Sleep disturbance is a clinically significant problem in nearly one half of patients with dementia and is a predictor of cognitive and functional decline. There are multiple causes of sleep disturbance in dementia and all represent diagnostic and management challenges. Impaired circadian rhythm is a disorder characterized by an increase in daytime sleep with reduced night-time sleep, a reversal of the sleep/wake cycle. Such disrupted circadian rhythm may occur early in Alzheimer's disease, worsen over time, and is associated with behavioural disturbance, including aggression. In addition to the timing of sleep, the structure of sleep may deteriorate, giving rise to poor sleep quality.