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. Greatly increased daytime 
sleepiness or striking insomnia at the very beginning of suspected dementia 
should thus prompt the search for other, possibly treatable causes of dementia. 
Neuropathological and neurophysiological studies support the hypothesis of a 
deteriorated hypothalamic suprachiasmatic nucleus (harbouring the biological 
clock) as a cause for the deranged circadian sleep-wake system in dementia. 
Management of sundowning behaviour includes restriction of daytime sleep, 
exposure to bright lights, and social interaction schedules during the day. The 
benzodiazepines and analogues usually not being sufficiently effectual, low 
doses of mild neuroleptics are often needed. Whether recent reports on efficacy 
of melatonin in elderly insomniacs also apply to demented patients is yet 
uncertain. The careful search and treatment of possible extracerebral 
physiologic factors causing reversible hypersomnia or insomnia is an important 
requisite. Polysomnographic studies are needed to recognise treatable sleep 
disturbance which could deteriorate or mimic dementia and sundowning. 
Particularly, a sleep-apnea-hypopnea syndrome must be searched for at the 
beginning of a suspected dementia, when successful treatment is still possible. 
Sleep studies should also identify periodic leg movements of sleep with restless 
legs and/or increased daytime sleepiness, and hyperkinetic parasomnias such as 
REM sleep behaviour disorder which may complicate or imitate sundowning.