The reason lowering the thermostat at night saves money is because you are lowering the temperature differential. As you lower the temperature differential, you lower the energy transfer rate. In other words, the hotter the house is the winter, the FASTER it will lose that heat to the outside. It doesn't cost you more to heat the house back up in the morning, unless you have some old inefficient 2 stage heat pump. but it still should be less energy than keeping the house warmer during the night.
Just remember, the bigger the difference in temps, the FASTER the energy transfer.
