In epidemiological surveys, cognitive decline has been found to be associated with both short and long sleep duration.Our goal was to objectively determine how total sleep time (TST) at night was associated or not with apathy or severity scores in patients with Alzheimer ‘s disease (AD).During an observational first step of a clinical trial, sleep was assessed in institutionalized patients with mild or moderate AD using actigraphy (MW8, Camtech, Cambridge, UK) for 14 consecutive 24-hour periods. Sleep parameters analyzed were: TST, time in bed (TIB), wake after sleep onset (WASO), sleep efficiency (SE) defined by the ratio TST/TIB, in percentage), the number and length of awakenings, the night fragmentation index, the interdaily stability, and intradaily variability indexes. Statistical association analyses were tested between these values and AD apathy and severity scores.208 individuals coming from 82 centers worldwide (France, Germany, Spain, Italy, Portugal, Poland, United States, Canada, and Australia) and≥50 years old participated. Their average TST was 7 hours and 35 minutes and the average WASO 58 minutes. TST and SE were significantly higher in patients with apathy and the number of awakenings was significantly lower. TST was also positively associated with functional disability (ADCS-ADL scores), but it was not found significantly greater in patients with a moderate AD severity compared to the mild.Despite several and long awakenings, TST was not shorter in patients with AD. TST was even significantly increased with disability and apathy.