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Aim Sleep disturbances are prevalent in various dementia subtypes but rarely investigated in early clinical stages. Although memory clinics have become an established institution for the early diagnosis of dementia, sleep assessment is not part of their routine diagnostics. This study aimed to examine whether subjective and objective sleep variables are related to cognitive impairment in patients referred to a memory clinic. Methods On two consecutive days, patients underwent routine diagnostic procedures, including a neuropsychological examination (consortium to establish a registry for alzheimer's disease), and had their sleep quality evaluated by the Pittsburgh Sleep Quality Index and overnight hand‐wrist actigraphy. Results Data of 31 patients (age, M ± SEM: 74.1 ± 1.5; 18 women, 13 men; Clinical Dementia Rating: 0–1) were analysed. One had been diagnosed with subjective cognitive impairment, 13 with mild cognitive impairment with or without depression, and 17 with dementia syndrome due to Alzheimer's and/or cerebrovascular disease. Compared to patients with subjective or mild cognitive impairment, dementia patients showed a significantly increased nocturnal acceleration magnitude; other differences in subjective and objective sleep measures were not significant. Comparing patients with subjectively poor (Pittsburgh Sleep Quality Index > 5: n = 9) and good sleep (Pittsburgh Sleep Quality Index ≤ 5: n = 22) yielded no differences in any neuropsychological and clinical variables. In contrast, patients with low actigraphically recorded sleep efficiency (<85%: n = 11) exhibited a significantly more impaired cognitive performance than those in the high sleep efficiency group (≥85%: n = 20). Correlation analyses demonstrated that actigraphically assessed disturbed sleep continuity accompanied by increased night‐time motor activity was substantially associated with cognitive impairment. Conclusion This study highlights that objectively assessed, but not self‐reported, parameters of disturbed sleep are closely related to cognitive dysfunction in the early stages of dementia of different aetiologies. Possible diagnostic and treatment implications are discussed.

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