OBJECTIVE: The Corona Virus Disease-19 (COVID-19) pandemic has evolved into the largest public health event in the world. Earlier COVID-19 studies have reported that the pandemic caused widespread impacts on mental health and sleep in the general population. However, it remains largely unknown how the prevalence of mental health problems and sleep disturbance developed and interacted in adolescents at different times in the epidemic. METHODS: 831 teenagers (aged 14-19) underwent a longitudinal follow-up study to evaluate the prevalence of mental health problems and sleep disturbance among adolescents before, during, and after the COVID-19 breakout in China and to explore the interaction between mental health and sleep across the three measurements. The chronotype, anxiety and depression level, sleep quality, and insomnia were investigated during each measurement. RESULTS: The adolescents had delayed sleep onset and sleep offset time, longer sleep duration during the quarantine than before and after the epidemic, whereas their chronotype tended to morning type during the epidemic. Yet, the highest prevalence of anxiety, depression, poor sleeper, and insomnia symptoms were observed before but not during the COVID-19 breakout. The females and adolescents who were eveningness type showed significantly higher anxiety and depression levels, poorer sleep quality, and severe insomnia status than the males and the intermediate and morning types. Sleep disturbance was positively associated with mental problems among three measurements. Pre-measured depression level significantly predicted sleep disturbance level at follow-ups. CONCLUSION: These findings suggested that adolescents' high prevalence of mental health and sleep problems occurred before the COVID breakout and decreased during and after the epidemic. Gender and chronotype were significant risk factors associated with affective and sleep disturbances. Depression positively predicted later sleep problems, but not vice versa.