My phone loses battery overnight if I forget to plug it in. It will always run out overnight and the cell standby would be 70% without even touching it. I'm also on wi-fi at home so I'm not sure whats the problem. It's been like this since I bought it 3-4 weeks ago.
If you click on.the cell standby entry what details does it give you? If it gives you a number for time without signal, that means wherever you are setting your phone overnight is a dead zone. Your options are A. Plug it in overnight. B. Move it some place with reception overnight C. Put it in airplane mode - though you won't receive calls or texts.