This study examined sleep–wake patterns in 3 matched comparison groups of preschool-aged children: children with autism (AUT), children with developmental delay (DD) without AUT, and children who are developing typically (TYP). Sleep was assessed via actigraphy and parent-report diaries for 7 consecutive 24-hr periods across 3 time points: at enrollment (n = 194), 3 months later (n = 179), and 6 months after enrollment (n = 173). At each recording period, children in the AUT group slept less per 24-hr period, on average, and were less likely to awaken at night than children in the other two groups. In contrast, children in the DD group had more frequent and longer duration nighttime awakenings than children in the AUT group. Overall, children in the 2 neurodevelopmentally disordered groups demonstrated more night-to-night variability in their sleep–wake measures than children in the TYP group.