Pulse — an open-source, Whoop-style readiness tracker for the Fitbit Air

Pulse is a private iOS app that reads your Google Fitbit Air data through the new Google Health API and turns it into daily readiness metrics — recovery, strain, sleep and a biological "Pulse Age". Everything is computed on device against your own baseline. No server, no subscription, no tracking.

Not affiliated with WHOOP, Google or Fitbit. "Whoop-style" describes the kind of metrics (recovery / strain / sleep readiness); it does not imply any connection. See Disclaimer. Licensed under Apache-2.0.

_{Today · Pulse Age & strain · Trends · Health monitor · Customizable layout — English & German, light/dark.}

No Fitbit at hand? Just want a look? Build it and tap Demo mode on the welcome screen: 120 days of realistic sample data, no Google account, no setup. That is what the screenshots above show.

What it does

• Recovery score (1–99 %) — from HRV, resting heart rate, sleep performance and respiration, each scored against your personal 30-day baseline.
• Strain (0–21) — cardiovascular daily load from heart-rate-reserve zones on a logarithmic scale (Whoop-style), including strain per workout.
• Sleep — need, debt, performance, consistency and efficiency, with a phase hypnogram.
• Health Monitor — resting HR, HRV, respiration, SpO₂ and skin temperature shown inside your personal baseline band (± 1.65 SD) with a warning status.
• Pulse Age — a biological age estimate from VO₂max, HRV, resting HR, sleep and activity, mapped against sex- and age-specific reference norms (calibrates over ~30 days).
• Trends — recovery vs. strain, sleep, HRV and resting HR over 7 / 30 / 90 days.

All data stays local on the iPhone (JSON in Application Support). No account on any server, no analytics.

Why the Google Health API (not the Fitbit Web API)?

The classic Fitbit Web API is being shut down in September 2026. Google is replacing it with the Google Health API (health.googleapis.com/v4, OAuth 2.0). The Fitbit Air already syncs into the Google Health app, so Pulse is built directly against the new API. Because it is new (v4, launched May 2026) and not every field name is finalized, Pulse decodes tolerantly (candidate keys, read-variant fallback) and logs every metric individually in an in-app sync log — a single missing data type never blocks the rest.

How the scores are built (in short)

• Recovery weights HRV 40 % · resting HR 25 % · sleep 25 % · respiration 10 %, each as a z-score vs. your 30-day baseline through a logistic curve; zones follow Whoop (≥ 67 green, 34–66 yellow, < 34 red). Missing components are re-weighted.
• Strain integrates time in heart-rate-reserve zones (Karvonen) and maps it logarithmically to 0–21.
• Pulse Age is anchored on VO₂max — the strongest single predictor of all-cause mortality (Mandsager, JAMA Netw Open 2018) — translated to an age via the FRIEND reference norms, blended with an HRV-based age and small capped corrections. It is an orientation, not a medical result.

The full methodology, sources and the Google Health data-type mapping live in SETUP.md.

Build it

Requirements: an iPhone on iOS 17+ with the Google Health app (Fitbit Air paired), Xcode 16+, and xcodegen (brew install xcodegen).

open Pulse.xcodeproj      # NOT the folder or Package.swift — pick the "Pulse" scheme

In Xcode: target Pulse → Signing & Capabilities → your personal team (a free Apple ID is enough) → connect the iPhone → ▶︎ Run. In the app, tap Connect Google Health, paste your OAuth client ID (iOS client, bundle id net.dehlwes.pulse, PKCE — no client secret), sign in, done. No Google setup? Start Demo mode for 120 days of realistic sample data.

Bring your own Google Cloud project. There is no shared backend or API key in this repository — Pulse uses PKCE and stores nothing server-side. Every user creates their own Google Cloud project and iOS OAuth client and stays subject to Google's API terms. The one-time setup (project, scopes, iOS client id) is documented step by step in SETUP.md.

Project structure

pulse/
├── Pulse.xcodeproj   # generated project — open this to build
├── project.yml       # xcodegen definition (regenerate only after changes)
├── App/              # iOS: SwiftUI, OAuth browser flow, assets, the widget host
├── Core/             # platform-neutral: models, metric engines, API client, sync
│   └── Package.swift # exposes Core as a library for the self-tests
├── PulseWidget/      # home-screen recovery-ring widget (App Group)
└── SelfTest/         # standalone SwiftPM package

Run the self-tests without Xcode (PKCE against the RFC-7636 vector, DTO decoding with Google Health fixtures, every metric engine end-to-end on demo data, store roundtrip):

cd SelfTest && swift run pulse-selftest

Roadmap

• Live "stress monitor" from intraday HRV/HR deviation
• Apple Watch complication
• Webhook subscriptions instead of polling
• Export (CSV / Health Connect)

Contributing

This is a personal hobby project, shared in the hope it's useful, and feedback is genuinely welcome. It's early, so you will find rough edges.

Found a bug or a number that looks wrong? Open an issue. A screenshot helps, and for sync problems the matching line from the in-app sync log (More → Sync log) usually pins down the fix on its own.

Pull requests are welcome too. Run the self-tests first (cd SelfTest && swift run pulse-selftest); CI runs the same command on every push.

Details, ground rules and what to include: CONTRIBUTING.md. Release history: CHANGELOG.md.

Disclaimer

• Not a medical device. Pulse is for personal, informational use only. The scores (recovery, strain, sleep, Pulse Age, health monitor) are orientations, not diagnoses. They are not validated against clinical standards and must not be used to make medical decisions. If you feel unwell, talk to a doctor, not an app.
• No affiliation. WHOOP is a trademark of WHOOP, Inc.; Google, Fitbit and Google Health are trademarks of Google LLC. This project is independent and not affiliated with, endorsed by or sponsored by any of them. Product names are used purely descriptively. The metric methodology is built from publicly published research (cited in SETUP.md), not from any company's proprietary algorithms.
• No warranty. Provided "AS IS", without warranty of any kind (see the License).

License

Licensed under the Apache License 2.0 — see LICENSE and NOTICE. Apache-2.0 is permissive (use, modify and redistribute freely, including commercially) and adds an explicit patent grant and a trademark clause.