This study is testing a new medicine, FT218, for people with idiopathic hypersomnia, a condition that makes you very sleepy during the day without a known cause. FT218 is a special version of sodium oxybate, a medicine taken at night. The study is "double-blind," meaning neither the participants nor the researchers know who is getting FT218 or a placebo (a non-active substance). Participants may already be using other sleep medicines. The study will last about 42 weeks, and there is an extra period where everyone will know they are getting FT218 to check its safety.

To join, you must have idiopathic hypersomnia, sleep over 7 hours on average each night, and if you're a woman who can have children, use contraception. You cannot join if you are pregnant, have other sleep problems, or have certain mental health issues.

• The study lasts approximately 42 weeks, with some parts where everyone gets the real medicine.
• You can be on other stable sleep or alertness medicines when you start.
• You cannot join if you've used FT218 before or have certain health issues.