Atypical baseline levels of autonomic activity and sleep difficulties have been previously reported in persons with autism and indicate a heightened level of sympathetic activity. This project seeks to determine the neurochemical systems involved in producing atypical autonomic responses in children with autism. In this study, researchers will measure circadian rhythms of cortisol and alpha-amylase and their relationship to tonic pupil size in two to five year old children with autism spectrum disorder (ASD). These measures will be taken on two separate days on a group of 20 children with a diagnosed ASD, 20 children with Down syndrome, and 20 typically-developing children. The determination of the neurochemical component responsible for atypical autonomic responses could lead to identification of a neural impairment in autism that is potentially primary to other impairments and could lead to pharmacological and/or behavioral interventions targeted at preventing this impairment from causing a cascade of effects. If successful, the development of non-invasive autonomic and salivary measures could also provide easy-to-use markers for screening and detecting autism at a very early age.