This study is testing a drug called pitolisant on patients with Prader-Willi Syndrome (PWS), a condition that often leads to excessive daytime sleepiness (EDS). The goal is to see if pitolisant can help reduce EDS in people aged 6 and older with PWS. The study also looks at how pitolisant affects behaviors like irritability, hyperphagia (overeating), social withdrawal, and hyperactivity.

Participants will first go through a Screening/Baseline Period lasting up to 45 days. They will then be randomly assigned to receive either pitolisant or a placebo (a pill with no active drug) for 77 days. In-person checkups are required on Days 29, 57, and 77. Afterward, there is an optional Open-Label Extension Period where everyone receives pitolisant for up to 441 days with visits on Days 113, 260, and 441. Follow-up visits occur 15 and 30 days after stopping the drug.

• Participation requires several in-person visits over a long duration.
• There is an option to continue receiving the drug after the main study period.
• Participants cannot join if they have certain sleep disorders or are in other studies.