Some sleep disorders and sleep problems are reported as risk of dementia. It is reported that 50-80% of idiopathic REM sleep behavior had a later conversion to a synucleinopathy, thus this parasomnia is regarded as early marker of Parkinson disease, dementia with Lewy bodies and multiple system atrophy. Obstructive sleep apnea syndrome causes reversible decline of cognitive functions in children and adults, and it increases the risk of dementia and mild cognitive impairment in elderly women. A decrease of amplitude of circadian activity rhythm and habitual long sleep duration (> or = 9 hours) are reported to increase the risk of dementia in elderly people. Some reports indicate that hypnotic use may be a risk factor of dementia. However, it is not cleared whether insomnia itself related with dementia or not, since no study evaluated the risk of insomnia without hypnotic treatment.