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 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 (CMS.gov). * Has a history or presence of any unstable or clinically significant medical condition (chronic pain condition that may impact sleep), 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. * Has recently taken (ie, within 1 month prior to screening), is taking, or plans to take any of the following: * A substance or medication contraindicated with XYWAV use, ie. specifically alcohol and sedative hypnotics * A medication with a known drug-drug interaction with XYWAV * A medication that may have similar EEG effects to XYWAV * Medications known to have clinically significant CNS sedative effects * Other medications, natural health products, or substances from which the participant experiences clinically significant sedation Additional Exclusion Criterion for Participants in the Narcolepsy \>9-Gram Cohort • Any past or current medical conditions or experience that, in the investigator's clinical judgment, would preclude further increases in the current oxybate medication dosage beyond 9 grams