Self-reported sleep data from 2238 monozygotic and 4545 dizygotic adult twin pairs indicated a significant hereditary effect on sleep length [overall heritability estimate (h2 = 0.44)] and on sleep quality (h2 = 0.44). When the data were examined in subgroups defined by sex, age (18–24 years and 25 or more years of age), and cohabitation status of the twin pair, the highest heritability estimates for sleep length were for twins living together aged 25 or older. For twins living apart the heritability estimates were statistically significant in all women and men aged 25 or older. For sleep quality significant heritability estimates were found for all groups except women living together.