  Power spectral density is an accepted measure of heart rate variability. Two
estimators of multifractal properties: Wavelet Transform Modulus Maxima and
Multifractal Detrended Fluctuation Analysis are used to investigate
multifractal properties for the three strongly physiologically grounded
components of power spectra: low frequency (LF), very low frequency (VLF) and
ultra low frequency (ULV). Circadian rhythm changes are examined by
discrimination of daily activity from nocturnal rest. Investigations consider
normal sinus rhythms of healthy 39 subjects which are grouped in two sets:
5-hour wake series and 5-hour sleep series. Qualitative arguments are provided
to conjecture the presence of stochastic persistence in LF range, loss of heart
rate variability during night in VLF range and its increase in ULF.
