Your rights do not rise and fall with the sun. All you are required to do is provide your drivers license and proof of registration.
The police are probably trolling for drunks, as the bars are closing. Still, the pizza delivery cars I see all have some sort of sign on the roof so the people who ordered them can see the car when it comes to their home. One would think the police would be able to spot those signs as well.
Merely being on the road at 2-4 a.m. is not probable cause to stop a vehicle. It is not your obligation to answer the question "Do you know why I stopped you?" at all, still less to do so with an attempt at mindreading. Doing so will almost certainly give the officer the excuse s/he needs to rationalize the stop. A simple "No" will suffice.
If you feel you are being unfairly targeted, keep a log of when, where and by what department you are stopped, together with the officer's name. If you can prove a pattern of pointless stops, a letter detailing them to the chief may - MAY - get the officer's efforts redirected to a useful activity. If you do get a ticket, you will have some evidence of harassment.
The foregoing is for general information purposes and does not establish an attorney-client relationship.