Circadian pattern of heart rate variability spectral indices, including hourly, 24-hour, night, day, morning, and sex-adjusted measures of low frequency (U), high frequency (HF), and LF/HF, was evaluated in healthy persons in 24-hour 3-lead electrocardiogram. HF showed circadian pattern with the greatest night values and LF/HF with the smallest night values. Peaks of hourly LF were found between 5 and 9 AM and between 4 and 6 PM. The smallest LF was between 11 Pm and 3 AM. Hourly HF peaked between 11 PM and 5 AM. The smallest HF was observed between midday and 2 PM. LF/HF peaked between 6 and 9 Am as well as between 4 and 6 PM with the smallest values between midnight and 5 AM. Sex adjustment was of no significance. In healthy subjects, HF and LF/HF have circadian pattern. Evaluation of all 5-minute intervals of 24-hour period seems to be a precise method of heart rate variability analysis. (c) 2005 Elsevier Inc. All rights reserved.