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.