The issue of whether a high carbohydrate intake affects sleepiness and sleep variables has been studied in normals but not in patients suffering from narcolepsy, despite anecdotal evidence that sugars may facilitate sleepiness in this population. This study investigated whether the intake of 50 g glucose exacerbated sleepiness in narcolepsy subjects. A double-blind cross-over study, involving 12 narcolepsy subjects and 12 matched controls, measured behaviour after a light lunch supplemented with a drink of either 50 g glucose or placebo (artificially sweetened drink). The main dependent variables were the performance and EEG measures from the Wilkinson Auditory Vigilance Task (WAVT) and sleep variables from a 45 minute nap. The results indicate that in the narcolepsy subjects glucose was associated with decreased wake duration, reduced sleep onset latency and more spontaneous and induced sleep stage changes during the WAVT, while the nap revealed an increased intensity of sleepiness after glucose as measured by the Polygraphic Score of Sleepiness. Eleven of the twelve narcolepsy subjects showed increased REM duration in the nap after glucose. The findings are discussed in relation to serotonin synthesis, basal sleepiness and possible irregularities in the action of insulin.