The aim of this study was to observe the effects of donepezil on both the cognitive function and sleep patterns in patients of Alzheimer's Type Dementia (ATD), especially to determine the relationship between the improvement of cognitive function and the amount of rapid eye movement (REM) sleep. A total of 12 patients (7 females, 5 males; age, 73.0 +/- 6.8) meeting the NINCDS-ADRDA (National Institute of Neurological and Communicative Disorders and Stroke-Alzheimer's Disease and Related Disorders Association) criteria of probable AD were studied. These patients presented with mild to moderate dementia, which was confirmed by a Clinical Dementia Rating score of 1 or 2. Following baseline examinations consisting of the Alzheimer's Disease Assessment Scale-cognitive component-Japanese version (ADAS-Jcog) and polysomnography (PSG), 5 mg of donepezil was administered to the patients at breakfast every day. All patients were reassessed 6 weeks later using the same examinations. With sleep patterns, the percentage of REM sleep to total sleep time increased after the administration of donepezil. In addition, it was also found that sleep efficiency was increased and sleep latency was shortened by this administration. Although the ADAS-Jcog score did not decrease significantly, there was significant positive correlation between the decrease of the ADAS-Jcog score and the increase in the percentage of REM sleep. These results indicate the increase action of REM sleep due to activate central cholinergic systems and the possibility to improve sleep conditions due to one-time administration after breakfast of donepezil in mild to moderate ATD. It is concluded that the increase in REM sleep may reflect the improvement of cognitive function in ATD patients.