Everything you enter — moods, symptoms, feeds, all of it — lives only in this device's browser storage and is never uploaded. During the beta we collect anonymous usage analytics: which screens and buttons get used, so we can improve the app. Never the content of what you track — not a mood, a score, a measurement, or a word you type. Feedback goes out only when you tap Submit, and it goes to that same analytics tool, where we read it. Back your data up now and then from Settings.
Export saves a JSON backup; Import restores it (handy for moving to a new phone). Clearing the browser's site data erases the app's memory, so export once in a while.