To investigate the dynamics of heart rate in the course of sleep and to relate cardiac activity to sleep intensity, the electrocardiogram was recorded concomitantly with the polysomnogram in healthy young males. Heart rate was assessed across consecutive non-REM sleep (NREMS)-REM sleep (REMS) cycles as well as within individual episodes of NREMS and REMS. Within a sleep cycle, heart rate was lower in the NREMS episode than in the subsequent REMS episode. A global declining trend was present over successive NREMS episodes and over successive REMS episodes. A rapid increase of heart rate at the NREMS-REMS transitions was followed by a slow decline that started within the REMS episodes. Heart rate variability was higher in REMS than in NREMS and showed an increasing trend over successive REMS episodes but not over successive NREMS episodes. EEG slow-wave activity (spectral power density in the 0.75-4.5 Hz band), an intensity measure of NREMS, declined across NREMS episodes and was not correlated with heart rate. The global trends and ultradian variations of heart rate may represent sleep state-dependent modulations and circadian variations of the autonomic nervous system, which are not fully reflected in the sleep EEG.