This chapter describes some of the commonly occurring sleep disturbances in PWS patients through a case study. OSA can have serious consequences ranging from hypersomnia, hypertension and heart failure to obesity-hypoventilation syndrome and death. Major risk factors for sleep-disordered breathing in PWS include craniofacial dysmorphism with small nasal and oropharyngeal spaces, obesity, and hypotonia. An attended, in-laboratory, nocturnal polysomnogram is recommended for the diagnosis of OSA in children. This case also highlights the critical importance of weight management in obese children with OSA and the initiation of continuous positive airway pressure in those individuals with OSA who are not appropriate surgical candidates. Hypersomnia in PWS is complex; is not always due solely to OSA and may not resolve with treatment of OSA alone. This suggests a central nervous system- or brain-related origin to hypersomnia in PWS. Central hypersomnias should be considered in those with persistent hypersomnia despite the therapeutic resolution of OSA. Diagnosis and treatment options for narcolepsy due to a medical condition are discussed.