A Multicenter Study of the Efficacy and Safety of JZP-258 in the Treatment of Idiopathic Hypersomnia (IH) With an Open-label Safety Extension

Inclusion Criteria: 1. Male or female between 18 and 75 years of age, inclusive, at the time of consent. 2. Have a primary diagnosis of IH according to the International Classification of Sleep Disorders ICSD-2 or ICSD-3 criteria. 3. At the Screening Visit and the Baseline Visit, subjects who are not on Xyrem at study entry must have ESS scores ≥ 11 (as assessed with a look-back period of 1 week). 4. If currently treated with Xyrem, must have documented clinical improvement of EDS after the initiation of Xyrem per Investigator's clinical judgment. 5. Average nightly total sleep time of ≥ 7 hours, per subject history. Average nightly total sleep time will be confirmed by Investigator's review of sleep diaries collected during the final 2 weeks of the Screening Period. 6. If currently treated with stimulants and / or alerting agents or nicotine replacement therapy, must have been taking the same regimen and dose for at least 2 months prior to screening and must agree to take the same dose leading up to and throughout the Double-blind Randomized Withdrawal Period. 7. Have used a medically acceptable method of contraception for at least 2 months prior to the first dose of study drug and consent to use a medically acceptable method of contraception from the first dose of study drug, throughout the entire study period, and for 90 days after the last dose of study drug. Exclusion Criteria: 1. Hypersomnia due to another medical, behavioral, or psychiatric disorder condition. 2. Evidence of untreated or inadequately treated sleep-disordered breathing. 3. Clinically significant parasomnias (eg, sleep walking, rapid eye movement sleep behavior disorder, etc.). 4. Current or past (within 1 year) major depressive episode according to DSM-5 criteria. Patients with depression under control are allowed per the judgment of the Investigator or the treating physician and the anti-depressant treatment has to be stable for at least 6 months prior to Screening and remain stable for the duration of the study. 5. Current suicidal risk as determined from history by presence of active suicidal ideation as indicated by positive response to item #4 or #5 on C-SSRS, or any history of suicide attempt. 6. Occupation requiring nighttime shift work or variable shift work with early work start times or other occupations that could affect the safety of the subject per the judgment of the Investigator. 7. Treatment or planned treatment with any CNS sedating agents, including but not limited to benzodiazepines or other sedating anxiolytics, sedating antidepressants, hypnotics, sedatives, neuroleptics, opoids, barbiturates, phenytoin, melatonin, ethosuximide, medications containing valproic acid or its sodium salt, or any other medication in which the subject experiences sedation are prohibited during the study. Treatment must have been discontinued within 2 weeks or 5 half-lives, whichever is longer, prior to enrollment. The Investigator must ensure that discontinuation from these medications is medically supervised. Subjects must abstain from these medications during the study. 8. Current or past substance use disorder (including alcohol) according to DSM-5 criteria, or the subject is unwilling to refrain from consuming alcohol, cannabinoids, or prohibited medications during the study.