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WHOOP 4.0: PPG Sensor and Recovery Science

WHOOP 4.0's sensor has 5 LEDs (three green, one red, one infrared) and 4 photodiodes. WHOOP computes HRV as RMSSD during sleep and uses it with resting heart rate, respiratory rate and sleep in its daily Recovery score.

Photograph of a black WHOOP 4.0 Onyx SuperKnit band, with the WHOOP wordmark on the clasp.
WHOOP 4.0 Onyx SuperKnit band: product photograph from WHOOP’s listing.

WHOOP 4.0, launched in September 2021, has an active sensor area with five LEDs (three green, one red and one infrared) and four photodiodes. The red and infrared LEDs support SpO2 (WHOOP 4.0 launch). WHOOP does not publish the sensor's sampling rate.

WHOOP calculates HRV as RMSSD measured during sleep, when movement, stimulants and daytime stress are lowest, and HRV is the most heavily weighted input to the daily Recovery score (WHOOP: heart rate variability). In a validation against ECG, WHOOP heart rate showed a bias of no more than 0.39% with limits of agreement of no more than 1.56%; for Ln RMSSD, limits of agreement were about ±5.93% with WHOOP's own filter. The authors concluded HRV should be interpreted against that level of error (Bellenger et al., 2021). One author's position was later sponsored by WHOOP.

In an independent six-device sleep laboratory study, WHOOP 3.0 matched polysomnography on 60% of epochs for sleep stage plus wake and on 86% for sleep versus wake (Miller et al., 2022). WHOOP's Strain score uses a 0–21 scale that combines cardiovascular and muscular load (WHOOP: Strain).

Sourced figures

Every number in this guide comes from the source linked beside it. Each study result applies to that study's devices, participants and protocol only.

What published studies show

Results quoted from 8 published validation studies, each against a clinical reference. A result applies to the model and protocol tested, not to every model the brand sells. From the wearable accuracy database (v1.0.0).

Heart rate at rest1 study

Only maker-funded evidence for WHOOP on this metric.

Heart rate accuracy across devices →

Heart rate during exercise2 studies

  • WHOOP 3.0High agreement only in squats; low in rows and burpees

    Study finding (no single number) · vs ECG chest strap (Polar H7/H10) · n = 29, Resistance exercise (5 exercises)

    Støve et al. (2022) Funding not reported

  • WHOOP 4.0 (upper arm)Strongest agreement (with Verity Sense) in graded treadmill

    Study finding (no single number) · vs ECG chest strap (Polar H7/H10) · n = 28, Rest, warm-up, burpees, Bruce treadmill

    Moghaddam et al. (2025) Funding not reported

Heart rate accuracy across devices →

Overnight heart rate2 studies

Heart rate accuracy across devices →

HRV (RMSSD)3 studies

HRV accuracy across devices →

Sleep stages (4-stage)4 studies

  • WHOOP (strap, 2.0 era)64% (κ 0.47)

    Epoch-by-epoch agreement · vs Polysomnography (PSG) · n = 12, Sleep laboratory, 10-day protocol, 86 sleeps

    Miller et al. (2020) Funding not reported

  • WHOOP (strap, 2.0 era)κ = 0.47

    Cohen's kappa · vs Polysomnography (PSG) · n = 12, Sleep laboratory, 10-day protocol, 86 sleeps

    Miller et al. (2020) Funding not reported

  • WHOOP (auto-detect)63%

    Epoch-by-epoch agreement · vs Polysomnography (PSG) · n = 6, Sleep laboratory, 9 nights, 54 sleeps

    Miller et al. (2021) Maker funded

  • Epoch-by-epoch agreement · vs Polysomnography (PSG) · n = 53, Sleep laboratory, 1 night, 9 h in bed, all devices worn together

    Miller et al. (2022) Maker funded

  • WHOOP 3.0κ = 0.44

    Cohen's kappa · vs Polysomnography (PSG) · n = 53, Sleep laboratory, 1 night, 9 h in bed, all devices worn together

    Miller et al. (2022) Maker funded

  • WHOOP 4.0κ = 0.37

    Cohen's kappa · vs Polysomnography (PSG) · n = 40, Sleep laboratory, 1 night

    Schyvens et al. (2025) Funding not reported

Sleep accuracy across devices →

Sleep vs wake (2-stage)4 studies

  • WHOOP 4.0> 90% (all devices)

    Sensitivity · vs Polysomnography (PSG) · n = 62, Sleep laboratory, 1 night

    Schyvens et al. (2025) Funding not reported

  • WHOOP (strap, 2.0 era)89% (κ 0.49)

    Epoch-by-epoch agreement · vs Polysomnography (PSG) · n = 12, Sleep laboratory, 10-day protocol, 86 sleeps

    Miller et al. (2020) Funding not reported

  • WHOOP (strap, 2.0 era)κ = 0.49

    Cohen's kappa · vs Polysomnography (PSG) · n = 12, Sleep laboratory, 10-day protocol, 86 sleeps

    Miller et al. (2020) Funding not reported

  • WHOOP (auto-detect)86%

    Epoch-by-epoch agreement · vs Polysomnography (PSG) · n = 6, Sleep laboratory, 9 nights, 54 sleeps

    Miller et al. (2021) Maker funded

  • Epoch-by-epoch agreement · vs Polysomnography (PSG) · n = 53, Sleep laboratory, 1 night, 9 h in bed, all devices worn together

    Miller et al. (2022) Maker funded

  • WHOOP 3.0κ = 0.44

    Cohen's kappa · vs Polysomnography (PSG) · n = 53, Sleep laboratory, 1 night, 9 h in bed, all devices worn together

    Miller et al. (2022) Maker funded

Sleep accuracy across devices →

Total sleep time1 study

  • WHOOP (strap, 2.0 era)+8.2 ± 32.9 min (n.s.)

