A clinically relevant sleep-wake disturbance is found in up to half the patients with dementia, and the sundowning agitation is a common cause of institutionalisation of demented geriatric patients. The circadian rhythm of demented patients is levelled off with increased daytime sleep and disrupted night sleep. Particularly in vascular dementia, Korsakow syndrome, Parkinson's disease, and depression the alteration of sleep architecture may be pronounced, whereas in Alzheimer's disease prominent hypersomnolence or insomnia is typically only found in later stages of the diseases