Apple Watch had a change in its heart rate monitoring functionality with WatchOS 1.0.1, which was considered to be a bug. With WatchOS 1.0.1, heart rate readings would only be taken when your wrist was completely idle instead of 10 minute intervals.
In response to the complaints about this bug, Apple updated its Apple Watch heart rate support page to convey the message that this is not a bug.
In addition, 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.
With WatchOS 2 GM, this behaviour has changed again and the readings have gone back to 10 minute intervals. The support page still shows the same description quoted above but the readings are more regular and useful now. Check out the screenshots below from my recent experimentation while using WatchOS 2 on my Apple Watch Sport.
The readings are now taken after every 10 minutes at an average, even when my arm is moving, specially when driving. With WatchOS 1.0.1, this wasn’t the case.
I prefer WatchOS 2’s approach for heart rate monitoring and hope Apple sticks to it instead of changing it again after the next update.