Using data overnight

Hi all,
Could anyone tell me why data is used overnight when the phone is connected to WiFi?
It happens every night.
I’ve turned off Background App Refresh.
Thanks in advance

Could turn both WiFi and Data off, or turn your phone off completely perhaps

Yes, I could, but I wonder why it is doing it?

Peeeep to confirm your phone is using mobile data overnight while connected to your wifi?

Hi Martin,
Yes, overnight wifi is on, but data is still used. Sometimes only very small amounts, e.g..1 mb, other times larger amounts, e.g. 132.6mb. It happens every night, several times.
It happens with my grandson's phone too.