Previous research has found that cardiac parasympathetic nervous system (PNS) activity increases and cardiac sympathetic nervous system (SNS) activity decreases during night-time sleep. This study aimed to examine in greater detail the time course of these changes in cardiac autonomic nervous system (ANS) activity. In the week prior to the experimental night, nine subjects maintained a constant sleep-wake schedule and experienced an adaptation night. Each subject's experimental night consisted of 2 h of presleep wakefulness, followed by a night of sleep, commencing at each subject's normal sleep onset time. One hundred and twenty beat blocks of presleep wakefulness and stable Stage 2 non-rapid eye movement (NREM) sleep across the night were selected. SNS activity was assessed using pre-ejection period, the amplitude of the T-wave in the ECG and the 0.1 Hz peak from the spectral analysis of the ECG. PNS activity was assessed using respiratory sinus arrhythmia (spectral analysis). Heart rate and respiratory rate were also measured. The results indicated a progressive decrease in SNS activity throughout sleep and a rise in PNS activity during the first half of the normal sleep period. The changes in PNS activity were similar, while the changes in SNS activity were altered, compared with a previous study in which stage of sleep was not controlled. This indicates a likely sleep stage influence on SNS activity, but not on cardiac PNS activity. These results are consistent with the concept of a primarily circadian, but not sleep, influence on PNS activity, and primarily a sleep, but not circadian, influence on SNS activity.