    Mean bias (signed) · vs Polysomnography (PSG) · n = 12, Sleep laboratory, 10-day protocol, 86 sleeps

    Miller et al. (2020) Funding not reported

Sleep accuracy across devices →

WHOOP 4.0: PPG Sensor and Recovery Science Comparisons

Whoop vs Apple Watch: What the Sleep-Lab Studies Found

Two sleep-lab studies tested WHOOP and Apple Watch on the same people on the same night, against polysomnography (PSG) for sleep and ECG for heart rate. Miller and colleagues (2022) tested WHOOP 3.0 and Apple Watch Series 6 in 53 adults; Schyvens and colleagues (2025) tested WHOOP 4.0 and Apple Watch Series 8 in 62 adults. Overnight heart rate was close to ECG for both. WHOOP 3.0 tracked overnight HRV more closely than Apple Watch S6. For sleep stages the two studies point in different directions, so neither device clearly wins there.

View Comparison →

Whoop vs Oura Ring: Which Tracks Sleep and HRV Better?

WHOOP is a screenless band worn on the wrist or body; Oura is a ring worn on the finger. Two peer-reviewed studies tested both on the same people. Miller et al. (2022; 53 adults, one night with polysomnography and ECG) found WHOOP 3.0 and Oura Gen 2 close for sleep staging (60% vs 61% epoch agreement) and WHOOP closer for overnight HRV (ICC 0.99 vs 0.63); WHOOP supplied its raw data and funds the lab's research. Dial et al. (2025; 13 adults, 536 nights at home against ECG, no competing interests) tested the newer hardware and found the opposite for HRV: overnight RMSSD error was 5.96% for Oura Ring 4 and 7.15% for Oura Gen3 versus 8.17% for WHOOP 4.0. Resting heart rate error was 1.94%, 1.67% and 3.00%.

View Comparison →

WHOOP 4.0 vs 5.0: PPG Sensor Upgrade and Recovery Changes

WHOOP 5.0 and WHOOP MG launched in May 2025. WHOOP says they are 7% smaller than 4.0, sample sensors 26 times per second and last 14+ days per charge, versus five days for WHOOP 4.0. MG adds a medical-grade ECG via the clasp, and WHOOP's top membership adds Blood Pressure Insights. No independent study has compared 4.0 and 5.0 accuracy.

View Comparison →

WHOOP vs Garmin: Sleep and Heart Rate Accuracy from a Published Study

In Miller et al. (2022, 53 adults), WHOOP 3.0 agreed more closely with ECG overnight (HR ICC 0.99 vs 0.41) and with polysomnography sleep stages (60% vs 50%) than the Garmin Forerunner 245. WHOOP supplied raw data and funds the lab.

View Comparison →
View all comparisons →

Research records

Device records collect datasets, papers, and the sourced figures on this page. For accuracy across the brand, read How accurate is WHOOP?.

Frequently Asked Questions

How does WHOOP measure HRV differently from other wearables?

WHOOP measures RMSSD during sleep rather than during the day, so each night's value is taken under comparable conditions. WHOOP's descriptions have weighted the calculation toward slow-wave sleep. HRV values are not directly comparable across brands, which use different measurement windows.

Is WHOOP suitable for clinical research?

WHOOP provides API access to derived metrics such as heart rate, HRV, sleep and recovery, but not raw PPG waveforms. For waveform-level analysis, a device with raw data access is more appropriate.

How does WHOOP Strain Score work?

Strain is a personalized 0–21 score that combines cardiovascular load, from continuous heart-rate data, with muscular load. WHOOP combines them with a proprietary non-linear formula, so each extra point takes progressively more effort (WHOOP: Strain).

Sources

  1. WHOOP. Introducing WHOOP 4.0 with Any-Wear Technology
  2. WHOOP. Heart Rate Variability: How to Read and Improve Your HRV
  3. WHOOP. How does WHOOP Strain work? Calculation, scores, and Strain Target
  4. Bellenger et al. (2021). Wrist-Based Photoplethysmography Assessment of Heart Rate and Heart Rate Variability: Validation of WHOOP. Sensors. doi:10.3390/s21103571
  5. Miller, Sargent & Roach (2022). A Validation of Six Wearable Devices for Estimating Sleep, Heart Rate and Heart Rate Variability in Healthy Adults. Sensors. doi:10.3390/s22166317

Compare with Other Wearables

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