Excessive daytime sleepiness (EDS), nocturnal impaired-awareness episodes, and abnormal sleep-related behaviors have complex and overlapping etiologies. Narcolepsy, obstructive sleep apnea (OSA), and epileptic nocturnal events may present with similar clinical manifestations. In patients with intellectual disability (ID), restricted self-reporting, incomplete caregiver observations, and diagnostic overshadowing may lead clinicians to misattribute underlying neurological or sleep disorders to preexisting cognitive impairment or psychiatric and behavioral problems, resulting in delayed diagnosis or misdiagnosis. We present an illustrative case of a 34-year-old woman with lifelong cognitive and adaptive impairments who was referred for long-standing snoring, EDS, and two nocturnal impaired-awareness episodes. An outside emergency department had previously considered “sleep apnea?” and “narcolepsy?” as possible diagnoses. After detailed collateral history-taking, overnight polysomnography (PSG), the Multiple Sleep Latency Test (MSLT), and two 24-hour ambulatory electroencephalography (EEG) recordings, she was diagnosed with epilepsy with focal to bilateral tonic-clonic seizures, moderate OSA, and mild ID. At the 6-month follow-up after concurrent initiation of levetiracetam (LEV) and continuous positive airway pressure (CPAP) therapy, no further nocturnal impaired-awareness episodes, limb jerking, or cyanosis of the lips were reported; her EDS also improved, with the Epworth Sleepiness Scale (ESS) score decreasing from 18/24 at baseline to 6/24. Rather than reporting a rare comorbidity alone, this case-based diagnostic analysis uses the index case to clarify the differential diagnostic approach to EDS and nocturnal events in patients with ID, highlights the key distinctions among narcolepsy, OSA-related sleepiness, and epileptic nocturnal events, and emphasizes the need for integrated management when OSA and epilepsy coexist.