It might very well be that the Apple Watch will need to be charged at night, and when it is charging you won’t be able to wear it. But that doesn’t mean it won’t be able to track your sleep pattern. I’m not wearing the iPhone on my arm, but I do use it to track my sleep behavior with an app called Sleep Cycle.
Every evening I put my iPhone in Airplane mode, connect the charger and start the Sleep Cycle app. Then I put the iPhone on the corner of my bed just below the sheets.
The accelerometer in the iPhone tracks movement in my bed and can guess, by the amount and type of movement, whether I’m awake, or sleeping, or dreaming. You will be able to do the same with your Apple Watch. Just leave it on the edge of your bed and it will keep track of how you sleep just fine.
I think this is even more attractive than wearing a bracelet or watch at night.