The present study was undertaken to investigate the characteristics of morphine in rat sleep patterns and also the effects of donepezil and memantine on somnolence caused by morphine. Electrodes were chronically implanted into the cortex and dorsal neck muscle of rats for electroencephalogram (EEG) and electromyogram (EMG) recordings, respectively. EEG and EMG were recorded with an electroencephalograph. SleepSigh ver.2.0 was used to analyse the sleep-wake state. Total times of wakefulness, non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep were measured from 10:00 to 16:00. Morphine at a high dose caused a significant decrease in sleep latency and total REM sleep time, although the drug at low doses caused significant increases in sleep latency and total awake time, and a significant decrease in NREM sleep time. Donepezil, memantine and methylphenidate antagonized the decrease in sleep latency caused by morphine. From these findings, it can be concluded that morphine caused somnolence, and donepezil and memantine are useful for somnolence caused by morphine, similar to methylphenidate.