Because cardio-metabolic diseases have strong nutritional determinants, the link between sleep and chronic conditions may be mediated by diet. The central hypothesis of this thesis is that short sleep drives risk of weight gain and cardio-metabolic diseases, and this is mediated through dietary factors. In this thesis, a cross-sectional analysis of the UK population’s sleep duration using the National Diet and Nutrition Survey Rolling Program showed that one third of adults aged 19 – 64 years do not achieve the recommended 7 – 9 hours of sleep per night. In addition, short sleep was associated with indicators of a poorer quality diet that further deviates from the UK dietary guidelines, characterised by lower intake of fruit and vegetables, fibre, micronutrients and cardio-protective fats (MUFA, n-6 and n-3 PUFA), as well as higher intakes of NMES and alcohol. In addition, short sleepers also had lower intake of protein. A cross-sectional investigation using wrist actigraphy to obtain objective measures of sleep in a university-based population showed no associations with diet, however, short sleep was associated with increased physical activity. To understand causal effects of sleep on energy balance, a systematic review and meta-analysis of intervention studies assessing the effects of sleep deprivation on energy balance was conducted. Sleep deprivation significantly increased energy intake by 385 kcal (95% CI 252, 517), without compensatory effects on energy expenditure. In addition, sleep deprivation caused an increased intake of fat at the expense of protein. To understand whether the reverse effects would be evident as a result of sleep extension, a lifestyle sleep extension intervention was undertaken in a feasibility study in free-living short sleepers. The sleep extension group significantly increased objective measures of sleep duration (h:min) including time in bed (0:55 (95% CI 0:37, 1:12)), sleep period (0:47 (95% CI 0:29, 1:05), and sleep duration (0:21 (95% CI 0.06, 0:36)) compared with the control group. Moreover, sleep extension led to significantly reduced intake of free sugars (-9.6 g (95% CI -16.0, -3.1)) compared with control. This thesis provides the foundation for further investigations into adherence to sleep extension over a longer duration and suggests that current dietary guidelines may benefit from the inclusion of advice on sleep.