Sleep and obesity have a bi-directional interaction. Central adiposity is a critical cause of disturbed sleep, mainly by driving obstructive sleep apnea (OSA) pathogenesis. We live in a society where there are many challenges to the length, depth and scheduling of our sleep. There is increasing evidence that abnormal sleep duration, poor sleep quality, inappropriate timing of sleep and even OSA are obesogenic and promote metabolic syndrome in children and adults. This is based on a large body of animal, epidemiological and experimental research. However, clinical intervention studies, on the other hand, can provide information on a causal effect of sleep loss and sleep disturbance on weight gain: energy intake and energy expenditure. Such studies include sleep extension, circadian realignment and OSA and other sleep disorder treatment trials. Here, the evidence is to some extent more fragmentary and limited by methodological issues. Delineating the extent that poor sleep as a risk factor for obesity requires a focused collaborative research approach.