Sleep Onset REM Period Narcolepsy

A 28-year-old male from Delhi presents with a 2-year history of irresistible daytime sleep attacks and sudden loss of muscle tone triggered by laughter. Overnight polysomnography confirms 7 hours of sleep with no obstructive apnoea. The Multiple Sleep Latency Test (MSLT) is performed with five 20-minute nap opportunities at 2-hour intervals. The diagram shows the sleep architecture during one of the nap periods. The structure marked **A** in the diagram represents a sleep-onset REM period occurring within 15 minutes of sleep onset. If two such events are documented across the MSLT naps, combined with a mean sleep latency of 6 minutes, what is the most likely diagnosis?