Along with some fixes and improvements, the first Apple Watch software update also modifies the behavior of the device's heart rate monitor.
The change is not for the better according to some Apple Watch owners. Instead of capturing heart rate every 10 minutes, the Apple Watch won't do so if you're in motion.
Over the weekend, Apple updated its support page for how the watch measures your heart ratenotes 9to5 Mac. Here's the relevant text with emphasis added by me to show the change:
You can check your heart rate any time using the Heart Rate Glance. And when you're using the Workout app, Apple Watch measures your heart rate continuously during the workout. This information, as well as other data it collects, helps Apple Watch estimate how many calories you've burned. 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. Apple Watch stores all your heart rate measurements in the Health app.
As noted, you can still use an Apple Watch to get your heart rate even when you're in motion. The Workout app will do just that.
But the all-day, continuous heart rate monitoring every 10 minutes that was an original feature of the watch? That's been ratcheted down for some reason.
There might be a little battery savings involved by not lighting up infrared LEDs the Apple Watch uses to measure heart rate. It can't be a dramatic amount, however. Besides, the watch's motion sensors will be drawing power to tell the heart monitor not to light up.
Apple does (and always did) note that motion can affect the reading of the heart rate sensor. This is true on most, if not all, of the smartwatches with such sensors available today.
So my best guess is that the Watch was getting readings that were simply too inaccurate throughout the day due to motion. In looking at my own data in the Health app, I've noticed some outlier readings on occasion: Rates in the 90 to 110 range when I know I wasn't active. My normal resting rate is in the low sixties, so something wasn't quite right for those readings.
Eliminating those readings affected by motion could be Apple's attempt at providing a more accurate view of your heart rate during the day. Accuracy is particularly important since Apple is using HealthKit to create an all-encompassing health profile for doctors, hospitals and individuals.