I already the know the common sense answer for this. A decrease in temperature difference between the inside of the house and outside of the house means the heat loss is less. Multiply the reduced heat loss by the amount of time the temperature is lower gives a daily (or hourly or whatever) energy savings. I have seen a rule of thumb that for every 1 deg C you drop the thermostat, you get a 2% savings (of course this is over that period of time).
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