Among the 16 monozygotic twin pairs described in the literature, only five pairs were concordant for narcolepsy, suggesting that non-genetic factors may predispose to narcolepsy. We report a completely discordant narcoleptic twin, with an onset of narcolepsy at the late age of 40 years. Monozygocity was confirmed. The onset of narcolepsy was associated with prolonged sleep deprivation and chronic emotional stress. It was noted that excessive daytime sleepiness preceded the onset of cataplexy, when narcolepsy was precipitated by prolonged sleep deprivation and emotional stress. Our case supports the multifactorial model with dominant human leukocyte antigen inheritance in the development of narcolepsy. Our observation implies that avoidance of poor sleep hygiene might prevent the development of narcolepsy, even in predisposed individuals.