The impact of COVID-19 pandemic on sleep duration and mental health among middle school students: a 3-year cross-sectional survey.

Background: Since the outbreak of COVID-19 in late 2019, and until the Chinese government downgraded the pandemic control measures to Category B management in January 2023, various epidemic prevention measures were implemented across regions based on the local spread of the virus. Correspondingly, educational formats shifted between online and offline teaching according to the pandemic situation. Changes in daily life and teaching methods, along with the high initial mortality rate of COVID-19, have had varying degrees of negative impact on the mental health (MH) of adolescents.