The investigation of sleep in twins represents one of the major methods for measuring the genetic contributions to sleep in humans. This paper reviews two twin studies in which the sleep EEG was recorded during three consecutive nights in young monozygotic (MZ) and dizygotic (DZ) male twins. The analyses, based on average values of repeated sleep recordings, indicate that a significant proportion of variance in stages 2, 4, and delta sleep appears to be genetically determined. Genetic influences on rapid-eye-movement sleep were found inconclusive, but this conclusion is limited by the relatively small size of the sample studied.