Acoustic wave velocity 3 Breathing disorders 3 Detrended fluctuation analysis 3 Entropy 3 HRV 3 Heart 3 Heart rate variability 3 Multiscale entropy 3 Photoplethysmography 3 Polysomnography 3 SampEn 3 Sampen Self-similarities 3 Sleep monitoring 3 Sleep research 3 Sleep studies 3 Spectrum analysis 3 Tachograms 3 Wave propagation 3 Wearable technology 3 breathing disorders 3 detrended fluctuation analysis 3 multiscale entropy 3 polysomnography 3 self-similarity 3 sleep 3 spectral analysis 3