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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. Sleep problems are common in autism spectrum disorder (ASD) and different factors can contribute to its occurrence in this population. Misalignment of the biological clock (our circadian system) has been described as one possible explanation. While there is a body of research on sleep problems, relatively less is known about circadian functioning and the specific population of autistic children with co-occurring attention deficit hyperactivity disorder (ADHD). Using an ambulatory circadian monitoring (ACM) system, which resembles a common watch, we gathered sleep parameters and the different rhythms obtained from measuring motor activity, light exposure and distal temperature in 87 autistic children and adolescents, 27 of whom were diagnosed with co-occurring ADHD, and 30 neurotypical children and adolescents as a comparison group. Autistic children and, especially, those with co-occurring ADHD showed greater motor activity during sleep which would be worth studying in future projects which could better define this restless sleep. Of note, we observed an atypical pattern of wrist temperature, with higher values in neurotypical children, followed by autistic children and, ultimately, those with co-occurring ADHD. Temperature is one of the most valuable factors evaluated here as it is closely connected to sleep-wakefulness and the hormone melatonin. Its special pattern during day and nighttime would support the hypothesis of an atypical secretion of melatonin in autistic individuals which would also link with the higher presence of sleep problems in this neurodevelopmental condition. [ABSTRACT FROM AUTHOR]