Longer sedentary time and insufficient sleep are common and potentially serious problems among adolescents and have substantially adverse effects on their physical and mental health. In this school-based study, we conducted an ecological momentary assessment using actigraphy to examine the within-subject association between sedentary time and sleep duration among 80 Korean adolescents aged 12-17. Objectively measured sedentary time and sleep duration were recorded over 5 days; participants also completed a self-reported questionnaire and sleep logs. Using a generalized estimating equation, analysis revealed that daily sedentary time was significantly and negatively associated with sleep duration ([beta] = -0.36, p = 0.028). Additionally, the interaction between age and sedentary time significantly affected sleep duration ([beta] = 0.03, p = 0.012). These findings suggest the need for school-based interventions that aim to reduce sedentary time and thereby improve sleep duration, helping adolescents, especially younger ones, to achieve a healthier lifestyle.