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Purpose of review Although several studies in the last years have evaluated obesity, obstructive sleep apnea (OSAS), and excessive daytime sleepiness (EDS) in patients with Prader-Willi syndrome (PWS), their pathophysiologies and interactions and the role of treatment with growth hormone are not completely understood. The present review analyzes the contributing role of obesity, OSAS, and sleep structure abnormalities in determining the EDS and the role of specific treatment in improving the clinical outcome. Recent findings The studies on sleep structure of PWS patients show abnormalities of rapid eye movement (REM) sleep and a decrease in non-REM sleep instability, corroborating the hypothesis of the presence of a primary disorder of vigilance and the similarities with narcolepsy. These sleep alterations might also be linked to the action of mediators of inflammation (i.e. adiponectin or cytokines) determined by obesity. Obesity and hypothalamic dysfunction could be responsible for the primary abnormalities of ventilation during sleep that, in turn, might contribute to EDS. Although EDS seems to resemble narcolepsy, PWS patients do not present the other typical symptoms of narcolepsy. Summary The most consistent hypothesis for linking the three different symptoms of PWS is a primary central hypothalamic dysfunction. Further research is needed to evaluate the contribution of the upper airway resistance syndrome in the pathogenesis of EDS, the role of the alterations of sleep microstructure, the relationships between PWS and narcoleptic phenotype, the involvement of orexin/hypocretin, and the effects of drugs acting on REM sleep and/or wakefulness.