Most organisms use internal biological (circadian) clocks to match behavioural and physiological processes to specific phases of the external world. Central to this is the synchronisation of internal processes across the brain and multiple organ systems. We are now keenly aware that disruption of our circadian system is detrimental to health and wellbeing. For example, environmental desynchrony (e.g. shift work) profoundly impacts human health, increasing cardiovascular disease and diabetes risk, many diseases show diurnal variation in severity of symptoms, and risk of clinically adverse events can vary across the day and night. In this seminar, I will discuss our work on the role played by the circadian system in driving daily rhythms in electrical activity in the heart, time of day vulnerability to arrhythmia and the impact of mistimed behavioural rhythms on cardiac function.