How reliable are the heart rate scores from your wearable?
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.
Is an ECG from your smartwatch actually useful?
For detecting atrial fibrillation, a smartwatch ECG can be useful, especially as a complement to medical monitoring. For other heart conditions, the evidence is still too limited.
Is a calcium score of my heart worth doing?
For people between 40 and 75 years old in a grey zone regarding statins, a calcium score is a well-supported tool for making a well-considered decision about medication together with your doctor.
Does my heart rate variability (HRV) tell me anything about my health?
A higher HRV is associated with better health and greater resilience. Slow breathing is the most accessible and best-supported way to work on this yourself.
Does your resting heart rate say anything about how fast you are ageing?
A structurally high resting heart rate, certainly above 80 to 90 beats per minute, is associated with a clearly higher risk of premature death. Exercising to lower your resting heart rate is worthwhile, but a very low heart rate at an older age is not always a good sign: if in doubt, discuss it with your doctor.
What does my CRP value (inflammation) say about my heart?
A high hsCRP value points to chronic inflammation and predicts a clearly elevated long-term risk of cardiovascular problems, even more strongly than well-known cholesterol values. It is an independent risk factor that deserves serious attention.
Are calcium supplements bad for your heart?
Calcium supplements may be associated with an increased heart risk, but not all studies confirm this. It is best to get calcium from your diet, and discuss with your doctor whether a supplement is truly necessary for you.