The ECG signals were automatically processed and analyzed by open-source HRV algorithms [18] . The standard HRV analysis has been well reviewed [19] . Briefly, time domain measures of HRV include the mean heart rate and standard deviation of the normal interbeat intervals (SDNN), the root mean square successive difference between adjacent normal interbeat intervals (RMSSD), and the percentage of adjacent intervals that varied by greater than 50 ms (pNN50) [20] . The SDNN assesses the overall variability of interbeat intervals. The RMSSD and pNN50 measure the short-term variation of interbeat intervals, which is mainly modulated by parasympathetic innervation [21] . Standard spectral HRV measures [19] include high-frequency power (HF; 0.15-0.40 Hz), low-frequency power (LF; 0.04-0.15 Hz), and very low-frequency power (VLF; 0.003-0.04 Hz). LF power is suggested to be modulated by both sympathetic and parasympathetic activities, whereas HF power is mainly modulated by parasympathetic activity [22, 23] . The LF/HF ratio is considered a measure of the shift of sympathovagal balance toward sympathetic activity [19, 24] . The physiological mechanism underlying VLF power is disputed but has been suggested to be mediated partly by the renin-angiotensin-aldosterone system or parasympathetic modulation [19, 25, 26] .