We examined the effects of low-dose oral risperidone (RIS) on nocturnal sleep in healthy participants. This study was performed in a placebo-controlled manner in 10 healthy male volunteers (mean age, 23.6 years), with administration of 0.5 mg of RIS oral solution or a placebo in the morning or evening for 2 consecutive days. Each night, polysomnography (PSG) was performed, and PSG data during non-rapid-eye movement (REM) sleep were processed by power spectral analysis. An evening administration of 0.5 mg RIS significantly increased total sleep time, sleep efficiency and sleep stage 3, and significantly decreased total waking time and waking after sleep onset (P < 0.05). A morning administration of 0.5 mg RIS significantly increased sleep stage 3 (P < 0.05). According to power spectral analysis, the evening administration of RIS significantly increased the theta power (P < 0.05) and decreased the beta power (P < 0.05) during non-REM sleep. The administration of 0.5 mg oral RIS increases sleep stage 3 and increases total sleep time following evening administration.