You just need to maintain a ambient temperature that is around 70-75 at night. It doesn't matter how you accomplish this, just that you do. You can use a space heater, nighttime bulb, etc. You just don't want them to get too cool (your monitor/tegu). Some can tolerate low nighttime drops, but I see no reason to let them go below 70 unless you are going to brumate them.

What species of monitor are you looking at?