National trends in sleep sufficiency and sleep time among adolescents, including the late-COVID-19 pandemic, 2009-2022: A nationally representative serial study in South Korea

•Over the 14-year period, both sleep duration among South Korean adolescents demonstrated a consistent downward trend.•A noteworthy decrease in sleep sufficiency was observed during the pandemic compared to the pre-COVID-19 period (2009–2019).•Regardless of the pandemic period, there was a decrease in sleep time, with no difference in the slopes of these decreases.•It is necessary for the society to solve not only the downward trend in sleep sufficiency but tackle the underlying cause.