Last night I was going to fly IFR to a class delta that sits just under the shelf of a bravo. Departure time was supposed to be 6 pm but passengers kept calling me and pushing time back until we finally left around 1 am. Very annoying. Anyway after takeoff I tried to get my IFR clearance in the air (weather was clear and a million) but could not get a response from approach. I was ****ed at my equipment and decided to just fly vfr around the area for an hour and come back. I was confused and just looked into the problem in the chart supplement. Turns out approach control is closed midnight to 6 am. It’s annoying to find this out because all I had to do was look a little harder before we left.it was probably all the scrambling because I didn’t want to waste precious Hobbs rental time. I am curious though, would I have been able to just call up center and are they open 24/7?