This is a double-blind, placebo-controlled, randomized withdrawal, multicenter study of the efficacy and safety of FT218. FT218 drug product is a once-nightly formulation of sodium oxybate for extended-release oral suspension. The study will enroll subjects who are diagnosed with idiopathic hypersomnia. Subjects will be eligible to enroll regardless of current treatment with oxybate therapy or stimulants/alerting agents at study entry. The estimated total duration of study for each subject is approximately 18 weeks, including the Screening period.

Inclusion Criteria:

• Primary diagnosis of idiopathic hypersomnia
• Total ESS score at Screening > 11 if not on prior oxybate
• Average nightly total sleep time of > 7 hours
• May use stimulants/alerting agents but dose and regimen must have been stable for 2 months prior to Screening, and remain stable until the double-blind, randomized withdrawal visit
• Females of childbearing potential must use highly effective contraception for 2 months prior to Baseline, throughout the study, and for 30 days after the last dose of study drug
• Males with female partners of childbearing potential must use condoms throughout the study and for 30 days after the last dose of study drug
• Willing and able to provide informed consent and comply with the requirements of the study

Exclusion Criteria:

• Pregnant, nursing or lactating females
• Hypersomnia due to another medical, behavioral, sleep, or psychiatric condition
• Untreated or incompletely treated sleep apnea in patients with an apnea-hypopnea index (AHI) ≥ 15 by American Academy of Sleep Medicine (AASM) 1A criteria
• Clinically significant parasomnias
• History or presence of seizures, head trauma, succinic semialdehyde dehydrogenase deficiency, uncontrolled hypothyroidism, and/or significant hepatic impairment
• History or presence of bipolar and related disorders, schizophrenia, schizophrenia spectrum disorders, or other psychotic disorders
• Ongoing or past (within 1 year) major depressive episode
• At risk for suicide or history of suicide attempt
• If not on oxybate at Screening, treatment or planned treatment with any central nervous system (CNS) sedating agents during study
• Current or past substance use disorder (including alcohol or cannabinoids)
• Excessive caffeine consumption (> 600 mg/day)
• Prior treatment with either FT218 or LUMRYZ

For the double-blind, randomized withdrawal portion of the study, the subject and all members of the study team will remain blinded to treatment assignment.