Sounds like one of your apps is set to check for updates at a particular time of day and hasn't been set to update over a WiFi connection only. Until you have worked out which app is the culprit, most likely an app backing up your photo's, contacts etc. to Google or a similar programme, turn off data and/or restrict background processes when you go to bed at 8.45pm
I had $20 disappear into thin air when I was in Singapore just recently, my fault because I didn't turn off data and the option on for WiFi uploading only when available. I've even had data turn itself on using up all my credit, most likely a rouge app that I gave permission without realising the implications.