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.

Key Inclusion Criteria:

1. The participant weighs greater than or equal to (≥) 40 kilograms (kg) and has a body mass index (BMI) between 16 and 38 kilograms per meter square (kg/m^2) [inclusive].
2. The participant has a documented, current diagnosis of IH.

Key Exclusion Criteria:

1. The participant has a current medical disorder associated with excessive daytime sleepiness (EDS) [other than IH].
2. The participant has medically significant thyroid disease.
3. The participant has a history of cancer in the past 5 years. (This exclusion does not apply to participants with carcinoma in situ [such as basal cell carcinoma] that has been treated and is stable, or who have been stable without further treatment. These participants may be included after approval by the medical monitor.)
4. The participant has any of the following viral infections based on a positive test result: Hepatitis B surface antigen (at screening), hepatitis C virus antibody (at screening), human immunodeficiency virus (HIV) antibody/antigen (at screening).
5. The participant has a clinically significant history of head injury or head trauma.
6. The participant has history of epilepsy, seizure, or convulsion (exception for a single febrile seizure in childhood).
7. The participant has a history of cerebral ischemia, transient ischemic attack (less than [<]5 years from screening), intracranial aneurysm, or arteriovenous malformation.

Takeda provides access to the de-identified individual participant data (IPD) for eligible studies to aid qualified researchers in addressing legitimate scientific objectives (Takeda's data sharing commitment is available on https://clinicaltrials.takeda.com/takedas-commitment?commitment=5). These IPDs will be provided in a secure research environment following approval of a data sharing request, and under the terms of a data sharing agreement.

IPD from eligible studies will be shared with qualified researchers according to the criteria and process described on https://vivli.org/ourmember/takeda/. For approved requests, the researchers will be provided access to anonymized data (to respect patient privacy in line with applicable laws and regulations) and with information necessary to address the research objectives under the terms of a data sharing agreement.