Polar HRV: Nightly Recharge, H10 and accuracy

Polar gives you HRV three ways: overnight from a watch or the Polar Loop band, as a 4-minute Orthostatic test Orthostatic test Orthostatic test A short morning test: lie or sit still for 2 minutes, then stand for 2 minutes, recording heart rate and HRV in both positions to see how your body handles standing up. More about Orthostatic test in the morning, and as raw beat-to-beat intervals from the H10 chest strap, which other apps can read. They use different sensors and different windows, so keep their numbers in separate series.

Documentation checked: 21 October 2026. Polar updates its watches and apps often, so details can change.

Polar Nightly Recharge and the ANS charge

Nightly Recharge has two parts. The ANS ANS Autonomic nervous system The part of the nervous system that runs the body without conscious control, including heart rate. HRV mostly reflects how it adjusts the heart from moment to moment. More about Autonomic nervous system charge shows how well your autonomic nervous system calmed down early in the night; the sleep charge compares your sleep score with your usual one. Both are set against your own last 28 days, and together they give a status from very poor to very good (Polar Support).

The ANS charge combines three measurements:

• Heart rate, which has “the biggest influence” on the score.
• HRV as RMSSD RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD , a standard measure of how much one beat-to-beat gap differs from the next (Task Force, 1996).
• Breathing rate Breathing rate Breathing rate How many breaths you take a minute. It moves HRV on its own: slower, deeper breathing makes the beat-to-beat swing larger, and below about 9 breaths a minute it shifts from the HF band into LF. More about Breathing rate , worked out from the same beat intervals, with the smallest influence.

The score runs from −10 to +10, and around zero is your usual level (Polar Support). Polar’s 2019 white paper adds that the usual band is roughly −2 to +2, that the limits come from the average and spread of your past 28 days with outliers removed, and that the ANS charge outweighs the sleep charge: an ANS charge much below usual puts the status in red whatever the sleep was like (Polar white paper).

What is in a readiness score?

From the makers' own documentation

Polar Nightly Recharge (ANS charge) −10 to +10, where zero is your usual level over the past 28 days. Needs three nights to start.

• ✓ HRV : named by the maker Heart rate variability (RMSSD) from roughly the first four hours of sleep.
• ✓ Resting heart rate : named by the maker Heart rate over the same window: "Heart rate has the biggest influence when forming the score."
• ✓ Breathing rate : named by the maker Breathing rate, "the smallest" influence.
• – Sleep : not named Not named by the maker.
• – Temperature : not named Not named by the maker.
• – Activity or load : not named Not named by the maker.

Also: A separate Sleep charge compares the sleep score with your usual level.

Polar's Nightly Recharge (ANS charge) names 3 of the six inputs: HRV, resting heart rate and breathing rate. Polar ranks the three inputs by influence but does not give the weights.

Source: Nightly Recharge recovery measurement (Polar Support)

A list of scores, one per device, with six inputs for each: HRV, resting heart rate, breathing rate, sleep, temperature and activity or load. Polar's Nightly Recharge (ANS charge) names 3 of the six inputs: HRV, resting heart rate and breathing rate. Polar ranks the three inputs by influence but does not give the weights.

You need continuous heart rate tracking switched on, more than 4 hours of sleep, and three nights before the first status appears (Polar Support). Until then the watch shows the heart rate, HRV and breathing details on their own (Polar user manual, Vantage V3). Polar lists Nightly Recharge for watches in the Vantage, Grit X, Pacer, Ignite and Unite lines, among others, and for the Polar Loop, a band with no screen (Polar Support; Polar, Loop).

Why only the first four hours

Polar’s developer documentation and white paper define the window: a 4-hour period starting 30 minutes after you fall asleep (Polar AccessLink API; Polar white paper). Polar’s FAQ says the first hours of sleep “reflect recovery more sensitively than measurements averaged from a whole night’s sleep” (Polar Support, FAQs). The white paper bases that on earlier studies and on Polar’s own unpublished findings.

The window matters when you compare devices. The early hours hold most of the night’s deep sleep (Polar Support, FAQs), and RMSSD differs from one sleep stage to the next (Herzig and colleagues, 2018), so a 4-hour value and the whole-night average most rings and bands report can differ on the same night. The model below shades both windows on one simulated night.

Same night, different numbers

A teaching model, not a measurement

Polar Oura

Sleep stages lights out to waking

• Awake
• REM
• Light
• Deep

HRV every 5 minutes (RMSSD) 20 to 60 ms

0h002h004h006h008h00

Polar

44.8 ms RMSSD

Roughly the first four hours after falling asleep.

Same night, same heart: Polar 44.8 ms and Oura 47.5 ms, because each device averages different hours.

Another night Night 1 of 5

The buttons work when JavaScript is on. The window for each device is listed in the description.

Two charts over eight hours of simulated sleep. The top chart is a hypnogram stepping between awake, REM, light and deep sleep, with deep sleep mostly in the first half and REM mostly in the second. The bottom chart is a line of HRV values, one per five minutes, that rise gently through the night and dip during wakeful minutes. One button per device shades the part of the night that device averages. Polar: 44.8 ms RMSSD. Oura: 47.5 ms RMSSD. Same night, same heart: Polar 44.8 ms and Oura 47.5 ms, because each device averages different hours.

