I'm using a 75watt IR bulb as well. I leave it on 24/7. I stopped in to see if anyone had any thoughts on that. With proper air restriction and 24watt Zoo-med heat mats at both ends of the tank the temperature is holding at about 84ºF in a 20 gallon long. I don't see why having a heat source present other than the UTH's would matter during the night cycle. The IR bulb and heat from the UTH's probably appears to be about the same (through labial pits) during the night hours.