OK, since we were at an impasse as to whether Gliders flew at night, I took the question to Avsim and started a thread. The conclusion, after a bit of back and forth is that there appear to be no regulations prohibiting night flying, just a bit of paperwork that needs to be filled out.
Your biggest obstacle will probably be your insurance company!
Its strongly suggested you equip your glider with lights if you do so, with the suggestion being that the lights be patterned on that of a regular private plane, though as posted above, a single red light is also acceptable.
Here is the Avsim thread:
http://forum.avsim.net/topic/465442-so-can-i-fly-a-glider-at-night/