BACKGROUND: The aim of this study was to identify the acute effects of ethanol on the relationship between sleep and heart rate variability (HRV) during sleep. METHODS: Ten healthy male university students were enrolled in this study. An alcoholic beverage was given to each subject at a dosage of 0 (control), 0.5 (low dose: LD), or 1.0 g (high dose: HD) of pure ethanol/kg of body weight. All experiments were performed at 3-week intervals. On the day of the experiment, a Holter electrocardiogram was attached to the subject for a 24-hour period, and the subject was instructed to drink the above-described dosage of alcoholic beverage 100 minutes before going to bed; polysomnography was then performed for 8 hours. Power spectral analysis of the HRV was performed using the maximum entropy method, and the low- (LF: 0.04 to 0.15 Hz) and high-frequency (HF: 0.15 to 0.4 Hz) components along with LF/HF ratio were calculated. RESULTS: As alcohol consumption increased, the heart rate increased and the spectral power of HRV measured at each frequency range decreased. Higher doses of ethanol also increased the LF/HF ratio compared with the measured ratio of the control group. CONCLUSIONS: Acute ethanol intake inhibits parasympathetic nerve activity and results in predominance of sympathetic nerve activity during sleep, in a dosage-dependent manner. The results of this study suggest that ethanol interferes with the restorative functions of sleep.