Sleep problems are prevalent in autism spectrum disorder (ASD) and have a range of causes, including circadian rhythm misalignment. However, little is known about sleep in autistic children with co-occurring attention deficit hyperactivity disorder (ADHD). A cross-sectional descriptive and analytic study was conducted in 87 autistic children and adolescents (10.22 ± 3.36), 27 of whom had a diagnosis of ADHD, and 30 children with typical development (9.07 ± 2.75). An ambulatory circadian monitoring device was used to record wrist temperature (WT), motor activity, sleep, and light intensity. Higher WT values were detected in the comparison group, followed by autistic children and, ultimately, participants with co-occurring ADHD. In addition, atypical daily WT patterns were observed in autistic children, with slope flattening during afternoon and evening hours and an absent or subtle postprandial peak. Participants with co-occurring ADHD showed greater motor activity during sleep. Our findings, especially concerning WT rhythm, appear to support previous findings on the etiology of sleep problems in autism, as WT is closely connected to sleep-wakefulness. We found specific sleep patterns in autistic children depending on the presence of co-occurring ADHD, particularly regarding sleep maintenance, which may be an area of future research.