The circadian system is a homeostatic system that coordinates the activities of the majority of our physiological and behavioural outputs, including sleep-wake behaviour. Although the exact functions of sleep are not completely understood, sleep is broadly proposed to be for resting the brain and the body. In addition, sleep has been demonstrated to be important for memory consolidation and learning. Changes in sleep-wake behaviour and the circadian system are common in healthy ageing. Many individuals experience a phase advance in their circadian system, and as a result tend to go to bed and wake early compared when they were younger. Many people also report reduced sleep durations compared to when they were younger. It is not clear whether shorter sleep periods are a normal part of ageing or a sign of an underlying disturbance to the sleep or circadian systems. The prevalence of insomnia is increased with increasing age, and associations between insomnia and an increased risk for depression have been reported. In addition, sleep-wake and circadian disruption are commonly reported as comorbidities in a number of neuropsychiatric and neurodegenerative disorders, including late life depression, Parkinson’s Disease, mild cognitive impairment (MCI) and dementia. Interventions to improve sleep-wake behaviour may also have significant benefits on a number of symptoms of some comorbid disorders, as well as overall health and quality of life.