Narcolepsy Sleep Architecture

A 32-year-old man presents with a 3-year history of irresistible daytime sleep attacks and sudden episodes of bilateral muscle weakness triggered by laughter at social gatherings. His wife reports he experiences vivid dream-like visions while falling asleep. Nocturnal polysomnography followed by Multiple Sleep Latency Test (MSLT) shows a mean sleep latency of 6 minutes with sleep-onset REM periods (SOREMPs) appearing within 15 minutes of sleep onset on 3 of 5 nap opportunities. The sleep architecture pattern marked **C** in the diagram is most consistent with this presentation. Which of the following best explains the underlying pathophysiology of the condition shown in **C**?