Why is my battery draining during the night?

[Masha91]

Hello!

I have Sony Xperia S and I just bought it from a friend and did a factory reset. It's working fine, everything is smooth but I noticed something about the battery.

I charged it to 94 percent last night and went to sleep and when I woke up, the phone showed 87 percent. Nothing was turned on (WiFi, Data, apps), the display itself was off and locked, so I'm not sure if this amount of drain is normal or not? I'd appreciate the help!