This study is testing a medicine called HBS-301 for adults with idiopathic hypersomnia (IH), a condition that makes people unusually sleepy during the day. The study is randomized, meaning participants are chosen by chance to receive either the real medicine or a placebo (a fake treatment) without knowing which one they're getting. It has several stages, starting with a screening period of up to 28 days to see if you qualify, followed by 8 weeks of treatment, and an optional year-long period where everyone gets the real medicine. After that, there's a 30-day check-up to make sure the medicine is safe.

• Study lasts up to more than a year including all periods.
• Participants will have regular visits and treatments.
• Some people might get a placebo during the first 8 weeks.