Apple fan blogger sites and the Apple Discussion site have reported a decreased frequency in the Apple Watch capturing the user’s heart rate after the update of the Watch OS to version 1.01. Under Watch OS 1.0 the user’s heart rate was recorded approximately every ten minutes. In addition, some users are reporting that they see no heart rate reading unless they trigger it manually by opening the Glance.
See: http://9to5mac.com/2015/05/22/apple-watch-heart-rate-bug/ and https://discussions.apple.com/thread/7050353?start=0&tstart=0. The Apple Support website was updated to clarify this change. Apple Support notes that you can check your heart rate any time using the Heart Rate Glance. Apple Watch also measures your heart rate continuously during a workout when the Workout app has been activated. The website further indicates that the Apple Watch attempts to measure your heart rate every 10 minutes, but won’t record it when you’re in motion or your arm is moving. Apple Watch stores all your heart rate measurements in the Health app. The Apple Support website also has a detailed explanation of how the heart rate sensor works as well as specific recommendations to keep readings as accurate as possible.