How to see your HRV on Polar

The ANS charge is a score, but the RMSSD behind it is shown too:

• On the watch. In the Nightly Recharge view, tap ANS charge. The details list “Heart rate variability (4 h average)” in ms, your 28-day average and a seven-day graph, beside heart rate, the average beat-to-beat interval beat-to-beat interval 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 and breathing rate (Polar user manual, Vantage V3).
• Seven-day trends. Firmware 6.0.18, released on 30 September 2026 for the Vantage V3 and Grit X2, added seven-day graphs of nightly HRV, heart rate and breathing rate on the watch (Polar Support, firmware updates).
• In the Polar Flow app. Choose Nightly Recharge from the menu and tap the ANS charge box for the detailed view (Polar Support).
• Through Polar’s API. Apps you connect to your Polar account receive the 4-hour RMSSD in ms and the 5-minute HRV samples behind it (Polar AccessLink API).

Where do I find HRV on my device has the same steps for other brands.

The Polar Orthostatic test and Recovery Pro

The Orthostatic test is a separate reading you start yourself. You lie or sit still for 2 minutes, stand when the watch tells you, and stay still for 2 more. It reports heart rate and RMSSD lying, heart rate and RMSSD standing, and the peak heart rate after you stand. Polar asks you to test at the same time each morning, after waking and before breakfast, in the same position. After two tests in 28 days you get feedback; after four, your latest result is compared with your own normal RMSSD range from the past four weeks (Polar Support, Orthostatic test).

It needs a Polar H9 or H10 strap (an older H6 or H7 also works). The four Polar watches with a wrist 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 sensor, the Vantage V3, Vantage M3, Grit X2 and Grit X2 Pro, can also run it without a strap: you rest a finger on the top-left button during the test (Polar Support, wrist ECG; Polar user manuals, Vantage V3 and Grit X2). Their separate 30-second ECG test shows HRV in ms too, and Polar states the wrist ECG is “not intended for any medical use” (Polar user manual, wrist ECG).

Recovery Pro builds on the test, adding questions about soreness, fatigue and sleep and your training history (Polar Support, Recovery Pro). Only your first successful test each day counts toward it (Polar user manual, Vantage V3). Polar suggests Recovery Pro if you are “highly performance-oriented” and Nightly Recharge if you are a regular trainer balancing training with other commitments (Polar Support, Recovery Pro). The two read HRV differently. The ANS charge treats higher HRV as better recovery, while Recovery Pro counts HRV above your normal range, as well as below it, as incomplete recovery (Polar white paper).

Overnight vs morning HRV explains why the two belong in separate series, and what is an orthostatic HRV test covers who it suits.

Polar H10 HRV accuracy

A watch shines light into the skin and times each pulse as it arrives at the wrist. The H10 measures “the electrical activity generated by the heart muscle” (Polar, H10): it is a one-lead ECG that times each R peak and sends the gaps between beats over Bluetooth. Polar’s FAQ calls ECG sensors like the H10 “more accurate, especially for heart rate variability (HRV) measurements”, and less affected by movement than optical sensors (Polar, Verity Sense).

1. Two electrode pads, wetted, pick up the heart’s electrical signal through the skin
2. The pod amplifies the signal and finds each R peak
3. R peaks: the gap from one to the next is the RR interval
4. The phone receives the RR intervals over Bluetooth, in steps of 1/1024 of a second

Three panels joined by arrows. First, the front of a torso with a strap around the chest just below the chest muscles; two rectangular electrode pads sit on the inside of the strap on either side of a small pod at its centre. Second, an ECG strip with four sharp spikes, the R peaks, with the gaps between them labelled 812 ms, 845 ms, 799 ms. Third, a phone whose screen lists the same three RR intervals in milliseconds.

Researchers use it as their yardstick. The 2025 study that tested Polar’s watch used an H10, recording at 1,000 samples a second, as the reference for every device (Dial and colleagues, 2025), and so do several studies in the device validation tracker. Checked against laboratory ECGs, it holds up:

• Lying, breathing deeply and standing: in 43 healthy adults, concordance of 0.99 or more and an average error under 1% for RMSSD (Blalock and colleagues, 2026).
• Reclined, in 37 trained adults: an average error of 2.16% and a bias of 0.4 ms against a 3-lead ECG (Johansson and colleagues, 2026).
• During exercise: in 10 adults doing five activities from easy to hard, 99.6% of beats timed correctly, and 99.4% at high intensity (Gilgen-Ammann and colleagues, 2019).

A review of wearables for 24-hour HRV concluded that the H10 “appears to be the most accurate wearable device when compared to criterion measures” (Hinde and colleagues, 2021). Each study used its own protocol, so the figures describe the strap and do not rank devices.

Recording clean RR intervals

The H10 works with Polar’s watches and apps and with other brands’ devices (Polar, H10). For HRV, pick an app that saves the RR intervals, since heart rate alone is not enough; Elite HRV, HRV4Training and Kubios read them over Bluetooth. The chest-strap test on this site reads them in Chrome or Edge, and the RR converter turns a Polar Flow session or a Polar Sensor Logger file into a plain RR list. Wet the electrodes, settle for a minute or two and measure at the same time each day; the chest-strap guide covers what goes wrong and how to fix it.

