If you were instead referring to the Workout app:
Apple advises that, even under ideal conditions, Apple Watch may not be able to record a reliable heart rate reading every time for everybody.
For the best performance from the heart rate sensor during workouts, the back of your watch needs to be in contact with your skin. A loose fitting band can affect performance by allowing your watch to lift from your wrist and/or by allowing clothing (for example, the end of a sleeve) to slip beneath the back of the watch.
The right fit should be neither too tight nor too loose, with room for your skin to breathe:
Apple suggests that you consider tightening your watch band beforehand and loosening it again afterwards.
The sensor is also likely to give better results for workouts that involve rhythmic (eg running) rather than irregular (eg boxing) movements. Other issues that can impact sensor performance include skin perfusion and wrist tattoos.
If your workouts involve flexing your wrists (such as when lifting weights, for example), it may help to move your watch a little higher up your arm, away from the flexing joint.
Check also that:
Wrist Detection is turned on:
- On your iPhone, in the Watch app, go to: My Watch (tab) > General > Wrist Detection.
Power Saving Mode has not been enabled:
- On your iPhone, in the Watch app, go to: My Watch > Workout - turn off Power Saving Mode.
More information:
Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support
Use the Workout app on your Apple Watch - Apple Support
If you continue to experience issues, for more consistent readings, you may wish to consider pairing an external heart rate monitor / Bluetooth chest strap to your watch:
Use Bluetooth accessories with your Apple Watch - Apple Support
