The day vs night values of variables that may be related to the autonomic nervous system were compared in 50 individuals without obvious heart disease. Using ambulatory electrocardiographic monitoring and heart rate variability analysis, it was found that in contrast to daytime, nocturnal heart rate and total heart rate variability were reduced and low (0-0.05 Hz) and high (0.2-0.35 Hz) frequency power of nocturnal heart rate variability were increased, whereas mid-frequency (0.05-0.2 Hz) power was reduced. In addition, nocturnal episodes of trapezoidal-shaped waveforms of heart rate increase were present primarily at night and ST was elevated to a greater extent at night. Many of these factors were related to age and tentatively, despite the small sample size, reflect an age-related diminished autonomic nervous system response. These results serve to establish a circadian basis for comparison when evaluating disease states such as ischemic heart disease and its prognosis.