Idiopathic Hypersomnia (IH) is a condition where people feel extremely sleepy during the day, especially in the morning, even if they sleep a lot at night. They may have trouble waking up in the morning, no matter how much they sleep (sometimes more than 11 hours per day), and they can't help feeling tired, even after taking daytime naps. Because of this sleepiness, they may have trouble focusing, thinking clearly, or keeping up with daily activities. They may also have symptoms like dizziness or feeling lightheaded. Orexin is a chemical made in the brain that helps keep a person awake and alert. TAK-360 acts like orexin. Previous studies have shown that medicines that act like orexin may keep people awake. The main aim of this study is to learn how safe TAK-360 is and how well adults with IH tolerate it. Researchers also want to find out if TAK-360 can help people with IH stay awake and how much TAK-360 is needed to do that. Participants will be randomly (by chance, like drawing names from a hat) chosen to receive either TAK-360 or a placebo. The placebo looks just like TAK-360 but does not have any medicine in it. Using a placebo helps researchers learn about the real effect of the treatment.