Battery life isn't an issue. You don't have to charge it at night. I sleep with mine on and never see the battery get below 30%. I typically charge it while getting ready for work in the morning, then after my daily workout, or if I am just sitting at my desk at work. It usually only takes about an hour to charge it from 50%, and mine rarely get's there.I think also they need to improve battery life. Currently you have to charge the watch at night soo that would make it hard to help track sleep.
I have the exact opposite experience. I find it totally comfortable to sleep with the AW on. I guess I am used to wearing a watch, and also wearing one to sleep, so the AW actually feels like it is not even there. Prior to the AW, I was wearing a Basis Peak which gave you incredible sleep metrics, along with continuous HR monitoring. Unfortunately, it was a bit clunky, and was pretty uncomfortable to wear while sleeping which is why I moved on from it.the health app has a ton of sections that have nothing to do with the Apple Watch, at least yet.
Personally, it's uncomfortable to sleep with the AW on, especially coming from a Fitbit Charge HR that I had been wearing the last 6 months.