Chronobiological changes due to school closures during the COVID-19 pandemic among adolescents in the DOrtmund Nutritional and Anthropometric Longitudinally Designed cohort study

<jats:title>Abstract</jats:title><jats:p>Due to the lockdown of schools as one of the COVID-19 control measures, adolescents have had the opportunity to re-organise their daily lives; e.g. some of them have adapted their bedtimes to the new situation during the lockdown in favour of their own chronotype. Hence, we investigated differences in chronobiological characteristics (e.g., the midpoint of sleep, sleep duration or social jetlag (SJL); i.e., a discrepancy between biological and social timing) before and during the pandemic lockdown to examine potential changes. We asked participants from the ongoing open cohort Dortmund Nutritional and Anthropometric Longitudinally Designed (DONALD) study to fill out the Munich Chronotype Questionnaire during the COVID-19 lockdown and received the information of participants (<jats:italic>n</jats:italic> = 66) during the pandemic. A reference group matched for age, season, and sex was randomly selected from the DONALD study to assess participants’ chronobiological characteristics prior to (<jats:italic>n</jats:italic> = 132) the pandemic. Analyses of covariance were applied to examine differences between the two groups reflecting the situation before and during the COVID-19 pandemic. Participants were aged 9–18 years (52% males). In the current examination, average sleep duration across the week was higher among adolescents during the pandemic (<jats:italic>β</jats:italic> = 00:30; <jats:italic>p</jats:italic> = 0.0006) and social jetlag was significantly lower (<jats:italic>β</jats:italic> =  −00:39; <jats:italic>p</jats:italic> &lt; 0.0001).</jats:p><jats:p><jats:italic> Conclusion</jats:italic>: Our results showed that the COVID-19 lockdown enabled adolescents to adapt their sleeping habits according to their naturally late chronotype, which led to a significant reduction in SJL. These observations are likely to be explained by the effect of school closure.<jats:table-wrap><jats:table><jats:tbody> <jats:tr> <jats:td colspan="2"><jats:bold>What is Known:</jats:bold><jats:italic>• Under ‘normal conditions’ without pandemic lockdowns, adolescents accumulate a lack of sleep due to social obligations such as school starts, resulting in social jetlag.</jats:italic>• <jats:italic>A late chronotype or exposure to social jetlag is a known risk factor for the development of chronic diseases.</jats:italic></jats:td> </jats:tr> <jats:tr> <jats:td colspan="2"><jats:bold>What is New:</jats:bold><jats:italic>• </jats:italic><jats:italic>The COVID-19 lockdown represents a ‘natural experiment’ that enables adolescents to adhere to their internal biological clock. Without the usual social obligations, social jetlag can be significantly reduced.</jats:italic></jats:td> </jats:tr> </jats:tbody></jats:table></jats:table-wrap></jats:p>