The circadian rhythm functioning and sleep patterns of 10 adults with Asperger syndrome were investigated using actigraphy. When compared with data from neurotypical adults, both statistical and clinically significant differences were found between the two groups, with the adults with Asperger syndrome showing marked abnormalities in both the quantity and the quality of sleep recorded. Examination of the actigraphic data indicated low sleep efficiency and high fragmentation as being characteristic of the sleep of participants with Asperger syndrome. These individuals also showed lower-amplitude circadian rhythms that were less strongly linked to environmental synchronizers, but no evidence of significant desynchronization of circadian rhythm. Possible mechanisms for these abnormalities and implications for clinical practice are discussed.