fashion. An optional sub-study extension phase is available to subjects who complete the first year of treatment and consists of continued open-label treatment for up to 3 years additional.