I've generally used a heat lamp during the day, and covered the entire vivarium at night with a towel, leaving space for ventilation. Whenever I do this, the humidity generally goes from 50%-60%. I have the light fixture lowered at night so that it is almost touching the towel, but enough space is left so that there's sufficient air to cool it.

As I didn't have the foresight to purchase an infrared bulb, I've had to deal with a light on in my room 24/7 - I have a moonlight bulb and an infrared bulb on the way now, however, and I hope that will be a more pleasant nighttime solution.