This paper reviews the characteristics of sleep disorders found in people at a greater risk of dementia: the elderly adult, patients with mild cognitive impairment (MCI) and those with neurodegenerative diseases. The frequency of sleep architecture modifications and circadian rhythm sleep disturbances increases with age. Although around 40% of older adults complain of poor sleep, true sleep disorders are far less prevalent in healthy older adults and are frequently associated with comorbidities. The sleep disorders observed in Alzheimer's disease (AD) patients are often similar to (but more intense than) those found in non-demented elderly people. Poor sleep results in an increased risk of significant morbidities and even mortality in demented patients and constitutes a major source of stress for caregivers. The prevalence of primary sleep disorders such as rapid eye movement (REM) sleep behavior disorders (RBDs), restless legs syndrome (RLS), periodic limb movements (PLMs) and sleep-disordered breathing increases with age. There are no published data on RLS and PLMs in demented persons but RBDs and sleep apnea syndrome have been studied more extensively. In fact, RBDs are suggestive of Lewy body dementia (LBD) and are predictive for neurodegeneration in Parkinson's disease. Obstructive sleep apnea (OSA) shares common risk factors with AD and may even be an integral part of the pathological process in AD. In MCI patients, the hypotheses in which (i) sleep disorders may represent early predictive factors for progression to dementia and (ii) MCI is symptomatic of a non-diagnosed sleep disorder remain to be elucidated. Guidelines for drug and non-drug treatments of sleep disorders in the elderly and in demented patients are also considered in this review. In healthy but frail elderly people and in early-stage AD patients, sleep should be more thoroughly characterized (notably by using standardized interviews and polysomnographic recording).