What are the best ways to help maintain a decent humidity level?

As of right now I have a 12x12x18 Exo-Terra Terrarium. I spray it a couple of times a day, but overnight it drops down to near zero! That's scary to me, considering they are WTF's, and all the stuff I have read about how important it is to maintain good humidity. I have covered half of the screen on top and added more moss, and give it a good spray right before bed in the hopes it would hold it over until I got up in the morning. This is only about 8 hours, sometimes less. At night I also keep an infared heat lamp on, but it is very low wattage and its just to maintain the temps in the terrarium.

Any ideas what might be causing the huge drop in humidity overnight and any ideas how to maintain good humidity through the night?