Verity Sense and OH1: optical, worn on the arm

Polar’s Verity Sense and OH1 use light, like a watch, but you wear them on the arm or, clipped to swimming goggles, at the temple (Polar, Verity Sense; Polar, OH1+). Polar says the OH1 gives no real-time HRV during exercise and that “Polar H10 is a more suitable device for HRV data” (Polar, OH1+). At rest the OH1 comes close. Against an H10 in 31 adults, the OH1’s 5-minute RMSSD read 3.21 ms higher on average lying down (intraclass correlation 0.955) and 8.05 ms higher sitting (0.834) (Coste and colleagues, 2025).

How accurate are Polar watches overnight?

Dial and colleagues followed 13 healthy adults at home over 536 nights, 206 of them with a Polar Grit X Pro. Because Polar reports only the first four hours, the researchers worked out the strap’s RMSSD over the same four hours for the comparison. Polar’s concordance with the strap was 0.82, with an average error of 16.32% and 7.27 ms. Its bias and percentage error were significantly larger than every other device’s, and the authors call its agreement poor (Dial and colleagues, 2025).

Overnight RMSSD against a Polar H10 chest strap

Dial and colleagues, 2025

• Polar
• Other devices tested

13 adults, 536 nights at home. The authors report the Polar Grit X Pro’s average bias and percentage error as significantly higher than every other device’s.

Source: Dial and colleagues, Physiological Reports, 2025. Every study we have checked, with its numbers, is in the device validation tracker.

A dot chart in two panels, one row per device tested by Dial and colleagues (2025) against a Polar H10 chest strap. The first panel shows concordance (CCC), where 1 is perfect agreement; the second shows the mean absolute percentage error (MAPE), where lower is closer. Oura Ring Gen 4: concordance 0.99, average error 5.96%. Oura Ring Gen 3: concordance 0.97, average error 7.15%. WHOOP 4.0: concordance 0.94, average error 8.17%. Garmin Fenix 6: concordance 0.87, average error 10.52%. Polar Grit X Pro: concordance 0.82, average error 16.32%. 13 adults, 536 nights at home. The authors report the Polar Grit X Pro’s average bias and percentage error as significantly higher than every other device’s.

The 95% limits of agreement spanned about 38 ms, so a single night could sit well away from the strap’s value. In a second analysis that compared each night with the person’s own usual values, closer to how Polar reads it, Polar’s concordance was 0.79, near the Garmin’s 0.77 and WHOOP’s 0.76.

Two older studies tested other Polar watches:

• Polar Vantage V, sleep laboratory: in a study of 53 healthy young adults over one night, with the ECG matched to Polar’s 4-hour window, the watch read 8.7 ms low on average, with an intraclass correlation of 0.65 (Miller and colleagues, 2022). The authors’ group receives funding and equipment from WHOOP.
• Polar Vantage V2, at home: researchers analysed the watch’s raw pulse intervals over the same 4-hour window against an H10. In 29 recreational endurance athletes, agreement for log RMSSD log RMSSD lnRMSSD The natural logarithm of RMSSD, with no unit. Taking the log evens out the spread of RMSSD values, which makes day-to-day changes easier to compare and analyse. More about lnRMSSD was very strong (intraclass correlation 0.931), with values running high, especially in people with low HRV (Nuuttila and colleagues, 2021). Polar partly funded the study.

The three studies used different models, windows and references, and the third analysed beats rather than the app’s number, so they do not rank anything. All three tested healthy adults. We found no published validation of overnight HRV from the Vantage V3, Grit X2 or Polar Loop.

How to read Polar’s numbers

• Look at the milliseconds before the score. Heart rate drives the ANS charge most, so a low charge can come from a higher heart rate while RMSSD barely moved.
• Read weeks, not nights. At an RMSSD of 50 ms, the Grit X Pro’s 16% average error is about 8 ms, so a one-night change of that size may be measurement error. The seven-day graph is a better guide than one morning.
• Know what 28 days hides. The usual level follows you. If you start tracking during a stressful stretch or an illness, that becomes your usual, and Polar’s white paper says Nightly Recharge cannot reveal chronic stress for that reason.
• Keep conditions the same. Same wrist, same tightness, continuous heart rate on. A new watch or a firmware update can shift the values; see can a software update change my HRV.
• Compare Polar only with itself. Its 4-hour RMSSD is not a whole-night average, so it will not match an Oura, WHOOP or Garmin number. HRV by device lists what each brand measures, and normal HRV explains what published values can tell you.

For other brands, see Garmin HRV Status, WHOOP vs Oura and Garmin vs WHOOP.

When to talk to a doctor

Neither the H10 nor a Polar watch diagnoses anything, and the ANS charge is not a medical test. Talk to a doctor if low HRV or a run of poor statuses comes with feeling unwell, lasting fatigue, palpitations, dizziness, fainting or chest pain, whatever the number says.