When it syncs

Every sync is the same job, whatever started it: work out which days changed, read them in full from Health, and send them to each destination that is not paused.

What starts a sync

• Health records something new. Health Relay asks iOS to wake it when data arrives: hourly at most for most types, daily for sleep, stand hours and mindful minutes. Many types arrive together, so wakes within five minutes of the last sync are skipped; their changes go with the next one.
• Every morning. The app asks iOS for a catch-up around 06:30.
• Opening the app, when the last successful sync is more than an hour old.
• Sync now on the Status tab, or pulling down to refresh.

iOS decides exactly when background work runs. It gives more time to apps you use, and less in Low Power Mode.

Turn on Background App Refresh

Since iOS 27, an app can read Health in the background only when Background App Refresh is on for it (Settings › General › Background App Refresh). With it off, Health Relay still works, but only syncs while you have it open.

Your iPhone must have been unlocked

iOS encrypts Health data while the phone is locked, and no app can read it. Changes recorded overnight (sleep, for one) are sent after you first unlock the phone in the morning. If a sync runs while the data is locked, the Status tab says so and the next sync picks the days up; nothing is lost.

Which days a sync sends

• Days with new data. Health Relay tracks every type with a bookmark, so it knows exactly which days gained samples since the last sync, including a late watch sync for last Tuesday.
• Today and yesterday, every time.
• The last 35 days, once a week. iOS doesn’t say which day a deleted sample belonged to, so a weekly sweep re-sends the recent past and deletions reach your destinations within a week.

Each day is sent whole: every type, every source, every sample the phone has for it. A receiver can replace what it held for that day, so edits and deletions on the phone land too. See Behaviour for the exact rules.

Re-send on demand

Status › Re-send recent days marks the last 7, 14 or 35 days to be sent again and syncs. Use it after fixing a receiver, or after deleting something in Health that you want gone everywhere now rather than at the next weekly sweep.

When a destination fails

Health Relay keeps each batch on disk until the destination accepts it, and retries with growing gaps (1 minute, doubling, up to 6 hours). A destination that refuses the token stops until you fix it; see Troubleshooting. Other destinations carry on regardless.