Hi can anyone help me with this issue? I have a flight early in the morning on Sunday and I would like to find out whether the night buses are active at this hour - so I want see their schedule... thanks in advance!

Look at the ZVV website, here.

Easiest way is to input your required destination with desired arrival time and see what options it will give you. If night buses are an option/running, then they will be included.

The schedule can be downloaded here. [1.5MB PDF] There aren't any night buses from Zürich to the airport - maybe because there are no night flights.

There is a 5:02 train though, which would put you at the airport at 5:14. How early do you need to be there?

There is a night bus to the airport, the N52.

No, there is a night bus from the airport (to Kloten, where you could catch the SN7 into Zürich.) Some of the night buses run both ways, most don't.