Last updated: August 18, 2026
Onyx is built on a simple privacy stance: your health data stays on your phone, except for the features that can't work without leaving it. This policy explains exactly what we do, what we don't do, and what you can control.
Your workouts, food log, saved recipes, recovery scores, sleep notes, profile, and settings are stored in your device's local database. There is no Onyx account and no Onyx-side login — we don't run a server that holds your profile.
If you turn on iCloud sync, some of this also lives in your iCloud account. See "iCloud sync" below, because it changes what happens when you delete the app.
With your permission, Onyx reads the metrics below. iOS asks in two stages: the core set when you first set up, and a second, smaller request the first time you open the Body tab — so you're not handed forty switches before you've seen the app work.
You can grant or revoke any individual metric at any time in iOS Settings → Privacy & Security → Health → Onyx. That screen is always the authoritative list.
Recovery and sleep Heart rate variability (HRV) · Resting heart rate · Heart rate · Walking heart rate average · One-minute heart rate recovery · Sleep stages and duration · Respiratory rate · Blood oxygen · Overnight breathing disturbances · Sleep apnea notifications · Wrist temperature deviation
Activity Steps · Active energy · Resting (basal) energy · Exercise minutes · Stand time and stand hours · Walking, running, cycling and swimming distance · Flights climbed · Time in daylight · Workouts and workout routes
Body Body mass · Body fat percentage · Lean body mass · Body mass index · Waist circumference · Height · VO₂ max
Movement quality Walking speed · Step length · Walking asymmetry · Double support percentage · Stair ascent speed · Six-minute walk distance · Walking steadiness
Clinical and lifestyle Blood glucose (from a CGM, if you use one) · Blood pressure · Dietary water · Caffeine · Alcoholic beverages · Mindful minutes
Heart notifications Irregular rhythm notifications · High and low heart rate notifications · Atrial fibrillation burden · Falls
Cycle tracking Menstrual flow. This is used for one purpose: so Onyx doesn't mistake normal luteal-phase changes in resting heart rate, HRV and body temperature for illness. It is never shared and never leaves your device.
Characteristics Biological sex and date of birth, used for age- and sex-based reference ranges (for example, where your VO₂ max sits against peers).