Gurman's sources say that the company has been testing the feature for several months that sites around Cupertino and Apple have been testing the feature for Apple Watch. the testing goes well, it is hoped to add the feature to "a future version of its smartwatch" in 2020.
It may be disappointing to have that long when competitors like fitbit have sleep tracking in wrist-based products today, But we can count on Apple's take on the feature is either more or more full-featured than current offerings.
It is also annoying that the report claims it's for a future version of the watch, not merely a future watchOS software update . Certainly, if there are third-party apps like AutoSleep or Pillow that do sleep tracking today, Apple could work on current hardware (the Apple Watch Series 4, notably, has more accurate sensors than prior generations, if that's the issue) . And if Apple is testing today, it's certainly not using the 2020 Apple Watch, it's using current hardware.
However, the hold-up might be around battery life. The Apple Watch lasts all day in normal use, but not much more ̵