Efficacy and Safety of ACT-541468 in Adult Subjects With Insomnia Disorder

Principal inclusion Criteria: * Signed informed consent prior to any study-mandated procedure. * Male or female aged 18-64 years (inclusive). * Women of childbearing potential must have negative pregnancy tests and use reliable methods of contraception up to 30 days after EOT. * Body mass index (BMI): 18.5 ≤ BMI (kg/m2) \< 32.0. * Insomnia disorder according to DSM-5 criteria. * Insufficient sleep quantity as collected subjectively in the sleep diary and validated objectively by polysomnography. * Insomnia Severity Index score ≥ 15. Principal exclusion Criteria: * Any current history of sleep disorder other than insomnia, or any lifetime history of related breathing disorder, periodic limb movement disorder, restless legs syndrome, circadian rhythm disorder, rapid eye movement (REM) behavior disorder, or narcolepsy. * Self-reported usual daytime napping ≥ 1 hour per day, and ≥ 3 days per week. * Caffeine consumption ≥ 600 mg per day. * Shift work within 2 weeks prior to the screening visit, or planned shift work during study. * Travel ≥ 3 time zones within 1 week prior to the screening visit, or planned travel \> or= 3 time zones during study. * Hematology or biochemistry test results deviating from the normal range to a clinically relevant extent as per judgment of the Investigator. * AST and/or ALT \> 2 × ULN and/or direct bilirubin \> 1.5 × ULN (except known history of Gilbert's syndrome). * Severe renal impairment (known or defined as estimated creatinine clearance \< 30 mL/min). * History or clinical evidence of any disease or medical condition or treatment, which may put the subject at risk of participation in the study or may interfere with the study assessments. * Any circumstances or conditions, which, in the opinion of the investigator, may affect the subject's full participation in the study or compliance with the protocol.