The heart rate sensors are not turned on constantly and only measure briefly throughout the day to find your heart rate when you aren't doing a workout. (Under a workout, it will "illuminate" for 100% of the time.) You can check that "illumination" by going into ECG or the heart rate app. (I have no idea on frequency.)
If you aren't receiving any data on your phone throughout the day, follow these troubleshooting steps below.
Close certain apps and then restart (normally) both your iPhone and your Apple Watch:
- Close the Watch app, Activity app and Health app (if open) on your iPhone
- Double-click the Home button, then swipe up on each app preview to close the apps.
- Close the Workout app, Activity app and Heart Rate app on your Apple Watch:
Turn both devices off together, then restart your iPhone first:
If you have previously used any third-party fitness tracking, sleep tracking or heart rate monitoring apps:
- Check whether there is an active or paused session within any of them.
- If there is, then end that session from within that app on your Apple Watch.
- If in doubt, it may help to remove any such third-party apps from your Apple Watch, at least temporarily.
People have said that Apple may have (not much evidence) reduced readings when moving constantly due to inaccurate readings, I have no idea why that is the case.