if you use a dimmer type unit, just because the air in the room may very a little bit...does not mean the heat element will get colder, because it will not. the ambient temperatures in the cages may fluctuate...but the heat element temps will not. so if you have your hot spot on 95 degrees, and the room temps drop six degrees, your hot spot will still stay 95 degrees...I don't get where some of you say that it will change...? a dimmer type unit, only regulates the amount of wattage that the heat element(or what ever you have plugged into it) gets....this does not change. a thermostat on the other hand, will gradually raise or lower the wattage on the heat unit to keep it at the set temp(for proportional thermostat). an on/off type will switch on a heat element once the probe temps drop to a set degree, and shut off once the probe reaches a set high temp. so a thermostat will change the temp of the heat element, not a dimmer/ramp type rheostat....that will leave the units temps set at what they are set at.