The study is designed to look at the usefulness and safety of AXS-12 or placebo for the treatment of Narcolepsy. The open-label treatment period is 24 weeks, which will be followed by a double-blind treatment period of 4 weeks, for a total of up to 28 weeks with 13 visits. Narcolepsy is a chronic sleep disorder characterized by overwhelming daytime drowsiness and sudden attacks of sleep. Study staff nor the participants will know if they are receiving a study drug or placebo. Participants will be male or female subjects 15-75 years old with a diagnosis of narcolepsy. Study drug will be administered as a pill.

The current study will investigate the safety, efficacy and pharmacokinetics of serdexmethylphenidate (SDX) compared to placebo in subjects with Idiopathic Hypersomnia (IH). The study may inform about the best dosing regimen, optimal dose range, duration of treatment, and secondary endpoints for a future Phase 3 study.