Why are you concerned about humidity levels at night?
Unless your dragon is sick or there are other factors of poor husbandry, there is no reason to worry about humidity levels unless its so humid in the tank that condensation is forming on the glass.
Humidity levels are going to fluctuate no matter what you do. The point is, it doesn't matter really as long as the rest of the husbandry is proper.
Despite what most think, it does get humid in the parts of Australia where dragons live too, it rains, and it gets cold. If dragons couldn't handle fluctuations in humidity or weather, I don't think they'd have survived long enough to make it into the pet trade
Now of course we should try to provide them with the best environment possible, but humidity is a non-factor in my opinion, and most owners get way too worried about it due to fears of URI's.