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The greater the difference between inside and outside temps, the faster the heat loss--it's not a linear relationship. In other words, a house will lose more than twice as much heat at a 40º difference than a 20º difference. So turning the thermostat down at night (when it's colder) will use less energy than keeping it at the same setting all day.
The the difference between the inside and outside temps determines the amount of heat loss (also considering the amount of insulation). But the way I see it, the few degrees between 68 and 70, or even 65 and 70, is not terribly significant when the outside temperature is dropping from 60, to 50, to 40, to 30, to 20, etc.
Given that, however, I keep the oil stove on about 69 during the day, and the timer sets it back to 60 at night. I keep my bedroom electric heater off during the day and the timer sets it to about 64 at night. That's about where I sleep best.