A Study to Investigate the Effect of XYWAV on Sleepiness, Polysomnography, and Functional Outcomes in Participants With Idiopathic Hypersomnia or Narcolepsy

Key Inclusion Criteria: Is 18 to 75 years of age (inclusive) at the time of signing the informed consent form (ICF) Has a primary diagnosis of IH meeting International Classification of Sleep Disorders, Third Edition (ICSD-3) criteria or narcolepsy (Type 1 or Type 2) meeting ICSD-3 or Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-V) criteria. If not currently taking oxybate medication, has clinically significant symptoms of Excessive daytime sleepiness (EDS) with an ESS score > 10 at study entry. If currently taking oxybate medication, has an ESS score > 10 at the Baseline polysomnography Visit (after washout period). If currently treated with anticataplectics and/or alerting agents, has been taking the same dosage for at least 1 month prior to Screening Visit 1 and has no current plans to adjust the dosage during the study period. Key Exclusion Criteria: Shows evidence of a previous untreated or inadequately treated sleep disorder considered by the investigator to negatively impact the conduct of the study, including sleep-disordered breathing, parasomnias, circadian rhythm sleep disorders, or restless legs syndrome determined by a previous sleep-laboratory diagnosis or interview utilizing modules of the Diagnostic Interview for Sleep Patterns and Disorders. Shows evidence of untreated or inadequately treated sleep-disordered breathing during Baseline Visit 2 polysomnography defined as an Apnea/Hypopnea Index (AHI) > 10, using the US Centers for Medicare and Medicaid Services rules. Has a history or presence of any unstable or clinically significant medical condition, behavioral or psychiatric disorder (including active suicidal ideation or current or past [within 1 year] major depressive episode), or history or presence of another neurologic disorder or surgical history that might affect the participant's safety and/or interfere with the conduct of the study, in the opinion of the investigator. Is being treated with or has planned treatment with any central nervous system sedating agents, including but not limited to benzodiazepines or other sedating anxiolytics, sedating antidepressants, hypnotics, sedatives, neuroleptics, opioids, barbiturates, phenytoin, ethosuximide, medications containing valproic acid or its sodium salt (eg, Depakene® and Depakote®), other sedating antiseizure medications, melatonin, muscle relaxants, general anesthetics, or any other medication from which the participant experiences sedation.