Chronotype, social jetlag and body composition in German students: what is the role of physical activity behaviour and did the lockdown affect this?

Introduction: Young adults have a later chronotype and are thus vulnerable for a discrepancy in sleep rhythm between week and weekend, called social jetlag (SJL). A later chronotype has been shown to be associated with a higher BMI, yet it is not clear whether this association is specific to visceral fat mass or skeletal muscle mass and in how far physical activity behaviour acts as a mediator. The COVID-19 lockdown in spring 2020 might have affected circadian rhythmicity including changes in physical activity behaviour. Objective: This study analyzed (i) whether chronotype or social jetlag associate with skeletal muscle mass or visceral fat mass, (ii) whether this may be attributable to physical activity behaviour and (iii) whether the lock-down resulted in chronotype-specific changes in physical activity behaviour. Methods: From September to January 2020, 320 students were recruited (age 18 to 25 y, 43% male) at Paderborn University. Chronotype was assessed by the Munich Chronotype Questionnaire and defined as midpoint of sleep on free days corrected for sleep debt on work days. SJL was calculated as difference between midpoint of sleep during university and free days. Body composition was assessed by BIA. Data on duration and frequency of any physical activity (leisure and sports) were inquired as well as data on frequency, timing and type of sport (strength or endurance). Multivariable linear regression analyses were used to relate chronotype or SJL to body composition;the relevance of physical activity behaviour for these associations were tested in a conditional model. In June 2020, 156 students participated in an online follow-up survey. Relations between changes in categories of physical and sports activity variables (increase, no change, decrease) and tertiles of chronotype were tested by the Mantel- Hanszel Chi-Square test. Results: Prior to lockdown the students' mean chronotype was 4:32 h:min (+/-1:04), the median SJL 1:07 h:min (Q1;Q3: 0:35;1:45). Chronotype and SJL were not independently related to skeletal muscle mass (P>0,44). A later chronotype and a larger SJL were associated with a higher visceral fat mass (P = 0.05 and P = 0.01). Frequency or duration of physical or sports activity explained the association with chronotype (all P>0.05). Associations with SJL were partly explained by sport type (p = 0.07). Mean chronotype hardly changed during the lockdown (median difference: 0:05 (h:min (Q1;Q3: 0:42;0:27)), whilst SJL was reduced by 0:25 h (h:min (Q1;Q3: 1:01;0:05)). Overall, timing of sport during the week - i.e. the timing freely selectable during lockdown - was unchanged in 2/3 of the participants. Changes in all physical activity parameters were unrelated to chronotype (all p>0.07). Conclusion: A later chronotype and a higher SJL are associated with a higher visceral fat mass. Physical activity behaviour accounts for the association with chronotype. In this healthy sample of students, the COVID-19 lockdown resulted in no chronotype-specific changes in the timing of physical behaviour nor its duration, frequency or type.