The etiologic role of genotype and environment in sleep pattern (daytime napping, habitual bedtime, and sleep duration) and subjective sleep quality and sleep disturbance was examined using a general population sample of 3,810 adult Australian twin pairs, aged 17-88 years. Genetic differences accounted for at least 33% of the variance in sleep quality and sleep disturbance and 40% of the variance in sleep pattern. There was no evidence for a decline in the importance of genetic predisposition with age. Short-term environmental fluctuations accounted for as much as 30% of the variance, and more stable nonfamilial environmental effects accounted for the remainder. No effect of shared family environment on sleep characteristics was found.