AIM: Sleep disturbances in patients with Alzheimer's disease have not been systematically evaluated, because sleep monitoring has proved difficult. Our goal was to quantitatively characterize sleep disturbances in patients with Alzheimer's disease. METHODS: The present study was a cross-sectional descriptive study, carried out in dementia care units in Japan. Participants were 63 patients with Alzheimer's disease (mean age 77.6 ± 8.0 years) with severe behavioral and psychological symptoms of dementia. A non-wearable actigraphy device placed under patient mattresses was used to measure sleep parameters: sleep time, time in bed, number of wakings and number of times they left the bed. Cut-off points for sleep parameters were based on the interquartile range of the data. RESULTS: Median nocturnal sleep time was 6.2 h (interquartile range 2.7 h). Median frequency of waking was eight times (5)/night. Median frequency of leaving the bed was five times (6), with a maximum of 31/night. We identified three types of sleep disturbance: frequent bed leaving, short sleep time and excessive sleep time. Multiple linear regression showed that duration of dementia was a significant predictor of frequent bed leaving at night (P = 0.042). CONCLUSIONS: Frequent bed leaving at night reflects severe sleep deprivation in patients with mild-to-severe Alzheimer's disease. Safety measures are required to prevent accidents related to sleep disturbances, regardless of dementia severity. Geriatr Gerontol Int 2019; 19: 130-134.