What Topol published and why it matters now
Topol's essay "The Big Holes in Wearable Heart Rate Variability And Readiness Scores" appeared in his newsletter Ground Truths ten days after Apple announced Apple Watch Series 12 on 9 September 2026. Apple added a 0–10 readiness score and says HRV is now measured up to 24 times more often, joining the composite scores Oura, WHOOP and Garmin already ship.
His verdict: "we're not ready for readiness scores." The scores are proprietary, their algorithms undisclosed, and they are "unvalidated against health outcomes."
Key findings from the review
Most wearables measure pulse rate variability optically, while the research linking low HRV to mortality used ECG. The two agree mostly at rest. The evidence behind his verdict:
- A validation of five wearables in only 13 healthy adults (536 nights) found overnight HRV errors from about 6% (Oura Gen 4) to 16% (Polar Grit X Pro), with WHOOP 4.0 and Garmin Fenix 6 in between (Physiological Reports).
- A 2026 systematic review of 43 studies found only 10 with enough data to pool, mostly at rest, and warns against reading the results as evidence that optical and ECG HRV are interchangeable (Sensors).
- A 2025 review cited by Topol catalogued 14 readiness and recovery scores from 10 manufacturers: HRV is an input in 86% of them, resting heart rate in 79%, physical activity and sleep duration in 71% each, and none of the companies disclosed its exact formula (review).
- In more than 46,000 UK Biobank participants, low measured HRV predicted higher mortality, while genetically predicted HRV did not; Topol's reading: HRV most likely reflects a person's physiological state and has no shown causal role (study).
- Topol names resting heart rate as the only metric shown consistently across devices to be highly accurate.
| Question | What the review reports | What to do with your data |
|---|---|---|
| Is a readiness score comparable across brands? | Each brand uses its own formula; a 90 on one device has no relationship to a 90 on another, and companies can change the algorithm without notice. | Compare yourself only with yourself, on the same device. |
| Which HRV reading is closest to the real thing? | Overnight, minimal motion, RMSSD, as a 2–3 week trend. | Build a baseline before reacting to one night. |
| Does raising HRV improve health? | Topol found no peer-reviewed evidence for it. | Use HRV to see what disrupts your nights, and keep health decisions with your doctor. |
Read your HRV as a trend against your own baseline
Longvai builds a personal control chart for each metric and flags drift beyond 2–3 standard deviations of your own range. Free plan: up to 2 sources.
How to read your own HRV after this review
Useful HRV is a personal, multi-week trend tied to events in your life. Exercise, sleep quality, stress and alcohol all move it, so one low night says little. Three rules follow:
- Use one device, overnight RMSSD, and 2–3 weeks of data before drawing conclusions (our HRV guide explains the metric).
- Read resting heart rate next to HRV, and log drinks, hard sessions, late meals and symptoms the same day.
- Test a suspected cause with an n=1 experiment, as in our protocol for measuring what a drink costs your HRV and sleep.
Topol also warns that fixating on these numbers can feed anxiety ("orthosomnia"); a weekly look at trends is enough.
Where Longvai fits
Longvai reads raw numbers from Oura, WHOOP, Garmin, Fitbit and Apple Watch (via Apple Health) and explains them against your own baseline. The Experiment Tracker runs randomized 2–6 week n=1 trials and reports Cohen's d, so you see whether a change beats your normal noise. Longvai is not a medical device. See our HRV explainer and how Longvai compares with device apps.
FAQ
Are readiness scores accurate?
The sensors are reasonably accurate in small studies of healthy adults at rest. The composite scores are proprietary and, per Topol's September 2026 review, unvalidated against health outcomes.
Can I compare my Oura readiness with WHOOP recovery or Apple readiness?
Brands use different inputs and undisclosed weightings, so the numbers are not interchangeable. Compare raw overnight HRV and resting heart rate within one device.
How reliable is wearable HRV?
It correlates well with ECG at rest in small studies of healthy adults. Overnight RMSSD is the closest proxy; 2–3 week trends beat single nights.
Should I worry if my HRV is low for a few days?
Hard training, poor sleep, stress and alcohol all lower it. If a drop persists without a clear cause or comes with symptoms, see a doctor.
Get answers you can check from your wearable data
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