The effects of cold exposure on heart rate variability (HRV) during sleep were examined. Eight male subjects slept under three different conditions: 3 degrees C, 50-80% relative humidity (RH) [3]; 10 degrees C, 50% RH [10]; and 17 degrees C 50% RH [17]. No significant differences were observed in HRV during rapid eye movement sleep (REM) and wakefulness. The ratio of the low frequency (LF) to high frequency component (HF) of HRV (LF/HF) significantly differed among the conditions during stage 2 and slow wave sleep (SWS) that decreased as the ambient temperature decreased. The normalized LF [LF/(LF + HF)] significantly decreased in 3 and 10 than in 17 during SWS. In low ambient temperature, predominant cardiac parasympathetic activity during stage 2 with no significant difference during REM and wakefulness may cause variations in HRV at transition from stage 2 to REM and wakefulness. These results may partly explain the peak in adverse cardiac events during winter.