Obesity has reached crisis proportions in industrialized societies [1]. Many factors converge to yield increased body mass index (BMI). Among these is sleep duration [2–10]. The circadian clock controls sleep timing through the process of entrainment. Chronotype describes individual differences in sleep timing, and it is determined by genetic background, age, sex, and environment (e.g., light exposure) [11–14]. Social jetlag quantifies the discrepancy that often arises between circadian and social clocks, which results in chronic sleep loss [11, 15]. The circadian clock also regulates energy homeostasis [16], and its disruption—as with social jetlag—may contribute to weight-related pathologies [17–19]. Here, we report the results from a large-scale epidemiological study, showing that, beyond sleep duration, social jetlag is associated with increased BMI. Our results demonstrate that living "against the clock" may be a factor contributing to the epidemic of obesity. This is of key importance in pending discussions on the implementation of Daylight Saving Time and on work or school times, which all contribute to the amount of social jetlag accrued by an individual. Our data suggest that improving the correspondence between biological and social clocks will contribute to the management of obesity.