The purpose of the study is to evaluate whether Lemborexant is more effective than placebo in shortening sleep onset latency in patients with delayed sleep phase syndrome (both type 1 and type 2). This will be tracked using sleep logs as well as actigraphy.

In this 2-year study, the investigators will examine if Lemborexant administered 5-10 mg nightly taken at desired bedtime (at least 2 hours prior to self-reported sleep onset habitual time) can improve the symptoms of Delayed Sleep Phase Syndrome.

Delayed sleep phase syndrome (DSPS) is a disorder in which a person's sleep is delayed by two hours or more beyond what is considered an acceptable or conventional bedtime. The delayed sleep then causes difficulty in being able to wake up at the desired time. In DSPS, bedtime is shifted later than the general population such that individuals have difficulty getting enough sleep to meet their sleep need before they have to get up for their daytime obligations (work, school, childcare, etc.). As a result, patients experience daytime impairment including daytime sleepiness and cognitive impairment. DSPS, if maintained in adulthood is associated with numerous deleterious health effects, although causality is not well established. The prevalence of this condition is approximately 7-16% among adolescents and young adults.

Inclusion Criteria:

- Participants will be required to be 18 years of age or older and have delayed sleep phase syndrome (DSPS). Questionnaires will be used to identify potential confounders and to confirm a potential diagnosis of DSPS based on ICSD3 criteria: a) Sleep is delayed by two hours or more beyond what is considered an acceptable or conventional bedtime for the subject (their desired bedtime). b) Subjects not able to fall asleep if trying to sleep before the later bedtime; c) This is interfering with their wishes/having a social impact. Concomitant medications will be allowed, though dosages will be required to remain fixed throughout participation in the study. The participant also needs to be willing and able to comply with all aspects of the protocol.

Exclusion Criteria:

• Clinically significant depression (PHQ-9 score of 10 or more), anxiety disorder (GAD- 7 score of 10 or more), substance use disorder, any other sleep disorder, or any medical disorder/therapy that could interfere with the trial
• Use of medications with significant effects on sleep-wake function (insomnia therapies, stimulants)- unless they are discontinued at least 5 half-lives prior to study participation. Non-sedative antidepressants or SSRI will be allowed if at a stable dose in the absence of concomitant severe depression or severe anxiety.
• Use of CYP3A inhibitors and CYP3A inducers, at least 1 week (or five half-lives, whichever is longer) prior to the first day of the baseline phase.
• Pregnancy (verified by urine pregnancy test on visits 1, 2, and 3) or plan to become pregnant in the next 3 months or currently breastfeeding.
• Shift workers or subjects working unusual hours.
• Transmeridian travel across more than 3 time zones 4 weeks prior to the screening phase.
• Transmeridian travel across more than 2 time zones during this trial (including the screening phase).
• Having a positive drug test or being unwilling to refrain from using illegal drugs or marijuana during this trial.
• Any clinically abnormal symptom or organ impairment found by medical history at Screening or Baseline and physical examinations, vital signs, or laboratory test results that require medical treatment.
• Impaired liver function (values for enzymes aspartate transaminase (AST) and alanine transaminase (ALT) > 1.5 times the Upper Limit of Normal).
• Known to be human immunodeficiency virus positive.

Data may be shared only with collaborators at Stanford University who are mirroring this study and recruiting half of the required participants and with Eisai Inc Ltd, the study sponsor that is providing the drug of investigation. Individuals will be assigned a unique coded identifier to limit risk of privacy loss

IPD from enrolled participants will be available to collaborators for the duration of the study (Mar 2023 - May 2027 (anticipated) until the study has ended.

Collaborator Emmanuel Mignot from Stanford University and study sponsor Eisai will be able to access the data as participants from UCSF and Stanford will be combined for the overall data analyses

Double Blind