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 Screening Visit 1. If currently taking oxybate medication, has an ESS score \> 10 at the Baseline Visit 2 polysomnography (after washout period). Note: Not applicable for participants entering the Narcolepsy \>9-gram cohort (see additional criteria below for the Narcolepsy \>9-gram cohort). * 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. Note: Alerting agents refer to either traditional stimulants (eg, amphetamines, methylphenidates) or wake-promoting agents (eg, modafinil, pitolisant, solriamfetol). Additional Inclusion Criteria for Participants in the Narcolepsy \>9-Gram Cohort * Is on a current treatment regimen including XYWAV at a dosage of 9 grams Key Exclusion Criteria: * Shows evidence of a previous untreated or inadequately treated sleep disorder considered by the investigator to…