Social jetlag

Social jetlag is the chronic discrepancy between an individual's endogenous circadian timing and the sleep-wake schedule imposed by social obligations such as work or school, quantified as the absolute difference in midsleep time between free days and workdays. The term was introduced by Wittmann, Dinich, Merrow, and Roenneberg in 2006 and measured in hours using the Munich Chronotype Questionnaire (MCTQ); approximately 69–70% of adults in industrialized countries exhibit at least one hour of misalignment. Late chronotypes are disproportionately affected because social start times are earlier than their biological clocks prefer. Roenneberg et al. (2012, Current Biology) demonstrated in a large European cohort that social jetlag was associated with an odds ratio of 3.3 (95% CI 2.5–4.3) for belonging to the overweight group, independent of sleep duration — one of the first large-scale human associations between circadian misalignment and adiposity. Proposed mechanisms include HPA-axis dysregulation with elevated cortisol, autonomic nervous system activation, appetite-hormone shifts (elevated ghrelin, reduced leptin), impaired insulin sensitivity, and reduced physical activity, all converging on metabolic dysfunction. Evidence for cardiovascular risk remains largely associational in humans; experimental misalignment studies confirm acute physiological changes, but randomized long-term intervention trials demonstrating that correcting social jetlag improves hard cardiometabolic endpoints are still lacking as of 2026.