Hi
Apple Watch measures your heart rate:
- Regularly when you are still (a resting heart rate).
- Continuously during workouts (recorded via the Workout app).
If you are not yet doing so, you might wish to use these settings:
- On your iPhone, in the Watch app, go to: My Watch (tab) > General > Wrist Detection - turn this on.
- On your iPhone, in the Watch app, go to: My Watch > Passcode > Turn Passcode on.
- Also on the Passcode settings screen, enable Unlock with iPhone.
After putting it on your wrist, your watch will then unlock after whatever you do first: either unlocking your watch manually (by entering the passcode) or unlocking your iPhone. It will then remain unlocked (including when the screen is asleep) until you remove it from your wrist. When you remove your watch, it will lock automatically.
It may also help to check your settings:
On your iPhone, in the Watch app, go to: My Watch > Privacy > Motion & Fitness - check that both Fitness Tracking and Heart Rate are enabled.
More information:
Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support
https://help.apple.com/watch/#/apd748b87e2a