Garmin Metrics
HRV on the Wrist: What Garmin, Polar and Whoop Measure
August 2, 2026
The short answer
Garmin, Polar and Whoop all print a number labelled HRV, and they are not measuring the same thing. Garmin averages 5-minute windows across your entire sleep period, Polar's Nightly Recharge reads roughly the first four hours after you fall asleep, and Whoop weights its nightly average toward your last slow-wave sleep stage.
Same underlying physiology, three different sampling decisions. That is why your Garmin number and your friend's Whoop number can differ by 20 milliseconds and neither device is broken.
One disclosure up front: our depth on Garmin far exceeds our depth on Polar or Whoop. We hold multi-year Garmin histories from consenting users and nothing equivalent from the other two, so the Garmin evidence below is ours, while the Polar and Whoop material is their own documentation plus one peer-reviewed study, each linked.
Why "HRV" names several different measurements
Heart rate variability is the variation in time between consecutive heartbeats. Your heart does not tick at a fixed interval; the gap stretches and shrinks under your autonomic nervous system. Measure those gaps and you get a series of numbers in milliseconds. What you do with that series is where brands diverge, and the 1996 Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology defined the vocabulary for it in European Heart Journal. Two time-domain statistics matter for wearables:
RMSSD is the root mean square of successive differences between beat intervals. It responds mostly to parasympathetic (rest-and-digest) activity and stabilises quickly, which makes it usable over short recordings.
SDNN is the standard deviation of all normal beat intervals in the recording. It captures total variability from every source, and its value depends heavily on recording length. A 5-minute SDNN and a 24-hour SDNN are not comparable quantities.
So three decisions sit behind any HRV number: which statistic, over which window, from which sensor. Optical wrist PPG infers beat timing from a light signal through skin; chest-strap ECG reads the electrical signal directly. Change any one of the three and the number moves. Consumer brands changed all three, independently, without agreeing on anything.
What Garmin documents
Garmin measures HRV from the optical sensor on the back of the watch while you sleep. Per Garmin's health-science material, which we covered in our breakdown of what Garmin HRV Status is doing, the watch splits the night into 5-minute windows and averages them across the whole sleep period, and treats that full-night coverage as an advantage over methods that sample only a slice.
The Fenix 7 owner's manual sets the timeline: a 7-day average appears after roughly a week, and the watch needs three weeks of consistent sleep data before showing a status. Of the four statuses, Balanced, Unbalanced, Low and Poor, only "Poor" is defined against an age-referenced population range. The rest compare your seven-day average to your own baseline.
Garmin's consumer pages never name the statistic. They say "the time between your heartbeats" and stop there. The RMSSD attribution comes from independent work: Dial and colleagues, in the 2025 validation study covered below, state that every device they tested reports HRV as RMSSD.
What Polar documents
Polar runs two HRV systems, and they use different sensors, different windows and different reporting.
Nightly Recharge is the wrist one. Polar's Vantage V3 manual states that its ANS charge component "is formed by measuring your heart rate, heart rate variability and breathing rate during roughly the first four hours of sleep." It reports on a scale from -10 to +10 where zero is your usual level, and needs three nights before showing anything. Polar names the three inputs but never names the HRV statistic; Dial et al. describe the Grit X Pro as using a 4-hour window after sleep onset, matching Polar's wording.
Recovery Pro is the chest-strap one, and methodologically it is the most conservative thing any of the three brands ship. Polar's Recovery Pro documentation names the statistic outright: resting heart rate variability (RMSSD rest) and standing heart rate variability (RMSSD stand), both from an Orthostatic Test that requires an H10 or H9 chest sensor rather than the watch. You need at least three tests in the previous seven days, and your normal range comes from your own mean and standard deviation over the past four weeks; with fewer than four tests in that period, Polar falls back to population norms.
That is the most honest thing any of these three companies has published about HRV: the statistic, the sensor, the cadence, and what happens when your own data is too thin. Worth knowing that you cannot take it with you, though. Our guide to exporting Polar data found Nightly Recharge among the derived metrics Polar excludes from its account archive by design.
What Whoop documents
Whoop is the only one of the three whose primary source names the statistic without ambiguity, and it does it in an API schema rather than a help page: the Whoop developer documentation exposes the field as hrv_rmssd_milli. RMSSD, in milliseconds, confirmed. On inputs, Whoop 101 says Recovery is calculated when you wake from "measurements from the previous day and your sleep, such as resting heart rate (RHR), heart rate variability (HRV), respiratory rate, sleep duration/quality, skin temperature, and blood oxygen."
The window is where Whoop differs most. Dial et al. quote Whoop's own description as a "dynamic average during sleep… weighted towards your last slow wave sleep stage each night." That is a correction we owe our own readers: when we compared Garmin, Whoop and Oura recovery scores we flagged the slow-wave-sleep detail as unconfirmed because Whoop's site returned 403 to us. It now appears, quoted, in a peer-reviewed paper. The widely repeated "last five minutes of slow-wave sleep" refinement still has no primary source I have been able to open.
The three methods side by side
| Garmin | Polar Nightly Recharge | Polar Recovery Pro | Whoop | |
|---|---|---|---|---|
| Sensor | Wrist optical | Wrist optical | H10/H9 chest strap | Wrist optical |
| Window | Entire sleep period, 5-min windows | ~first 4 h after sleep onset | On-demand orthostatic test | Sleep, weighted to last SWS stage |
| Statistic named by the brand | No | No | Yes, RMSSD | Yes, RMSSD (API field) |
| Compared against | Your 7-day average vs your baseline | Your usual level, -10 to +10 | Your own 4-week mean and SD | Your own moving averages |
| Setup time before a status appears | 3 weeks | 3 nights | 3 tests in 7 days | Not documented |
Two things fall out of that table. None of the four numbers is a population score; every one is calibrated to you. And the only column naming both its statistic and its sensor is the one that requires a chest strap.
