A Study Evaluating the Efficacy and Safety of Xywav Expanded Dosing vs Placebo in Participants With Narcolepsy or IH

The trial has 3 treatment periods: open label titration and optimization period (OL-TOP), stable dose period (SDP) and the double-blind randomized withdrawal period (DBRWP). Participants will be evaluated by their disease cohort (Narcolepsy and IH). Once eligibility to participate in the trial is confirmed, eligible participants will begin OL-TOP. Participants with narcolepsy and IH will be assigned to receive either a once-nightly expanded dosing of Xywav or a twice-nightly expanded dosing of Xywav. Assignment is dependent on participant's standard oxybate treatment and the treating investigator's decision. During OL-TOP, participants' Xywav dosing will be adjusted until they achieve a stable dose. This period can last up to 12 weeks. Once a stable dose is achieved, participants will begin the SDP. During the SDP, all participants will receive the stable Xywav dose for 2 additional weeks. Afterwards, the participants will begin the DBRWP. During this period, participants will be randomized to either continue on their stable dose of Xywav or receive placebo for 2 more weeks. In total, the participants will receive treatment in the trial for up to 16 weeks.