This is probably more detailed than necessary, but since some asked....
I work at a Class C airport with separate Tracon and Cab, though all controllers, (except trainees) are certified and do work both. Along around 10:30 pm or so, the Midshift, (two people) shows up. One goes to the Cab, the other the Tracon. Along around 11:00 pm, providing traffic is light enough, and the WX is reasonable, we call the Center and they make a computer entry that directs all the flight plan strips to the tower printer, then we make a computer entry in our own ARTS computer to direct track control of all targets to one of the Tower positions. We have two D-BRITE radar displays in the cab with separate controls. One I'll range out to around 10 miles beyond our airspace boundry, and the other I'll leave at about a 20 mile range, so if I have to do some "fancy vectorin" close in, I have a better display with less target overlap.
Note, a D-BRITE display, if you haven't seen one, is about a 20" square, often mounted from the ceiling of the Cab. When I run the range out to encompass all our airspace plus a little, 1 mile on that display equals about 3/16th of an inch, so two aircraft 3 miles apart are just 9/16ths of an inch apart, or a little over a half inch. Don't expect me to get real fancy along around 2:00 am by vectorin' two airplanes a half inch >.....< apart....
Where was I? Oh yeah, we'll then call up all the normal Approach and Departure Freqs on the position we'll use in the Cab, and also set up the Clearance Delivery freqs for the satellite airports on a second position. (Using hand mikes now, we ditch the headsets.) Then call the Tracon for a briefing on what he/she has going, and take over the approach control in the Tower Cab.
Then the person downstairs will spend about 20 minutes closing out the facility, position and pay logs, close out the traffic count and delay logs etc. Afterwards, we simply spell each other on the one position in the cab, so one can eat or take a nature break etc. Helps a LOT having two person mids. They get real sleepy and dreary working by yourself, which we occasionally have to do if one sicks out. (no overtime on our mids...) Typically, the last airline flight arrives around 1:00 am, but may be later if WX delays at the Hubs have pushed everyone's schedules way out of whack. Afterwards, it's mostly just check haulers and an occasional Medivac or Police helo. Often the airport authorities will schedule some maint. on one of the movement areas, and there will be a detailed (almost hour long) inspection of all airfield lighting.
Along around 5:45 am, the whole process is reversed as the early shift arrives.
Busier facilities will keep the radar room open all night with three people. One acts as relief for the two. Then you have the facilities like MEM which are running lots of traffic all night long, and have a full compliment on the midshift. Also, some of the bigger airports are "split" facilities, where the Cab and Tracon are separate ratings, and none of the controllers are rated in both.