Inclusion Criteria:

• Males or Females, age 18 to 70 years old.
• Onset of hypersomnia between age 10 and 30 years of age.
• Subjects with a diagnosis of IH, as determined by the Patient Selection Committee, with findings including:
  • A mean total nocturnal sleep time of ≥ 7 hours as demonstrated on the most recent 7 daily entries in subject’s sleep log or actigraphyA mean total nocturnal sleep time of ≥7 hours as demonstrated on the most recent 7 entries of a two week A mean total nocturnal sleep time of ≥7 hours as demonstrated on the most recent 7 entries of a one or two week (depending on the subject’s prohibited concomitant medication status) sleep log or actigraphy and
  • sleep log or actigraphy; and
  • A MSLT with a mean sleep latency of ≤ 8 minutes with <2 SOREMPs.
• An Epworth Sleepiness Scale score of ≥ 11 on all screening ESS assessments.
• Bedtime and wake time variability during work days of less than 2 hours, respectively.
• Bedtime no later than 12 midnight on two or more nights in any given week.
• A minimum 3-month history of daily irrepressible need for sleep or daytime lapses into sleep.
• Mean Mental fog score of ≥ 6 in the pm entry of the Idiopathic Hypersomnia Symptom Diary (IHSD)from the 7 most recent daily entries.
• Able to comply with requirements for concomitant medications and consumption of caffeine, alcohol, tobacco and nicotine.
• Females with a negative pregnancy test AND who are non-lactating.
• Sexually active females of childbearing potential must be willing to use a highly effective method of birth control from start of Screening until at least 3 months after last dose of study drug [defined as one that results in a low failure rate (i.e., less than 1% per year) when used consistently and correctly, such as implants, injectables, combined oral contraceptives, some intrauterine contraceptive devices (IUDs) or partner vasectomy. Non-childbearing potential is defined as > 1 year post-menopausal or tubal ligation.
• Sexually active males must have had a vasectomy or use condoms AND female partners of male study participants must use one of the following methods: barrier contraceptive (e.g., female condom, cervical cap, diaphragm); hormonal (oral, injectable, transdermal patch); or IUD.

Exclusion Criteria:

• History of any disorder causing hypersomnia other than IH including Narcolepsy Type 1 (Cataplexy is present or low CSF hypocretin-1 concentration), Narcolepsy Type 2, any Circadian Rhythm Sleep-Wake Disorder, or severe periodic limb movement disorder (PLMAI >10/hr).
• Evidence of circadian-rhythm disorder, nighttime shift work or need to travel more than 3 time zones during the course of the study.
• Sleep apnea syndrome based on:
  • A previous polysomnogram with an AHI > 15; or
  • A positive Berlin Questionnaire for Sleep Apnea. If a Berlin questionnaire is positive, a home or in laboratory sleep study needs to be done with an AHI > 15.
• Subjects who currently require the use of a continuous positive air pressure (CPAP) machine.
• Obese subjects with BMI > 35 kg/m2.
• History of or current seizure disorder or history of syncope, unexplained loss of consciousness or seizure in the past 3 years as well as any past history of benzodiazepine and/or barbiturate and/or alcohol-related withdrawal seizures.History of or current seizure disorder or history of syncope, unexplained loss of consciousness or seizure in the past 3 years.
• Columbia-Suicide Severity Rating Scale (C-SSRS) findings consistent with significant history of or current suicidal ideation or behavior.
• Subjects who fail to wash out any medications for IH or any other prohibited medications described in Section 6 of this protocol (Prior and Concomitant Medications) within 2 weeks or 5 half-lives, whichever is longer, prior to Screening Visit B.
  • a. A positive toxicology result during Screening or Baseline Visits (A positive toxicology result at Screening Visit A due to a documented concomitant medication will not exclude the subject if the Screening Visit B retest is negative).
• Clinically significant abnormal findings from physical, electrocardiogram (ECG) or laboratory assessments at Screening including:
  • Significant hepatic impairment (ALT, AST or total bilirubin > 1.5 × ULN);
  • Renal impairment (estimated creatinine clearance <50 mL/min by Cockcroft Gault)
• History of or current significant pulmonary, cardiac, neurological or psychiatric disease, substance dependence, porphyria, malignancy (with exception of local cutaneous squamous or basal cell carcinomas or local cervical squamous cell carcinoma resolved after resection) or hypothyroidism (unless euthyroid at Screening Visit and treated with a stable dose of medication for at least 3 months prior to the Baseline Visit).
• The Investigator should discuss any subject with TSH outside the normal range with the Medical Monitor prior to enrollment.
• Participation in a clinical drug trial within 4 weeks of Screening Visit.