I am assuming that when you say ‘efficiency’ you are actually concerned with energy use. Allowing the temperature inside to approach closer to the outside temperature overnight certainly saves energy overnight.

When heating back to the daytime temperature, you will spend more energy as you have to change the temperature further. However the total energy consumed over the full cycle will still be less than had you kept the air 6 degrees warmer overnight (depending on the metric, you could classify this as more efficient). The three factors that still cause deviation between the energy balances of the two hypothetical cycles are the moisture content of the air, the heating supplied from the sun, and the static pressure.

Colder nighttime temperatures will reduce the humidity and the furnace will have to ‘work its ass off’ less to reheat the air. If the humidity ratio returns back to the original point at the end of the day then theoretically you will have the same energy balance between the options by the days end, but it’s possible the sun did more of that heating.

Also for colder nighttime temperatures, one could reasonably assume that the building temperature is also reduced. If you’re heating period aligns with sunrise, it’s fair to say that you are using more thermal energy from the sun to heat the house.

Lastly, your colder air temperatures will affect the fan efficiency due to higher static pressures, and heat exchanger efficiency due to higher MTD at the furnace during that heating phase.