We investigated whether donepezil, a cholinesterase inhibitor, can be used to treat sleep disturbances in patients with dementia with Lewy bodies (DLB). Sleep disturbances were evaluated with the sleep disturbances item of the Neuropsychiatric inventory (NPI) and an actigraph in 16 DLB patients and 24 normal elderly control (NC) subjects. The presence/absence of nine kinds of sleep symptoms, such as dream enactment, were also evaluated in the DLB patients. The DLB patients were then given 5 mg/day donepezil for 14 weeks and evaluated again. Eight of the 16 DLB patients had some sleep disturbances before taking donepezil. The actigraphy data indicated that average activity count per minute in sleep (AAC), which reflects body activity at night, was significantly higher and total sleep time was significantly longer in DLB patients than in NC subjects. The NPI sleep disturbances score significantly improved and the number of DLB patients who had sleep disturbances decreased after taking donepezil. The actigraphy results indicate that the sum of all wake epochs within the sleep period, which reflects the degree of fragmented sleep, and the AAC decreased in the DLB patients after donepezil treatment. These results indicate that donepezil treatment reduced sleep disturbances in DLB patients.