A reference for what in particular? Yes, there are third party apps like Sleep++ that do this, but there is nothing native. Those third party apps also can't change system settings likes adjusting notifications. That means that your wrist will vibrate every time you receive an email overnight. The idea that this is intentional to discourage sleep tracking was just conjecture by me.
I think there's a big difference between actively discouraging and just not having a native app.
As other commenters have mentioned your phone can automatically going to do not disturb mode which will also set the watch thus turning off all notifications, but honestly the idea of my watch buzzing every time I get an email even during the day sounds terrible.
I have an Apple Watch and I love it, but I don't have that many things notify me on it.
