When the Apple Watch first launched, the device recorded your heart rate every ten minutes, as originally advertised. After an update to Watch OS 1.0.1, users noticed that the readings became more sporadic.
Apple is now responding to the complaints by saying that the sporadic readings are intentional, and they are not caused by a bug in the watch’s software. The company has updated the heart rate information page on their support website, which was first spotted by 9to5Mac, to say 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.”
The update to the operating system does not affect the Workout app, which is still able to record your heart rate during runs or other physical activities.
It is still unclear as to why Apple changed how the heart rate monitor functions, however, it seems like the company is trying to avoid inaccurately high readings caused by motion. If your heart rate is recorded when you are seated or stationary, the values should be a lot more consistent.