Why the chest strap is still the reference
ECG measures ventricular depolarisation directly. Optical PPG at the wrist infers beat timing from a blood-volume waveform several steps downstream, degraded by motion, perfusion, strap tightness and limb position. Schaffarczyk and colleagues (2022, Sensors, n=25) concluded that "linear HRV measurements derived from the Polar H10 sensor chest strap device recorded with the Elite HRV app correspond closely with measurements taken with a reference ECG in terms of RR intervals and HR", which is why validation work uses an H10 as the criterion rather than as another consumer device.
The best head-to-head available is Dial et al. (2025, Physiological Reports): 13 adults slept in five wearables simultaneously against a single-lead Polar H10 ECG at 1000 Hz, across 536 nights. For overnight RMSSD against ECG, by concordance correlation coefficient and mean absolute percentage error: Oura Gen 4 at 0.99 and 5.96%, Oura Gen 3 at 0.97 and 7.15%, Whoop 4.0 at 0.94 and 8.17%, Garmin Fenix 6 at 0.87 and 10.52%, Polar Grit X Pro at 0.82 and 16.32%. Thirteen people, specific model years, and site is confounded with brand because a ring sits on a finger and the rest sit on a wrist. Read it as evidence that wrist optical HRV lands within roughly 8 to 16 percent of ECG on a good night, not as a brand league table.
What our own Garmin data shows about the number you get
Our study of consenting users' Garmin histories, published in full at our recovery-signals research page, asked how much independent information Garmin's recovery numbers carry once you know the HRV.
Body Battery against overnight HRV: median within-athlete correlation of 0.674 across 82 athletes, interquartile range 0.574 to 0.793. Every one of the 82 was positive, and 97.6% cleared 0.3. Body Battery against sleep score ran similarly, median 0.683 across 85 athletes, again 100% positive. So the accuracy question for Body Battery is partly a question about HRV.
The positive control is what makes me trust the pipeline. HRV against resting heart rate is a genuinely inverse pair, and it came out inverse: median −0.696 across 82 athletes, 95.1% below −0.3, not a single athlete positive. A pipeline that finds a strong negative where physiology demands one is detecting real relationships rather than manufacturing agreement.
Cohort caveat: these are self-selected Garmin users who connected a third-party training-analytics app, likely fitter and more data-curious than average, and more consistent about wearing the watch overnight. Six accounts were excluded as non-independent. We hold no Polar or Whoop data, and nothing here should be read as measuring those platforms.
What to actually do with the number
Stop comparing your absolute value to anyone else's. This is the part I will argue for. Your HRV in milliseconds is a function of your age, your genetics, your sensor, your measurement window and your brand's summary statistic, and only one of those five is about your training. Comparing your 48 ms against a training partner's 71 ms tells you approximately nothing, and comparing either against a published "normal range" tells you less, because those ranges mostly come from short supine ECG recordings rather than a night of wrist PPG. Your baseline trend is the only part of this that travels.
Do not switch platforms mid-block and expect continuity. A device change resets the window, the statistic and the sensor at once. Budget three weeks of blindness on Garmin, three nights on Polar Nightly Recharge, and an undocumented period on Whoop.
Protect the window before you read the number. Garmin averages the whole night, so a short or broken one gives the algorithm less to work with, while Polar's four-hour window makes your sleep-onset time matter more. Consistent sleep timing is the cheapest accuracy upgrade available, and it shows up across how Garmin tracks athletes' sleep too. Then read the direction rather than the digit, which is the same logic behind Garmin's Training Readiness score.
If you want a number you can defend, buy a chest strap. Polar's Recovery Pro is the only documented consumer route to RMSSD from an ECG signal, at a fixed body position, on a schedule you control.
Frequently Asked Questions
Why is my Garmin HRV different from my Whoop HRV?
Different windows over the same night. Garmin averages 5-minute windows across the entire sleep period; Whoop weights toward your last slow-wave sleep stage. Two different slices of one night produce two different averages, so there is no reason to expect the digits to match. The trends should still move together.
Do Garmin, Polar and Whoop all report RMSSD?
Whoop and Polar's Recovery Pro name RMSSD in their own documentation. Garmin and Polar's Nightly Recharge do not name a statistic publicly; the RMSSD attribution for those comes from Dial et al. (2025), not from the manufacturers.
Is there a normal HRV range I should aim for?
Not one that transfers between devices. Published population ranges mostly come from short supine ECG recordings, which is a different measurement from an overnight wrist average. Compare yourself to your own baseline over weeks.
Does Polar's Nightly Recharge use the same measurement as Recovery Pro?
No. Nightly Recharge reads the wrist sensor during roughly the first four hours of sleep. Recovery Pro uses RMSSD from an Orthostatic Test taken with an H10 or H9 chest strap while you are awake.
The interesting question is not which brand's HRV is right. It is why each of them chose a different window and then printed the output under the same three letters. Garmin picked the whole night for coverage, Polar the first four hours for the settling curve, Whoop deep sleep for signal cleanliness. Every one of those arguments is defensible, and every one produces a number that only means something inside its own system. Until a brand publishes its statistic and its window on the same page, the honest reading of your HRV is a direction, not a value. Our respiration-rate explainer covers another metric with the same problem, and the methodology behind our studies is public for the same reason.
