MDMA (ecstasy) is an illicit drug which has pharmacological actions on the serotonin system, leading to a number of physiological and behavioral changes. Research conducted in both animals and humans has focused on how ecstasy use affects systems or functions in which serotonin has a regulatory role including mood, sleep and circadian rhythms. In this paper we review the evidence with respect to changes in sleep and circadian rhythms following ecstasy use. Studies of the subjective measurement of sleep have suggested that there are changes in sleep quality and duration following ecstasy use, while research utilizing objective measures including polysomnography has highlighted changes in sleep architecture following ecstasy use. Collectively these findings suggest that there are consequences associated with ecstasy use, and the implications of these findings for ecstasy users will be examined. Finally, preliminary evidence from the animal literature implicating ecstasy as having specific effects on the circadian system will be reviewed. A discussion of the limitations of the current evidence for such a claim is presented, and possible directions for future research are explored.