The chronicity of sleep disturbance and its relation to co-occurring symptoms in children with autism spectrum disorder (ASD) are not well understood. The current study examined longitudinal relations among sleep and co-occurring symptoms in a large well-characterized sample of 437 children with ASD assessed at baseline and follow-up (M = 3.8 years later). Twenty-three percent experienced worsening sleep problems over time, while 31.5% showed improvement. Path analysis indicated that sleep problems at baseline predicted later development of ADHD symptoms in younger children and somatic complaints in older children. For younger children, sensory over-responsivity predicted future sleep problems. Findings suggest that sensory over-reactivity may contribute to sleep problems in some children with ASD, and that sleep problems may result in poor daytime functioning.