OBJECTIVES: To examine the circadian pattern of cardiac autonomic modulation (CAM) and its attributes in general population. METHODS: We obtained 24-h beat-to-beat RR data using a high resolution 12-lead Holter ECG in a community-dwelling sample of 115 non-smokers. We performed heart rate variability (HRV) analysis on the normal RRs from each 5-min segment to obtain time-specific HRV indices: high (HF; 0.15-0.40 Hz) and low (LF; 0.04-0.15 Hz) frequency powers, standard deviation of RR intervals (SDNN), and the square root of the mean of the sum of the squared differences of the adjacent RR intervals (RMSSD). For each individual, we fit the segment-specific HRV data to a cosine periodic function, and estimated 3 individual-level cosine function parameters to quantify the circadian variation: the mean (M), amplitude (A), and acrophase (theta). We then used a random-effects meta-analysis to summarize the M, A, and theta, and their 95% confidence intervals (CI). RESULTS: The mean age was 56 (SD 8) years, with 63% female and 76% white. The averages of M, A and theta (95%CI) of log HF were 3.59 (3.43-3.76) ms(2), 0.61 (0.54-0.68) ms(2), and 3:10 (2:25-3:55) AM, respectively, and that of RMSSD were 22.3 (20.5-24.1) ms, 6.5 (5.4-7.5) ms, 3:45 (2:55-4:35) AM, respectively. Older age is associated with lower mean of HRV. Males have higher oscillation amplitude than females. The acrophase of LF/HF was earlier in females than in males, and in younger individuals than in older individuals. CONCLUSIONS: The circadian pattern of CAM can be quantified by 3 cosine parameters of HRV, which are correlated with age and gender.