iPhone graph of - Average sleep per week or month
Autosleep works for me really well the last 6 months. 


AutoSleep Tracker for Watch by Tantsissa https://itunes.apple.com/us/app/autosleep-tracker-for-watch/id1164801111?mt=8


I’m much more inclined to get the best data wearing my watch at night, but it does work well if you use charging your phone as proxy for when you lie down and when you get up. Of course you can manually start and stop your rest periods, but for me that is the last hint I want to do before and after resting.