How reliable are the heart rate variability and readiness scores from your wearable actually for your health?
A good wearable measures resting heart rate reliably, but HRV scores and readiness are less accurate than they appear. Use them as a signal over the longer term, not as a hard daily diagnosis.
How much you can trust your wearable depends strongly on what you are measuring, which device you are using, and how you apply that measurement. Resting heart rate is by far the most reliable measurement: Oura Gen 3 and Gen 4 are within 2% of an ECG reference. WHOOP 4.0 performs slightly less well (deviation ~3%), Garmin and Polar have larger errors, with the Polar Grit X Pro sometimes deviating 16% or more on HRV. With Polar, that error can also vary wildly from night to night, meaning that even if your average looks reasonable, an individual measurement can be far from reality.
The heart rate variability (HRV) reported by your wearable is also technically a different measure from what is recorded in medical research. Wrist sensors measure light signals rather than electrical heart signals; the outcomes are comparable but not identical. At a low sampling frequency, which is the case for many watches, the deviation can reach 20%. Oura scores best here for accuracy as well (error ~6-7%). A chest strap performs well at rest, but during intense exercise the more advanced HRV metrics become unreliable even for the better-performing chest strap.
Tracking sleep is reasonably accurate for total sleep time: all brands studied estimate this with 86-89% accuracy. If you want to know how much deep sleep or dream sleep you have had, accuracy is much worse, only 50-65%. That is too little for meaningful conclusions on a per-night basis.
The readiness score, which wearables base on HRV and sleep, barely predicts how fit you will feel the next day. In a study of military personnel, nightly HRV explained only 3% of the differences in perceived physical fitness; mental fitness was not predicted at all. A small study of police officers showed that sleep duration was a slightly more consistent predictor than HRV, but also that the associations varied strongly from person to person.
The practical conclusion: your wearable does a good job of measuring resting heart rate, especially with Oura or Apple Watch. HRV and sleep-stage classification are useful as a signal over weeks or months, but do not place too much trust in a single night or readiness score. Energy expenditure is unreliable across all brands, including the Apple Watch. For medical decisions, no consumer wearable is a substitute for clinical equipment.
This answer is based on a combination of comparative studies in which wearables were evaluated alongside an ECG reference (in 13 healthy adults over 536 nights), a systematic summary of studies on the Apple Watch (56 studies combined), a systematic review of multiple wearables under laboratory conditions, and smaller studies involving military personnel and police officers. The evidence is strongest for resting heart rate and HRV (multiple comparisons, fixed reference equipment). For readiness and sleep stages, the studies are smaller and participant numbers are low, which means the findings are not broadly generalisable.
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