I tried to find the source for this claim too in the past. The nearest I can find is that it implies the HVAC system is sized correctly: if the system is optima

I tried to find the source for this claim too in the past. The nearest I can find is that it implies the HVAC system is sized correctly: if the system is optimally sized, it should need to run most of the time to stay at the desired temperature. If it changes the temperature too fast, then it may end up with wild temperature swings that overshoot the desired comfort level and waste energy that way. In certain situations, with good insulation and building design, this waste can apparently exceed the savings from a few percent lower degrees for part of the day. If I understand correctly, this is a bigger problem for heat pumps that take a long time to spool up/down so simple bang-bang hysteresis control has poor performance in this scenario. Edit: as another poster mentioned, for heat pumps (like AC, but frequently not furnaces), it is also much more efficient to run them when it is cool outside (the morning), and then maintain that temperature through the day as a cold reservoir, than to try to expel the heat when it is hot outside. Which is probably a better explanation for the effect.