Blog
Heart Rate Variability is a genuinely useful signal and a genuinely noisy one. These pieces explain what it can tell you, where it falls apart, and why a number only means something next to your own recent history.
HRV is a readout of beat-to-beat timing, not a stress meter and not a health grade. What the number is really made of, and the everyday things that move it before your mood does.
2 September 2026 · 8 min read
Healthy adults sit at wildly different absolute HRV values. Age, anatomy and how your watch samples all shift the scale — which makes cross-person comparison close to meaningless.
2 September 2026 · 8 min read
A walk through the arithmetic Calmline uses, why the baseline is a median, and how to tell a real signal from a day the sensor simply did not get a clean look at you.
2 September 2026 · 9 min read
Calmline reads HRV on Apple Watch and scores it against the median of your own recent history — on-device, with no account and no server. It isn't on the App Store yet; the waitlist on the home page is the way in.
Calmline