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. 2019 Oct 29;9:15510. doi: 10.1038/s41598-019-52029-4

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© The Author(s) 2019

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Spectral density analysis of the mean flow velocity: Representative cases. To analyze the periodic component of the mean flow velocity (MFV) variation, a fast Fourier transformation was applied and the graphs of spectral density according to the frequency domain were drawn for a 33-year-old woman (upper panel), a 65-year-old man (middle panel), and a 60-year-old man (lower panel). As the result, a peak was observed in the very-low–frequency (VLF, 0.003–0.04 Hz) band during the sleep status (right panels), which was not prominent during the waking status (left panels).