Apple Watch OS 1.0.1 appears to have reduced how often the device measures your heart rate in the background. While this likely has a positive effect on battery life, it has the side effect of making heart rate data less accurate.
The Verge noticed that users in MacRumors' forums and Apple's own support boards were complaining about the topic. A check with our own heart rate data in Apple's iOS Health app confirms a marked decrease in the number of samples.
In the image below, we have screenshots of Heart Rate data from the Vitals section of Your Data in the Health app. In the image on the left we have data from May 13th, several days before Apple Watch OS 1.0.1. Note how the data consistently shows entries every ten minutes, except for a 3:10 when I apparently took several manual readings.
On the right, we have data from Thursday, and that data is all over the place. 50 minutes, 30 minutes, 20 minutes, and a few 10 minute windows.
Health App Screenshots of this author's heart rate data.
The watch icon indicates the data came from Apple Watch.
What we don't know yet is if this is purposeful, if Apple deliberately cut back on the sample frequency, or if something broke in the update. The company has yet to comment on the subject.
And the reality is that this is important to a subset of users. Fitness fiends an anyone paying close attention to their health may be intently interested in having an accurate look at their heart rate data over time. For the rest of the world, manual checks and infrequent automatic checks might be more than enough for them to get what they need out of this feature.