Author	L.K. McDonald
Author	A. Fisher
Author	C. Llewellyn
Author	J. Wardle
Abstract	The rising prevalence of obesity poses a considerable threat to population health. Shorter nighttime sleep has emerged as a novel risk factor for overweight and obesity, and the association appears to be stronger at young ages. Experimental studies in adults suggest that increasing energy intake drives weight gain during periods of sleep curtailment. However, there have been few studies addressing the association between free-living sleep and weight in children. Using data from the Gemini twin birth cohort, the objective of this thesis was to advance the understanding habitual sleep behaviour in early childhood and how it may operate to influence the development of adiposity. Study 1 identified the predictors and pathways to shorter sleep at age 15 months, demonstrating that multiple environmental factors are associated with shorter sleep in children, with several operating through a later bedtime. Studies 2 and 3 examined the association between sleep and energy intake at age 21 months. Study 2 identified a linear relationship between shorter sleep and energy intake, before an association between sleep and weight was observed. Study 3 demonstrated that shorter sleeping children consumed more calories at night only, and predominantly from milk drinks. Study 4 showed that shorter sleep was significantly associated with weight at age 5 years; and at this age shorter sleep was associated with higher food responsiveness, which could partly explain the association with a higher weight. Findings from study 5 highlight the role of the home environment, demonstrating a stronger association between sleep and weight among children living in higher risk home food environments. Overall, the results of this thesis highlight the importance of an early bedtime, and strongly suggest that shorter sleep in early life may lead to a greater propensity to over-consume. Shorter sleeping toddlers may consume more because of parents' inclination to feed to soothe at night, but changes in sensitivity to food stimuli may increase food intake and weight in older children with a greater autonomy over their eating behaviour. Implications, limitations and avenues for future research are discussed.