The HLTH Journal·The Science
How the HLTH Band measures HRV, and what our accuracy study found
A number that does not reflect what is actually happening in your nervous system is worse than no number at all.
HLTH Science Team· May 2026 · 5 min read
Your HRV reading is only as good as the device that takes it. A number that does not reflect what is actually happening in your nervous system gives you false confidence in decisions that should be based on real data.
Key takeaways
- Overnight HRV matched the Polar H10 chest strap at an R² of 0.93 and an RMSE of 3.6 ms.
- The band reads about 1.2 ms lower on average: a stable offset, not random noise.
- No funnel shape in the Bland-Altman plot, so it does not get less reliable at high or low readings.
- Accuracy narrows above roughly 160 bpm, which applies to every wrist-worn optical sensor.
How HRV accuracy is actually measured
Accuracy testing for wearable devices is not as simple as comparing two numbers side by side. A device can produce the right number once by chance. What matters is whether it produces consistent, reliable numbers across hundreds of measurements, and how far those numbers drift from a validated reference device. The standard method for this is called Bland-Altman analysis. It tests whether two measuring devices are interchangeable in practice by plotting the difference between paired readings against their average. If a device is accurate, the differences cluster tightly around zero with no pattern. A funnel-shaped cloud means the device becomes unreliable at higher values. A tilted cloud means it reads systematically high or low. Neither pattern appeared in our results. The second measure is RMSE, which stands for Root Mean Square Error. It gives a single number representing the average gap between the two devices across all paired readings. For HRV, an RMSE below 5 ms is generally considered acceptable for trend monitoring.
The reference device
To test a wrist-worn device, you need something more accurate to compare it against. We used the Polar H10 chest strap, which captures heart electrical signals rather than optical readings from the skin. The Polar H10 is the most widely used reference standard in wearable accuracy research and has been validated against clinical ECG equipment in independent studies. Wrist devices read blood flow through the skin using light sensors. Chest straps read electrical signals from the heart directly. The gap between these two measurement methods is what accuracy testing quantifies. Comparing a wrist optical device against a validated chest strap is the accepted methodology in this field.
What the study found
We ran a 30-day study across 12 adults, ages 24 to 51, collecting 1,440 paired data points across four measurement conditions. Both devices were worn simultaneously throughout. HRV was measured overnight during sleep, which is the window where wrist-based optical sensors perform most reliably and where the data carries the most meaning for recovery and health monitoring. The overnight RMSSD results against the Polar H10 reference device were an R² of 0.93, an RMSE of 3.6 ms, a mean bias of -1.2 ms, and limits of agreement from -6.8 to +4.4 ms. The Bland-Altman plot showed no funnel shape and no proportional bias. The differences between devices were evenly distributed across the full range of HRV values. The device does not become less reliable at high or low readings.
Where accuracy narrows
We want to be direct about where the accuracy decreases. At high-intensity exercise, above approximately 160 beats per minute, optical wrist sensors face a known challenge. Movement creates signal interference that competes with the pulse reading. Our study showed an R² of 0.88 at high intensity with wider limits of agreement above 160 bpm. This is not a limitation specific to the HLTH Band. It applies to every wrist-worn optical sensor on the market and is a documented characteristic of the underlying technology. The HLTH Band is designed for continuous health monitoring, overnight HRV measurement, and recovery tracking. For those use cases, the accuracy data is strong. It is not designed for real-time interval training feedback, and we do not position it as such.
Why overnight is the right measurement window
The HLTH Band measures HRV during rest and sleep rather than continuously throughout the day. This is a deliberate choice. Movement during daily activity introduces noise into optical wrist readings that reduces HRV accuracy. Removing that noise source by measuring overnight gives you the most stable and informative data available from a wrist-worn device. The overnight window is also when parasympathetic nervous system activity is at its highest, which is when HRV data is most meaningful for understanding recovery and cardiovascular health patterns over time.
