Pulse rate variability

Heart rate variability, strictly speaking, comes from the heart’s electrical signal: the RR intervals RR intervals RR interval The time from one heartbeat to the next, measured in milliseconds between the R peaks of an ECG. HRV is calculated from a series of these intervals. More about RR interval on an ECG ECG ECG A recording of the heart's electrical activity, taken with electrodes on the skin. ECG-based RR intervals are the reference standard for measuring HRV. More about ECG . Most wearables do not record that signal. They use an optical sensor ( PPG PPG PPG An optical method that shines light into the skin and detects the pulse of blood with each heartbeat. Most watches, rings and phone-camera apps use it to estimate HRV. More about PPG ) to detect the pulse of blood in the skin and time the gap from one pulse to the next. Variation in those gaps is pulse rate variability, or PRV.

Apps generally show PRV under the label “HRV”, and at rest that is mostly fair. A review by Schäfer and Vagedes compared PRV with ECG-based HRV across many studies. At rest, the agreement was good enough to use PRV as a stand-in, although PRV tended to slightly overestimate short-term variability, which the authors linked to interactions between breathing and the circulation. During physical activity, and with some mental stressors, agreement got worse, sometimes far worse. The studies used such different methods that the review could not give one overall error figure.

For your own numbers, a reading taken while still, such as during sleep or a morning spot check, is a fair estimate of your HRV. A reading taken while moving is much less reliable. Devices also differ in where the sensor sits and how their software finds each pulse, so two of them may not agree even on the same night.

The lesson Why your devices disagree on HRV covers this in more detail, and Can a phone camera measure HRV? looks at one common PRV source.