Overnight data usage

Please can anyone help. I've read previous topics on this. I've been with Lebara for a couple of weeks and every night a small amount of data is being used. I'm assuming this is due to wifi dropping out. I've done some research and amended all recommended settings, Mobile data is off, data saving on etc etc. Is there one particular setting that causes this to happen?
I'm using an android phone.
Thanks, Ali

Dont forget data update is not instant can be hours apart from when it appears on app.