Strong observational evidence demonstrates that sleep of short duration, and to a lesser extent sleep of poorer quality, increases the risk of obesity in children. However, the mechanisms that explain this well-established and consistent finding are not well understood. While it is feasible that being tired makes children less physically active, existing research has been limited by study design and analysis chosen. The more likely reason is that being tired changes what, why, or how children eat. This talk will summarise current knowledge regarding the links between sleep and obesity in children, and highlight ongoing research in this relatively new and intriguing area.