Although data usage is showing at night it does not necessarily mean the phone was using data at that time. Data usage for the day is usually added together and goes onto the bill in one go.
If the phone was using data it's more than likely accessing an application or completing updates. I would recommend checking your phone data application to try and see which app is using data or download an application.
To avoid this happening turn off your mobile data at night and only use a WiFi connection. If you are have WiFI assist turned on, the phone can still using data if the WiFi connection drops. Depending on the phone you are using if you check your settings, this can be turned off.
