Heart rate variability (HRV), defined as the variation in the time interval between consecutive heartbeats (Task Force of the European Society of Cardiology and the North American Society of Pacing & Electrophysiology, 1996), is a well-established surrogate marker of cardiac health and emotional well-being (Thayer et al., 2012). HRV may be characterized as a time-domain or frequency-domain measure. Time-domain measures are predominantly variations of the standard deviation of normal to normal R-R intervals (SDNN) reflecting overall HRV, although the root mean square of successive differences in heartbeats (RMSSD) is a metric that indexes parasympathetic influences of the automatic nervous system on cardiovagal tone (Russell et al., 2017; Task Force of the European Society of Cardiology and the North American Society of Pacing & Electrophysiology, 1996). In contrast to time-domain measures, the frequency-domain approach quantifies the distribution of the power spectral density of heart rate into