SlowpulseSlowpulse says your health data never leaves your iPhone. That is the kind of claim every app makes, and almost none of them let you check. This one you can check in about ten seconds.
The ten-second test
- Put your iPhone into Airplane Mode.
- Open Slowpulse.
- Look at your stress score. It is there.
- Start the guided breath. It runs, haptics and all.
- Open your history. All of it is still there.
Nothing degrades, because nothing was ever being fetched. An app that computed your score on a server would show you a spinner, a cached value, or an error. Slowpulse shows you the same thing it always shows you, because the work happens on the device in your hand.
You do not have to trust the sentence “your data stays on your device.” You can turn off the network and watch the app carry on as if nothing happened.
Why there is no server to need
The reason this works is not clever engineering. It is that the problem is small.
Slowpulse reads a handful of numbers Apple Health already holds on your phone: your resting heart rate, your heart rate variability, your active energy, your exercise minutes. It averages the first two over your recent history to establish what is normal for you. Then it compares today's reading against that average, and discounts the result if your active energy and exercise minutes suggest you have simply been exerting yourself.
That is the entire method. It is arithmetic over a few dozen numbers. It runs in microseconds on a watch, let alone a phone. There is no model in a data centre that could do it better, because there is nothing to do better — the hard part was deciding to compare you against yourself instead of against a population, and that decision does not need a GPU.
What the category usually does instead
Most apps in this space upload heart rate variability. Not out of malice — uploading is simply what makes the rest of the product possible. A dashboard you can open on the web needs your data on the web. A leaderboard needs everyone's data in one place. A recommendation engine needs a corpus to draw recommendations from. Once any of those is on the roadmap, the data has to leave.
Slowpulse does not have those features, so it never acquires the reason. That is the whole trick, and it is worth being honest that it is a consequence of scope rather than a feat of engineering.
It is worth knowing what is at stake in that choice. Heart rate variability read over weeks is unusually revealing: it moves with your sleep, your drinking, your travel, your illnesses and your bad days at work. It is not a step count. A long record of it is a fairly intimate document, and the safest place for an intimate document is a device you physically hold.
What Slowpulse reads
| Data | Why it is read |
|---|---|
| Heart rate | The current reading, compared against your baseline |
| Resting heart rate | Establishes your personal baseline |
| Heart rate variability (SDNN) | The main signal, compared against your baseline |
| Blood oxygen | Shown alongside your other current readings |
| Active energy burned | So exertion is not read as stress |
| Exercise minutes | So exertion is not read as stress |
| Stand hours | Shown as daily activity context |
All of it is read through Apple's own Health permission dialog, all of it can be withdrawn at any time in the Health app, and none of it is written back. There is no account to create, no email to hand over, and no password, because there is nothing on the other end that would need one.
Being precise about what the test proves
Airplane Mode shows that Slowpulse does not need the network. Strictly, it does not prove that the app never sends anything when the network is available. Anyone telling you otherwise is overselling it, so here are the checks that go further:
- iOS App Privacy Report. Settings → Privacy & Security → App Privacy Report. Turn it on, use Slowpulse for a week, and read the list of domains it contacted. It should stay empty.
- The App Store privacy label. Listed on the Slowpulse product page, and binding — Apple requires it to be accurate.
- Watch the traffic yourself. Point the phone at a proxy you control and use the app normally. This is the strongest check available to someone outside the project, and it is the one we would run.
We would rather point at those than ask you to be impressed by a marketing sentence. The claim is only worth making if it is falsifiable.
Slowpulse has no account, no server, no analytics and no third-party SDKs. It makes no network requests of any kind. The only thing it talks to is Apple's own payment system, and only when you choose to buy something.
The one exception, stated plainly
Purchases go through Apple's In-App Purchase system, which is a network transaction by definition. Apple handles it end to end. We never see your name, your card, or your billing address — only anonymised sales totals in App Store Connect, which cannot be tied to a person or to any health reading.
Not a medical device. Does not diagnose, treat or monitor any condition.