Which HRV metric measures what?
Heart rate variability (HRV) is the variation in time between successive heartbeats. The standard metric definitions come from the 1996 Task Force standards paper and are summarized in Shaffer and Ginsberg's 2017 overview. Time-domain metrics such as RMSSD and pNN50 are calculated from the gaps between beats, and frequency-domain metrics split that variation into bands.
| Metric | Domain | Definition |
|---|---|---|
| RMSSD | Time | Root mean square of successive RR interval differences |
| pNN50 | Time | Percentage of successive RR intervals that differ by more than 50 ms |
| SDNN | Time | Standard deviation of normal-to-normal (NN) beat intervals |
| LF power | Frequency | Power in the 0.04–0.15 Hz band |
| HF power | Frequency | Power in the 0.15–0.4 Hz band |
Recording length matters. Shaffer and Ginsberg cover "24 h, short-term (~5 min), and ultra-short-term (<5 min) HRV" and "caution that 24 h, short-term, and ultra-short-term normative values are not interchangeable." A 60-second reading from a watch shouldn't be compared with a published 24-hour norm.
Sources: Task Force of the ESC and NASPE, Circulation 1996; Shaffer & Ginsberg, Front Public Health 2017 (Tables 1–2 and abstract).
PPG vs. ECG for HRV
Most wrist and ring wearables estimate HRV from an optical photoplethysmography (PPG) pulse signal instead of an electrocardiogram (ECG). Strictly, PPG-derived variability "is often called pulse rate variability (PRV)," and a 2013 review of studies comparing it with ECG found it is a reasonable stand-in at rest but not during movement or stress.
| Factor | PPG (optical) | ECG |
|---|---|---|
| What it times | Pulse arrivals at the skin (pulse rate variability) | Heartbeats from the heart's electrical signal |
| Role in validation | The method being tested | "The gold standard technique" |
| At rest | "Sufficient accuracy," though short-term variability is often "somewhat overestimated" | Reference |
| During activity or mental stress | Agreement with ECG impaired, "often to an inacceptable extent" | Reference |
| Sensor position and algorithm | Effects "not conclusive" across studies | Not applicable |
Source: Schäfer & Vagedes, Int J Cardiol 2013 (abstract).
What affects accuracy
Three things decide whether a wearable HRV number can be trusted for your use. The first is movement and stress during the recording, which the Schäfer and Vagedes review found can break agreement between PPG and ECG. The second is recording length, because 24-hour, 5-minute and under-5-minute values are not interchangeable. The third is who is being measured: Shaffer and Ginsberg "stress the importance of measurement context, including recording period length, subject age, and sex." For a study or product, that means validating HRV against ECG in the conditions your users will actually be in, as laid out in our sensor accuracy and validation guide.
Wellness claim vs. device claim
Showing HRV as a general fitness or stress-and-recovery number is usually a wellness feature. Using HRV to detect, diagnose or manage a disease can make the product a medical device that FDA regulates. Where that line falls depends on your intended use and claims; our medical device decision guide walks through FDA's General Wellness guidance. This page is not regulatory advice.
Related resources
Frequently asked questions
What is the difference between RMSSD and SDNN?
Both are time-domain HRV metrics. RMSSD is the root mean square of successive differences between beat intervals, so it reflects beat-to-beat change. SDNN is the standard deviation of all normal beat intervals in the recording, so it depends heavily on how long the recording is.
Is HRV from a smartwatch as accurate as an ECG?
Not in all conditions. A 2013 review of studies comparing PPG with ECG found sufficient accuracy at rest, with short-term variability often somewhat overestimated, and much worse agreement during physical activity and some mental stressors.
Can I compare my wearable HRV to published norms?
Only if the recording length and context match. Published 24-hour, 5-minute and under-5-minute norms are not interchangeable, and age and sex also shift values.
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