Sleep disturbance and irregular hormone profiles are extremely common in children with autism spectrum disorder (ASD), suggesting impairments in the circadian timing system. Though similar sleep and circadian disturbances have been observed in adolescents and adults with ASD, early development of these functions has not yet been examined. Since ASD is only diagnosed reliably after 2-3 years of age, the proposed research will take the innovative approach of examining sleep and hormone rhythms in infants at greater risk for developing ASD because they have an older sibling with the disorder. Sleep behaviors and hormone fluctuations will be characterized within high- and low-risk infants at three different ages that represent important milestones in the early development of the circadian system: 3, 9, and 12 months. Patterns of light exposure, an important environmental factor that regulates both sleep and circadian rhythms (but has been largely ignored in research to date) will also be examined. Utilizing a battery of combined circadian and photic measures within individuals is currently the only way to obtain information regarding the mechanistic basis for disturbances in humans; however, such a strategy has not yet been employed in ASD. Furthermore, all proposed methods can be applied even at the earliest of ages and are relatively inexpensive, non-invasive, and not subject to performance or behavioral variability. This study will allow us to identify the mechanisms underlying sleep and circadian disturbances in ASD. In addition, establishing when and how these abnormalities first present will increase our understanding for the role of sleep and the circadian system in ASD. Examination of high-risk infants may reveal the development and origins of circadian atypicalities, providing insight into risk factors, etiology, and potential early biomarkers of ASD. Ultimately, better understanding the sleep and circadian dysfunction often co-occurring with ASD may help guide treatment strategies and minimize the negative impact of these disturbances on both the children and their families.