Subjects will complete 25-hours of hourly salivary, plasma sampling biweekly. Melatonin will be measured in urine, for children under the age of 3. Sleep and behavioral questionnaires will be completed by parents and/or teachers. This study involves an optional, 1-year melatonin treatment sub-study, in which the children will take 0.1 - 3mg daily. A second optional sub-study involves a study of the child's sleep through polysomnography, in order to identify possible sleep disorders.