OBJECTIVES: To identify sleep disturbances in participants with subtypes of dementia and explore clinical correlates. DESIGN: Cross-sectional. SETTING: Outpatient clinics in western Norway and the Mayo Clinic Study of Aging, Olmsted County, Minnesota. PARTICIPANTS: One hundred fifty-one community-dwelling western Norway residents referred for geriatric medicine, geriatric psychiatry or neurology evaluation and 420 participants without dementia from the Mayo Clinic Study of Aging. MEASUREMENTS: Dementia was diagnosed according to Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, criteria. The Mayo Sleep Questionnaire, an interview to detect sleep disturbances, was administered to diagnose probable rapid eye movement (REM) sleep behavior disorder, probable periodic leg movements during sleep, probable restless legs syndrome, probable sleepwalking, probable sleep-related leg cramps, probable obstructive sleep apnea, and excessive daytime sleepiness. Insomnia was assessed using the Neuropsychiatric Inventory, an interview to detect neuropsychiatric symptoms in dementia. RESULTS: Seventy-one percent of the participants with dementia and 55.7% of control participants had sleep disturbances (P=.001). Most frequently reported in the mild dementia participants were insomnia (29.9%), probable sleep-related leg cramps (24.1%), excessive daytime sleepiness (22.6%), probable restless legs syndrome (20.7%), and probable REM sleep behavior disorder (18.5%). There were more sleep-related problems reported in participants with Lewy body dementias (LBD) (dementia with Lewy bodies and Parkinson's disease dementia) than in those with Alzheimer's disease (P=.008). Having any sleep disorder correlated with depression (P=.03) and anxiety (P=.02). CONCLUSION: Sleep problems are common in dementia, particularly in subjects with LBD, and are associated with psychiatric symptoms. Further research is needed to understand these associations and identify treatment strategies.