What Is a Normal HRV? Personal Baselines, RMSSD and SDNN

02Find the name behind the number

RMSSD and SDNN can both be displayed in milliseconds, but they summarize different aspects of beat-to-beat variation. A proprietary recovery score can combine several inputs and is a different quantity again.^[1][2]

03Why do online normal-value tables disagree?

Reference studies have used recordings from seconds to 24 hours, different ages and different criteria for “healthy”. Even studies using the same metric differ in time of day, equipment, activity and handling of medication or other influences. A population average is not an individual diagnostic cut-off.^[1]

The 2025 review searched through November 2022. Its publication date should not be mistaken for a search that included every study through 2025. It supports caution about a universal range, while compatible reference studies may still be useful for a defined population and protocol.^[1]

04How to make a personal comparison more useful

Keep the metric and device consistent. For a deliberate resting measurement, also keep time of day, posture, resting period and recording length comparable. For an overnight device, use the manufacturer’s defined overnight series consistently. Record changes in device, firmware or method so that a jump is not automatically attributed to your body.^[2][3]

Look at repeated valid readings alongside sleep, illness, activity and how you feel. Do not discard a low value just because it is unwelcome; identify missing or poor-quality recordings separately. Device-specific baseline windows differ, and one week is not a validated universal minimum for a stable baseline.

05The same 40 ms can lead to different questions

These are invented values, not reference ranges or health thresholds.

06Can a wearable be compared with an ECG?

Agreement is a question for a particular device, algorithm, metric and setting. A 2025 validation study compared five wearables with a chest-strap ECG reference across repeated nights in 13 adults. Agreement varied by device; hundreds of nights did not mean hundreds of independent participants.^[3]

That study tested basic nocturnal HRV measures, not the validity of every composite readiness score. Its reading note also retains a numerical inconsistency in one device’s bias and agreement limits. It should not supply a precise personal correction factor.^[3]

07A one-week observation sheet

Use the optional sheet to organize existing readings, not to chase a higher score. Write the metric, device and recording conditions first; then note a daily value, signal-quality issues and relevant context. Leaving a blank for missing data is better than turning it into zero.

At the end of seven days, ask whether the measurements were comparable and whether an apparent change coincided with a changed routine or method. The sheet does not calculate a diagnosis, a normal range or a validated recovery score. Seven days may be insufficient to describe your usual variation.

The CSV opens in a spreadsheet and contains no upload function. Keep completed copies private; there is no need to enter your name or medical details.

• Download the one-week HRV observation sheet (CSV) ↗
• Why does HRV drop? ↗
• What do wearable recovery scores mean? ↗

08What can you do with a change?

First check the measurement. Then consider the pattern and context. A persistent unexplained change or concerning symptoms deserves clinical interpretation rather than a conclusion based on a watch score. A reassuring HRV value should not override symptoms.

If you are testing a routine, choose a relevant outcome too: for example, sleep continuity, tolerability of a usual activity or a specified training measure. Raising HRV alone does not prove better recovery or a treatment effect.^[2]

• Can HRV measure recovery? ↗
• Breathing exercises: what does the evidence support? ↗

09Sources and limits

This guide uses the reference-value review, psychophysiology recommendations and the linked wearable validation study. A 2026 methodological review also argues for clear reporting of signals, windows, respiration and artefacts; its checklist is author-proposed, not an externally validated clinical score.^[4] The observation sheet is TAP’s educational aid, not a validated instrument.

10Funding and interests

Brozat and colleagues report no external funding and no conflicts. Laborde reports German Sport University Cologne HIFF 920147 and no commercial or financial conflicts. Dial reports US Air Force Research Laboratory S-194-11T-001 and no competing interests. Kazdagli’s funding and interest declarations have not been fully verified. Consulted sections and remaining uncertainties are recorded in the source notes.