In a study just published on-line in the journal Twin Research and Human Genetics -with Dr. Saskia te Velde as first author- we assessed to what extent lack is associated with genetic and environmental factors. Lack of sleep is associated with physical and mental health. Lack of sleep is an important predictor of overweight, and as we published in an earlier study: many children across Europe sleep fewer than 8-9 hours and their sleep duration is associated with overweight and obesity. A twin-family design was used to gain insight into the genetic and environmental contributions to variation in sleep duration; data came for the Netherlands Twin Registry. The study sample consisted of 6,319 adolescent twins and 1,359 non-twin siblings, aged 12 to 20 years Results showed that the prevalence of shorter than optimum sleep duration, that is, less than 8 hours per night, was high, with the highest prevalence rates in later adolescence. The contribution of genetic and environmental factors to individual differences in sleep duration was dependent on age. Variation in sleep duration at the age of 12 years was accounted for by genetic (boys: 34%, girls: 36%), shared environmental (boys: 28%, girls: 45%), and non-shared environmental factors (boys: 38%, girls: 19%). At the age of 20 years, the role of genetic (boys: 47%, girls: 33%) and non-shared environmental factors (boys: 53%, girls: 67%) was more pronounced. It can be concluded from the results that individual differences in sleep duration were accounted for by genetic and non-shared environmental factors throughout adolescence, whereas shared environmental factors account for a substantial part of variation during early adolescence only.