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BACKGROUND: Sleep duration and physical activity have been associated with internalizing problems. However, genetic confounding and measurement error may introduce bias. We assessed genetic confounding in the associations of modifiable lifestyle with internalizing problems using device-based and questionnaire assessments to estimate shared genetic risk across different assessments in adolescents.METHODS: In this preregistered study in the Adolescents Brain Cognitive Development cohort, we included European adolescents with both self-reported and device-based sleep duration (N = 2283) and moderate/vigorous physical activity (MVPA, days/week, N = 2772). We used the Brief Problem Monitor to assess self-reported internalizing problem scores. Genetic sensitivity analyses were conducted to assess genetic confounding by combining polygenic scores and molecular-based heritability of internalizing problems.RESULTS: Longer sleep duration was associated with lower internalizing problems using both self-reported (-0.15-SD, SE = 0.02-SD) and objective (-0.10-SD, SE = 0.02-SD) assessments. More frequent MVPA was associated with lower internalizing problems using both self-reported (-0.09-SD, SE = 0.02-SD) and device-based (-0.06-SD, SE = 0.02-SD) assessments. Substantial genetic confounding (81%) was found between self-reported sleep duration and internalizing problems, predominantly among boys. No clear evidence of genetic confounding was found in the associations of device-based sleep duration or either measures of MVPA with internalizing problems.CONCLUSION: The observed negative relationship between reported child sleep duration and internalizing problems may be partly due to genetic confounding, particularly among boys. This genetic influence likely captured reporting measurement error of shared method variance. The impact of adolescent sleep duration on internalizing problems may be overestimated by self-reports, especially among boys, whereas associations of physical activity were not genetically confounded.