I don't have a citable source handy here, so it's just my word, but when I interned with Apple last year there were a few "town hall" events where interns could ask questions of senior management (Cook, Ahrendts, Cue, Maestri, etc). At one of them (I think the Tim Cook one but I'm not 100% certain) someone asked about smartphone addiction. He pointed out that the Apple Watch actually plays a role. It gives you a tap when you get a notification, but the tap isn't too intense and doesn't go for more than a moment. In user studies, they found that people were much more likely to simply dismiss notifications from the Watch than if they weren't wearing one.
The effect of this was that people weren't unlocking their phones as often, and by merely avoiding unlocking the phone people were not as tied to using it. It used to be the case for me that I would get a notification, unlock my phone and attend to the notification, and then immediately afterwards open Reddit or Facebook or something because, well, I was already on my phone so why not?
Since getting an Apple Watch, I've noticed I do this much less. Maybe part of it is that I'm more cognizant of the problem, but another part is that it's much easier to simply disregard the notifications since they're more fleeting now.
I'm sure this sounds like a sales pitch for the Watch to some people, but I don't mean it that way. I just thought it was very interesting when Cook (I think) pointed out that this was a deliberate consideration with the Watch and then I was able to anecdotally confirm its effectiveness myself. I wish I had an actual source to cite for this, but notes are forbidden in the town hall events.