In version 1.0 of the Apple Watch software the device would measure your heart rate every 10 minutes. Following an update to version 1.0.1, users noticed that their heart rate would be tracked at irregular intervals.
An updated support document explains why this is happening:
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.
It's unclear why Apple has made the decision to abandon a heart rate check when you're in motion. If regular checks are important to you, one way to accomplish this is by starting a workout in the Workout app. This will cause the watch to record your heart rate at ten second intervals.
Read More [via 9to5Mac]