Direction and magnitude of social jetlag are associated with obesity: Results from a sample of hospital employees.

Social jetlag, the discrepancy between social and biological timing, has been suggested to disturb metabolic functions. However, the relationship between social jetlag and obesity has been inconsistent in other studies. In this study we examined the association between social jetlag and obesity among day and shift workers. We invited 2508 day workers and 1383 shift workers from a hospital worker health cohort to participate in a 2018-2019 survey on their sleep behaviors. Shift-specific social jetlag was quantified using the Munich ChronoType Questionnaire, and body mass index was measured during annual physical examinations. The distributions of shift-specific social jetlag were illustrated, and logistic regression analysis was used to examine the association between social jetlag and obesity. We found that high level of social jetlag (odds ratio [OR] = 1.26, 95% confidence interval [CI] = 1.08-1.47) and positive social jetlag (OR = 2.25, 95% CI = 1.30-3.90) during evening shifts were associated with obesity after adjustment for age, sex, health behaviors, and sleep quality. During night shift periods, sleep time varied greatly on free days, but the participants slept at similar times, namely 16:00, on workdays. In conclusion, phase advance on workdays and high levels of social jetlag were associated with obesity. Sleep timing should therefore be recommended according to the relative phase of individuals' preferred sleep time and work time.