HRV breathing app

What HRV breathing is

Heart rate variability is the small change in the time between one heartbeat and the next. The Cleveland Clinic describes it as the amount of time between your heartbeats fluctuating slightly, driven by the two halves of the autonomic nervous system. It isn't your heart rate. Two people with a resting pulse of 60 can have very different HRV, and what counts as normal for one of them may not be normal for the other.

HRV breathing means breathing slowly and evenly, at about five to six breaths a minute, because that's where the heart rate swings most with each breath. Lehrer and Gevirtz, reviewing HRV biofeedback in 2014, put the average at about 5.5 breaths per minute, varying by person around 0.1 Hz. Concretely, it's coherent breathing: 5.5 seconds in, 5.5 out, no holds.

So an HRV breathing app has two jobs. Pace you at that rhythm, to the second. And show you HRV without lying about what a phone and a watch can actually measure. The pacing is the easy half. The second half is this page.

What Refresher measures, and what it reads

Heart rate is measured. With an Apple Watch on and heart rate tracking switched on in Settings, the app keeps the first reading of the session and the last. The difference is on the completion screen and in your history. The reading needs both ends and at least three samples. Without them the row is left out.

HRV is read from Apple Health. Apple Watch computes it and writes a handful of samples a day into Apple Health. Refresher draws the last 30 days of those, live from HealthKit, with the days you practised marked on the same chart. The chart doesn't appear at all until there are two days of data. There's no empty state and no nag to connect a Watch.

The Cleveland Clinic notes that most wrist-worn trackers aren't sensitive enough to detect HRV accurately, and that chest straps do better. That's one more reason the app shows HRV as a slow trend. A number built on a few daily samples from a wrist sensor is something to watch over weeks.

What it will not claim

The app never says what breathing does to your HRV. There's an audit script in the codebase that searches every listing, in every interface language, for a causal verb next to a variability noun, and blocks the release if it finds one. The chart draws two series on one axis, your Apple Health SDNN and your session days, and computes no correlation between them. That is the whole of what the app says about the two.

It's a pacer with a chart and makes no medical claim. It paces breathing and shows you numbers that Apple Watch and Apple Health already hold. If a doctor is tracking your HRV, their equipment is the one to read.