Lemborexant in Delayed Sleep Phase Syndrome

Inclusion Criteria: 1. 18 years of age or older. 2. Diagnosed with delayed sleep phase syndrome (DSPS) meaning that: 1. Sleep is delayed by two hours or more beyond what is considered an acceptable or conventional bedtime for the subject (their desired bedtime). 2. Subjects not able to fall asleep if trying to sleep before the later bedtime; 3. This is interfering with their wishes/having social impact. 3. Concomitant medications will be allowed, though dosages will be required to remain fixed throughout participation in the study. 4. The participant also needs to be willing and able to comply with all aspects of the protocol. Exclusion Criteria: 1. 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 (assessed by the Alliance Sleep Questionnaire- ASQ), or any medical disorder/therapy that could interfere with the trial (this will be verified through interview and analysis of the ASQ). 2. 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. 3. 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. 4. 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. 5. Shift workers or subjects working unusual hours. 6. Any risk of suicide within 6 months of screening period or throughout the trial (accessed by the Investigator and by the C-SSRS questionnaire). 7. Transmeridian travel across more than 3 time zones 4 weeks prior to the screening phase. 8. Transmeridian travel across more than 2 time zones during this trial (including the screening phase). 9. Having a positive drug test or being unwilling to refrain from using illegal drugs or marijuana during this trial. 10. Any clinically abnormal symptom or organ impairment found by medical history at Screening or Baseline and physical examinations, vital signs, ECG findings, or laboratory test results that require medical treatment. 11. Impaired liver function (values for enzymes aspartate transaminase (AST) and alanine transaminase (ALT) \> 1.5 times the Upper Limit of Normal). 12. Known to be human immunodeficiency virus positive. 13. Has a QT interval corrected using Fridericia's formula interval (QTcF interval) \>450 ms demonstrated on repeated ECGs (repeated only if initial ECG showed corrected QT interval (QTc) \>450 ms) at Screening or Baseline.