OBJECTIVES. Associations between sleep and internalizing problems are complex and poorly understood. To better understand these covarying difficulties, genetic and environmental influences were estimated by using a twin design.

METHODS. Three hundred 8-year-old twin pairs reported on their anxiety and depression by completing the Screen for Childhood Anxiety Related Emotional Disorders and the Children's Depression Inventory. Parents reported on their children's sleep problems by completing the Child Sleep Habits Questionnaire.

RESULTS. Children reported by their parents to have different types of sleep problems self-reported more depression symptoms than those without. The correlation between total sleep-problem score and depression was moderate. That between sleep problems and anxiety was smaller and was not examined further. The association between sleep problems and depression was mainly explained by genes, and there was substantial overlap between the genes influencing sleep problems and those influencing depression. There was smaller influence from environmental factors making family members alike, and environmental factors making family members different decreased the association between sleep problems and depression.

CONCLUSIONS. A range of sleep difficulties are associated with depression in school-aged children, and the overall association between the 2 difficulties may be largely influenced by genes.

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