Assessing Social Jetlag and Chronotype

At age 38 years, the Munich Chronotype Questionnaire was used to assess social jetlag as well as sleep duration and chronotype.^{29 (link)} Social jetlag, the discrepancy between our internal timing and external timing, was measured by subtracting each participant’s midpoint of sleep on work days from their midpoint of sleep on free days (MSF). Sleep duration was calculated by averaging the sleep duration on work days and free days, assuming five work days and two free days a week as standard. Chronotype, the preference in sleep timing, was assessed using sleep-debt-corrected MSF (MSF_sc) (see Ronneberg et al.^{17 (link)}). A detailed protocol for calculating the complete set of Munich Chronotype Questionnaire variables can be found elsewhere.^{14 (link)} Social jetlag was significantly correlated with chronotype (r = 0.40, P < 0.01), but not with sleep duration (r = −0.04, P = 0.28). The mean social jetlag among participants in our cohort was 0.88 h, with a standard deviation of 0.96 (n = 815) (see Supplementary Figure 1). All analyses were conducted using the absolute value of social jetlag.