Cairn reads your health and fitness data from the phone's health store — Apple
Health on iOS, Health Connect on Android — and saves it as plain, open-format
files (Open mHealth / IEEE 1752.1 JSON) in your own Nextcloud. You stay the sole
owner of your data: there is no central server and the developer stores nothing.

Requirements:

Android 14 or newer. Cairn reads through Health Connect, which is part of the
system only from Android 14; on Android 8-13 it is a separate Google app from
the Play Store, which a Play-less device cannot install — so the app would
find nothing to read.
A Nextcloud of your own is optional: without one, Cairn still reads and shows
your data, it just keeps the files on the phone.

Features:

Reads heart rate, steps, body weight, sleep and workouts — read-only. Cairn
never writes to your health store.
Syncs the files to your own Nextcloud over WebDAV, with optional background
sync.
In-app dashboard with a sleep-stage deep-dive and a BMI computed from your
latest weight.
Available in English and German.

Privacy:

No advertising, no analytics, no trackers, no third-party services.
Your data lives only on your device and in the Nextcloud you control — never
in any developer or third-party cloud.
HTTPS only. The Nextcloud app password is stored in the operating system's
secure storage and can be revoked at any time.

Cairn is an aggregator and viewer, not a medical device. It makes no diagnostic
or treatment claims.