The amount of time between heartbeats is controlled by the rate of depolarization of the sinoatrial node. This rate is controlled by a combination of sympathetic and parasympathetic inputs. For this reason, analysis of heart rate (HR) and heart rate variability (HRV) derived from intervals between normal heart beats provides insights into changing cardiac autonomic function. Increased sympathetic control of the heart from one time point to the next tends to increase HRs, decrease the amount of vagally modulated sinus arrhythmia measured as high-frequency (HF) spectral power, and increase the ratio between low-frequency (LF) spectral power and HF (the LF/HF ratio). Conversely, a shift toward greater vagal control of the heart tends to produce decreased HRs, increases in HF power, and decreases the LF/HF ratio. Clear circadian cycles of HR and HRV are seen in most subjects and correspond to periods of sleep and activity. The magnitude of these cycles is well captured by traditional HRV indices such as deviation of all normal-to-normal (NN) intervals. Cosinor analysis also provides a rough estimate of the magnitude of circadian rhythms. We have observed that HR and HRV fluctuate over time in a cyclic manner in most subjects. Although these cycles are especially prominent during the nighttime, they are seen in the daytime as well. We have developed a number of measures that quantify ultradian properties of HR/HRV cycles and would permit comparisons between subjects. We applied these measures during overnight polysomnography and have shown that ultradian rhythms of cardiac autonomic modulation have a variable and often weak correspondence with traditional sleep stages . In the current study, we applied the methodology developed for the nighttime recordings to 24-h recordings in young healthy subjects and compared these results to those obtained during the nighttime only in the same subjects and in an older group of cardiac patients. Our purpose was to explore the presence and magnitude of ultradian cycles of HRV during the entire 24-h period and during sleep.