STUDY OBJECTIVES: We examined whether small body size at birth and prenatal tobacco or alcohol exposure predict poor sleep and more sleep disturbances in children. DESIGN: An epidemiologic cohort study of 289 eight-year-old children born at term. MEASUREMENTS AND RESULTS: Sleep duration and efficiency were measured by actigraphy for 7 consecutive nights (mean = 7.1, SD = 1.2). We used both continuous measures of poor sleep and binary variables of short sleep and low sleep efficiency (