Recently, a multifractal-multiscale approach to detrended fluctuation analysis (DFA) was proposed to evaluate the cardiovascular fractal dynamics providing a surface of self-similarity coefficients α(<i>q</i>,τ), function of the scale τ, and moment order <i>q</i>. We hypothesize that this versatile DFA approach may reflect the cardiocirculatory adaptations in complexity and nonlinearity occurring during the day/night cycle. Our aim is, therefore, to quantify how α(<i>q</i>, τ) surfaces of cardiovascular series differ between daytime and night-time. We estimated α(<i>q</i>,τ) with -5 ≤ <i>q</i> ≤ 5 and 8 ≤ τ ≤ 2048 s for heart rate and blood pressure beat-to-beat series over periods of few hours during daytime wake and night-time sleep in 14 healthy participants. From the α(<i>q</i>,τ) surfaces, we estimated short-term (<16 s) and long-term (from 16 to 512 s) multifractal coefficients. Generating phase-shuffled surrogate series, we evaluated short-term and long-term indices of nonlinearity for each <i>q</i>. We found a long-term night/day modulation of α(<i>q</i>,τ) between 128 and 256 s affecting heart rate and blood pressure similarly, and multifractal short-term modulations at <i>q</i> < 0 for the heart rate and at <i>q</i> > 0 for the blood pressure. Consistent nonlinearity appeared at the shorter scales at night excluding <i>q</i> = 2. Long-term circadian modulations of the heart rate DFA were previously associated with the cardiac vulnerability period and our results may improve the risk stratification indicating the more relevant α(<i>q</i>,τ) area reflecting this rhythm. Furthermore, nonlinear components in the nocturnal α(<i>q</i>,τ) at <i>q</i> ≠ 2 suggest that DFA may effectively integrate the linear spectral information with complexity-domain information, possibly improving the monitoring of cardiac interventions and protocols of rehabilitation medicine.
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