Recent work has established an association between overweight/obesity and sleep duration, suggesting that short sleep duration and timing of sleeping may lead to overweight. Most of these studies considered sleep-length rather than any other aspects associated with the sleep and wake rhythm, e.g. chronotype, which is a measure of timing of sleeping (‘when to sleep’; based on the midpoint of sleep). The objective of this study was to assess the influence of different factors of the sleep-wake cycle and of co-variates on the Body Mass Index in a cross-sectional questionnaire study. Nine hundred and thirteen pupils (406 boys, 507 girls) from Southwestern Germany participated in this study. Mean age was 13.7 ± 1.5 (SD) years and range was between 11 – 16 years. We found that chronotype (β = .079) and social jetlag (β = .063) showed a significant influence on Body Mass Index (BMI), while sleep duration did not. Social jetlag is the absolute difference between mid-sleep time on workdays and free days. Further, screen time (in front of TV, computer, β = .13) was positively related with BMI. Self-efficacy on nutrition (β = -.11), a psychological variable important in health-behaviour models, showed an influence with high scores on self-efficacy related to lower BMI. A high BMI was correlated with low fast-food consumption (β = -.12) suggesting that adolescents with high BMI may exert some control over their eating.