Every claim on this page was written against the shipped 1.0.1 source, not against the marketing. Where the app is vague or silent, this page says so.
Three different things
- The pacer guides your breathing. It measures nothing, and it works on an iPhone on its own.
- Live Pulse shows your heart rate during a session on Apple Watch. It is heart rate, not HRV.
- The trend plots heart-rate variability samples that your watch wrote to Apple Health, over weeks. The app reads them; it does not produce them.
Most confusion in this category comes from sliding between the second and the third as if they were one feature.
The one Health type we read
HRV Breathe reads exactly one thing from Apple Health: heart rate variability SDNN, in milliseconds. That is the type Apple Watch writes. It does not read RMSSD, which is what Oura and Whoop report in their own apps, and it never converts one into the other. If your HRV history came from a device that writes RMSSD rather than SDNN samples, the app will not see it.
Writing to Health is separate and off until you turn it on. When you do, the app writes Mindful Minutes, and on iOS 18 and later a State of Mind entry if you choose. It never writes HRV.
Live Pulse, and the swing number next to it
Live Pulse is off until you switch it on, and it is part of the paid tier. When it runs, the Watch app opens a workout session for one reason: a workout is the only way an app can stream heart rate continuously on Apple Watch. The workout is always discarded at the end. Nothing is written to your activity rings or to Health.
Readings arrive about once a second. The first one typically takes ten to twenty seconds to appear, and if nothing has arrived after twenty-five seconds the app says the reading is unavailable rather than showing you a stale number.
The second figure, the swing, is the difference between the highest and lowest heart rate within a single breath, averaged over your last three breaths. A breath with fewer than four readings in it is thrown away. This is a visible rise and fall in heart rate, and it is not HRV: it comes from roughly one-per-second workout data, not from beat-to-beat intervals. Treat it as something to watch during a session, not as a score.
Why most sessions end without an HRV number
Apple Watch takes HRV samples a few times a day, on its own schedule, mostly when you are still. It does not take one because you started breathing.
So the “before” value is not a reading taken at the start of your session. It is the newest sample already sitting in Health, and it can be up to 24 hours old. At the end, the app looks again. If no newer sample has arrived, it stores nothing and shows no comparison, and the completion screen points you at your trend instead. That is the common case, not an error.
This is also why the app leads with how the session felt rather than with a number. A before-and-after pair that depends on whether your watch happened to sample mid-session is not evidence of anything.
The trend card, and the exact rule behind it
The trend compares your last seven days against the seven before them. It names a direction only when both of these hold:
- each week contains at least five samples;
- the difference clears a statistical test against your own variability (a Welch standard-error ratio of at least 2.2) and the change is at least five percent.
Clear both and the card reads Improving or Needs Attention with the percentage. Clear neither, and it reads Stable. Without five samples in a week it reads Not enough data. Those four states are the whole vocabulary; there is no score.
Two honest limits. The app pools every source that writes SDNN into one average: it does not de-duplicate overlapping samples, reject outliers, or separate your watch from another device. And a trend is a description of your data, not a verdict on your practice: plenty of things move HRV, and breathing is one of them.
Find Your Resonance
The Watch exercise paces five stages of ninety seconds each, at 7.0, 6.5, 6.0, 5.5 and 5.0 breaths a minute. It discards the first two breaths of every stage, measures the heart-rate swing across the rest, and needs at least three usable stages to return anything. It then names the rate with the largest average swing and offers to make it your default.
That is an observation about a handful of minutes on one morning, not a clinical determination of your resonance frequency, and the app does not call the result optimal. Run it twice on different days and you may get different answers. The resonance frequency page explains how the laboratory version differs.
Where the data goes
- Your sessions, check-ins and preferences sync through your own private iCloud when you are signed in, and stay on the device when you are not. We have no access to that database.
- Your raw Apple Health history is queried on the device and never uploaded.
- Purchases go through RevenueCat, which receives subscription information and no physiological data.
- In-app usage counters stay on the device and are deleted after 90 days. No HRV value, heart rate or check-in is sent to any analytics service, in the app or on this website.
What we do not claim
- That the app measures HRV. It reads what your watch measured.
- That the per-breath swing is HRV, or that it is comparable to SDNN or RMSSD.
- That a session produces a before-and-after result. Usually it cannot.
- That a rising trend was caused by breathing.
- That any of this is a medical measurement. HRV Breathe is a general wellness app. It does not diagnose, treat or monitor any condition.