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Anxiety symptoms and sleep problems typically emerge during adolescence and are frequently intertwined. However, there is a dearth of knowledge concerning their reciprocal influence and whether physical activity might play a protective role in this relationship. The present study aims at filling this gap exploring also the moderating role of sex. 915 13-year-old Swedish adolescents (56% girls) answered a survey conducted four times: at ages 13 (T1), 16 (T2), 19 (T3), and 22 (T4). A random intercept cross-lagged panel model was used. At within-levels, sleep problems and anxiety symptoms had a bidirectional positive association in middle adolescence. Vigorous physical activity and anxiety symptoms showed a reciprocal negative association from middle adolescence. Vigorous physical activity and sleep problems were reciprocally associated only in late adolescence. Associations were the same for girls and boys. This study demonstrated that the relations between anxiety symptoms, sleep problems, and vigorous physical activity cannot be understood without adopting a developmental perspective and that middle adolescence is a crucial period to plan interventions to reduce anxiety symptoms and sleep problems.