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HRV Time-Domain Measurements for Smart People

Updated: Mar 24

Female athlete running hurdles

Note: Athletes use the RMSSD to monitor workout intensity and their recovery. Graphic © Josep Suria/

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What Does Heart Rate Variability (HRV) Mean?

HRV is the organized fluctuation of time intervals between successive heartbeats defined as interbeat intervals. HRV is associated with executive function, regulatory capacity, and health . . . Cardiac vagal control indexes how efficiently we mobilize and utilize limited self-regulatory resources during resting, reactivity, and recovery conditions (Shaffer, Meehan, & Zerr, 2020).

The oscillations of a healthy heart are complex. HRV indexes how efficiently we mobilize and utilize limited self-regulatory resources to maintain homeostasis. HRV plays a vital role in regulatory capacity, executive functions, health, and performance. A healthy heart can rapidly adjust to sudden challenges due to the cooperation of interlocking and better-calibrated control systems. HRV is crucial to health, performance, and resilience. Behavioral interventions like aerobic exercise, healthy breathing, compassion, and mindfulness meditation are powerful strategies for increasing HRV.

Dr. Gevirtz explains HRV © Association for Applied Psychophysiology and Biofeedback.

A healthy heart is not a metronome (Shaffer, McCraty, & Zerr, 2014).

What Are HRV Time-Domain Metrics?

We can measure heart rate variability (HRV) using time- and frequency-domain metrics. Time-domain metrics quantify the amount of variability in measurements of the interbeat interval (IBI), which is the period between successive heartbeats. Frequency-domain values quantify absolute or relative power distribution into four frequency bands. An IBI is also called an R-R interval because it is the time between adjacent R-spikes. Graphic © arka38/

Interbeat Interval

We measure the time intervals between successive heartbeats in milliseconds (ms). The software starts counting after detecting the first beat and calculates the first IBI in ms after detecting the second beat. This process is repeated until the end of the epoch (data collection period). Graphic adapted from Dr. Richard Gevirtz.

Interbeat Intervals

Note: the numbers in boxes are IBIs measured in milliseconds.

A Few Cautions

We cannot compare HRV time-domain values (e.g., see RMSSD and SDNN below) obtained during slow-paced breathing to resting norms. Brief resting measurement periods underestimate 24-hour time-domain values. Nunan et al. (2010) found that published brief values for healthy adults were lower than Task Force (1996) norms. Although "gold standard" metrics like 24-hour SDNN predict cardiac risk, short-term SDNN does not. Finally, consumers should exercise caution in interpreting smartphone time-domain values because apps either do not artifact or perform limited data clean-up, inflating RMSSD and SDNN values. To address this problem, export text file data from the device software to additional analysis software.