The Big Apple proportional controller has a night-drop feature; you need to supply the appliance timer. Other models are available. See MG Reptiles (He advertises in the classifieds alot.)
You can use the existing thermostat and plug it into a timer. That will give you no heat output though. The nice thing about the night-drop feature built-in is that you can select how much of a temp drop you want, rather than all or none.
I have always liked the idea of 2 heaters per cage, and shutting off one at night like Chris mentioned. With few snakes, I can afford to have 2 heaters per cage, but I realize not everyone is setup for that.