Hello,
We completely understand what you are trying to say.
However, it is important for you to understand that there is no way to transfer the Data across network, if the Computer machines are not Turned ON.
So, in your case, if you want to have the Transfer done during off hours, the users would have keep the Machines Turned ON and leave.
They can surely LOG OFF their machines but NOT TO TURN their machines OFF.
Hope that helps!!