There is a strong relationship between Alzheimer's disease (AD) and sleep problems, and a sleep condition is informative for evaluating the AD status. In the present study, we evaluated subjective sleep problems in AD and mild cognitive impairment (MCI) with self-check questionnaires and objective sleep problems with a convenient home-based portable device, WatchPAT. A total of 63 subjects with normal cognition (NC) (n = 22), MCI (n = 20), and AD (n = 21) were cross-sectionally investigated. AD patients showed a better self-check Pittsburgh sleep quality index (PSQI) score (*p < 0.05) than NC and MCI patients. On the other hand, WatchPAT analysis showed a significantly reduced rapid eye movement (REM) sleep (*p < 0.05) and increased light sleep in AD patients (*p < 0.05) compared with NC subjects, and mildly reduced REM and increased light sleep in MCI subjects. The present study revealed a gap between the subjective self-check sleep questions and the objective WatchPAT analysis in AD patients. Thus, a home-based sleep study with WatchPAT is a useful tool to detect an objective sleep problem in AD and the risk of MCI conversion into AD. (C) 2020 Elsevier Ltd. All rights reserved.