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The causes of the ongoing metabolic disease pandemic are complex. Changes in human population genetics proceed slowly, so the recent increase in metabolic disease prevalence likely reflects environmental and behavioural changes. Our circadian (~ 24-hour) systems optimise our biology according to time of day. Artificial stimuli like electric lighting and around-the-clock food access enable waking behaviours, such as eating, at times at which our circadian systems prime us to sleep. Such mistimed behaviours may contribute to metabolic disease, as exemplified by increased risk of diabetes in shift workers. However, few researchers have concurrently explored associations between sleep, diet composition and timing, and metabolic health. Furthermore, many dietary analysis methods used have not been validated, precluding accurate inferences about diet-disease relationships. And few studies have assessed the metabolic effects of interventions to resynchronise the circadian system each day. I first helped validate myfood24, an online dietary recall tool, by completing all of the lab work for dietary protein and sugar biomarkers. myfood24 has comparable validity to the gold-standard recall method. Subsequent analysis of the myfood24 data showed that consuming calories later relative to sleep is associated with overweight and obesity. Next, analysis of a public database indicated that longer sleepers had lower body mass indices, smaller waists, and favourable blood lipid profiles. Finally, we assessed whether long-term supplementation of melatonin (a hormone that synchronises the circadian system) influences metabolic health, sleep, and diet in adults predisposed to diabetes. Contrary to our hypotheses, melatonin had few effects. This project helped validate myfood24, a tool that could unveil diet-disease relationships in future studies. These studies might have key public health implications. Our findings also strengthen the notions that mistimed eating and insufficient sleep contribute to obesity. Finally, this project shows that melatonin may not substantially influence metabolic health in relatively healthy adults.