Open-label Study to Investigate the Pharmacokinetics and Safety of Tasimelteon in Children and Adolescents.

This study is an open-label, single dose, non-controlled study to evaluate the PK and safety of tasimelteon in children and adolescents from 3 years to less than 18 years of age who were legally blind and met the diagnostic criteria for CRSWD Non-24 per DSM-V or who were diagnosed with a Neurodevelopmental Disorder (like ASD and SMS) and had a nighttime sleep complaint.

Inclusion Criteria:

1. Males or females 3 to <18 years of age who are legally blind [defined as having a visual acuity of 20/200 or less in the better-seeing eye with best conventional correction (glasses or contact lenses) and/or a visual field of 20 degrees or less in the better-seeing eye], males or females 3 to <18 years of age with SMS and with a nighttime sleep complaint and males or females 3 to <18 years of age with ASD and with a nighttime sleep complaint;
2. Weigh at least 10 kg;

3 Diagnosis of SMS determined by a prior positive genetic test result as indicated by parent/guardian; Diagnosis of ASD as indicated by parent/guardian; or a diagnosis of Non-24 as determined by DSM-5 diagnostic criteria for the Circadian rhythm sleep-wake disorder, Non-24-hour sleep-wake hour type:

1. A persistent or recurrent pattern of sleep disruption that is primarily due to an alteration of the circadian system or to a misalignment between the endogenous circadian rhythm and the sleep-wake schedule required by an individual's physical environment or social or professional schedule;
2. The sleep disruption leads to excessive sleepiness or insomnia, or both;
3. The sleep disturbance causes clinically significant distress or impairment in social, occupational, and other important areas of functioning.

Exclusion Criteria:

1. For blind subjects only: Subjects who have a probable diagnosis of a current sleep disorder other than Non-24-Hour Sleep-Wake Disorder that is the primary cause of the sleep disturbance based on clinical investigator medical judgment;
2. For blind subjects only: History (within the 12 months prior to screening) of psychiatric disorders including ADHD, Neurodisabilities, Major Depressive Disorder, Generalized Anxiety Disorder, Axis II Disorders, delirium or any other psychiatric disorder, that is not being successfully treated or has not been resolved and that in the opinion of the clinical investigator would affect participation in the study or full compliance with study procedures;
3. History of intolerance and/or hypersensitivity to melatonin or melatonin agonists;