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. 2019 Mar 1;10:115. doi: 10.3389/fphys.2019.00115

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Copyright © 2019 Castiglioni and Faini.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Scale coefficients of heart-rate variability during Wake and Sleep, for original and surrogate series. Coefficients calculated by Equations (22), combining DFA₁ and DFA₂ estimates by Equations (23); for surrogate data, the figure shows the average α surface over 100 series generated shuffling the phases of the Fourier spectrum of the original series. To more easily compare Wake and Sleep conditions, which are characterized by different heart rates, the τ axis represents the temporal scale in seconds; τ is obtained multiplying the box size, n, with the mean R-R interval (see Figure 8).