Multifractal analysis of the day and night characteristics of heart rate variability

The RR heart rate variability characteristics of normal subjects were studied to compare the multifractal characteristics during day and night. The RR intervals were obtained from 24-hour digital Holter ECG recordings of normal subjects, starting at midday. Each 24 hour period was split into 4 blocks of 6-hour which were analysed separately. The overnight period was fully contained in the third block, between midnight to 6 a.m. The 6-hour blocks were analysed using a wavelet transform multifractal formalism. Each spectrum peaked between the Hurst exponent (h) limits of 0.07 and 0.29. In one subject the overnight spectrum did not have a consistent overnight peak and could not be analysed. In the subjects with well-defined spectral peaks, the overnight peak position of h tended to be greater than that for any of the other three 6-hour periods, indicating that the fractal characteristics of the RR intervals differ during the overnight sleep period.