Increased daytime sleepiness, hypersomnia, is a very common symptom in the general population and may cause serious problems in their lives. Hypersomnia may be idiopathic or may occur secondary to other aetiologies. Infections and vaccinations may also be related to hypersomnias. A 74-year-old man complaining of muscle weakness after severe diarrhoea was admitted and diagnosed with Guillain-Barre syndrome (GBS). He also complained of increased daytime sleepiness since the emergence of his weakness, for which all metabolic and endocrine aetiologies were excluded. A full night polysomnography and multiple sleep latency test revealed objective daytime sleepiness and tree episodes of rapid eye movements during sleep periods. Because of the temporal association, the patient was diagnosed with secondary narcolepsy secondary to post-infectious GBS.