It's not a watch. I wear the watch and before I go to bed, use the mi band. The Apple watch is too big and heavy to be used as a sleep tracker. Maybe that's why no native app has appeared yet?
If Apple released a lightweight band to track sleep only, I would buy one. Maybe something like Beddit? I don't know but a watch shaped device is not the solution. Using the Miband for the first night I didn't have a numb arm and I didn't even have the Apple Watch tight either.
Wow, that's strange. I wear my Watch everyday, even at night when i sleep...no issue on my end. I have SS 42mm with Nike band.