Background Sleep disturbances in Alzheimer disease (AD) may affect behavioral and psychological symptoms of dementia (BPSD). Our aim was to elucidate the associations between sleep disturbances and other BPSD at different stages of AD. Methods This investigation was part of a multicenter‐retrospective study in Japan (J‐BIRD). Eligible for final analyses were 684 AD patients. Global severity of dementia was estimated using the Clinical Dementia Rating (CDR) scale. BPSD were assessed using the Neuropsychiatric Inventory (NPI). We analyzed the relationships between sleep disturbances and BPSD at different stages of AD according to the CDR score. Results Among the 684 AD patients, 146 (21.3%) had sleep disturbances. Patients with very early AD (CDR 0.5) and sleep disturbances had significantly more BPSD than those without sleep disturbances, as indicated by the higher prevalence of the following four NPI items: anxiety, euphoria, disinhibition, and aberrant motor behavior. In AD at CDR 2, (moderate AD) only one NPI item (irritability) was affected, while none was affected at CDR 1 (mild AD) and 3 (severe AD). Multiple regression analyses were performed in those with AD having various CDR scores. At CDR 0.5, the presence of sleep disturbances was associated with a high total NPI score (β = 0.32, p < 0.001). However, other factors, including cognitive decline, age, gender, and years of education, were not significantly associated with the NPI score. At CDR 1 and 2, no factor was significantly related to BPSD. Conclusion Sleep disturbances were strongly associated with other BPSD in the very early stage of AD. Copyright © 2016 John Wiley & Sons, Ltd.