Repetitive thought has been focused upon as a transdiagnostic risk factor for depression, anxiety, and poor physical health. Among the forms of repetitive thought, rumination and worry are considered to play important roles in the onset and maintenance of insomnia. However, there have been few attempts to clarify the similarities, differences, and interaction between the functions of rumination and worry in sleep problems. Furthermore, no study has investigated the prospective relationships between these two forms of repetitive thought and sleep disturbance. In the present study, we examined the prospective associations between repetitive thought and subjective sleep quality, measured by a self-report questionnaire. A total of 208 undergraduates participated in a 2-wave longitudinal survey with an interval of 3weeks between assessments. Hierarchical multiple regression analyses showed that baseline rumination predicted reduction in the follow-up assessment of subjective sleep quality, controlling for levels of depressive and anxious symptoms. This main effect of rumination was qualified by the levels of worry; for individuals with higher levels of worry, rumination was associated with greater reduction in subjective sleep quality. These results suggest that both rumination and worry have unique associations with sleep and that their interaction is especially important in sleep problems.