OBJECTIVE: To estimate genetic and environmental influences on parasomnias and dyssomnias and their association in 8-year-olds. DESIGN: Parents of twins completed the Children's Sleep Habits Questionnaire. SETTING: Families were primarily tested at the Institute of Psychiatry, London, England. PARTICIPANTS: A total of 300 pairs of 8-year-old twins and their parents participated in the study. MAIN OUTCOME MEASURES: Sleep difficulties in children. RESULTS: Individual differences in parasomnias and dyssomnias were largely explained by genes (accounting for 50% and 71% of the variances, respectively). The rest of the variances were mainly due to nonshared environmental influences. A moderate association was found between parasomnias and dyssomnias (r = 0.42), which was mainly accounted for by genetic influences (87%). The genetic correlation between parasomnias and dyssomnias was moderate (r = 0.61). In contrast, the nonshared environmental correlation was small (r = 0.10). CONCLUSIONS: The decomposition of the association between parasomnias and dyssomnias suggests that there may be different manifestations of shared underlying genetic risks for sleep problems partly dependent on nonshared environmental influence.