The 24-hour sleep–wake cycle is generated by two oscillatory processes: an endogenous hypothalamic circadian pacemaker and a sleep- and wake-dependent homeostat. These processes combine to maintain a consolidated bout of sleep at night and relatively stable waking function across the day. They also combine to determine ‘diurnal preference’—whether one is a ‘lark’ or an ‘owl’—a reflection of the phase relationship between the circadian and homeostatic processes. These processes are affected directly by light, either through resetting of the circadian pacemaker or its direct alerting effects. Sleep deficiency and circadian disruption have been associated with a higher risk of chronic disease, although the methodology for assessing these exposures is not optimal. Both sleep and the circadian system also have myriad influences on other aspects of our physiology, behaviour, and metabolism; therefore, steps should be taken to reduce their potential confounding effects in epidemiological studies.