We investigated the relationship between electroencephalogram (EEG) activity and autonomic nervous system function using spectral analyses of EEG and heart rate variability (HRV) in healthy subjects during sleep. Eleven subjects were enrolled in this study. From EEG, the spectral edge frequencies (SEFs including SEF50, SEF90 and SEF95) were calculated. From electrocardiogram (ECG), the spectral powers of low-frequency band (LF: 0.04-0.15 Hz), high-frequency band (HF: 0.15-0.4 Hz) and the ratio of LF to HF (LF/HF) were calculated. During sleep, each set of data was obtained as the average of a 5-min measurement. We found that SEFs and LF/HF or LF decreased simultaneously and periodically, suggesting simultaneous depression of EEG activity and relative sympathetic activity, and SEFs significantly correlated with LF/HF and LF in all subjects during sleep, but not with HF. The existence of a clear correlation of SEFs with LF or LF/HF may offer a simple approach to estimate the relationship between EEG activity and autonomic nervous system function during sleep.