Overnight data usage ?

My phone appears to be using lots of data overnight (around 3am) quite regularly.
I am on wifi at home so no idea why this is happening. It is pushing my monthly usage right up to my limit (10GB), which should never happen as with all previous suppliers I’ve used around 4-5GB.
Anyone else had this ?
Thanks

I just came on to ask the same question! Let's hope we get some answers.