Suppose it's not a cold night, and your heating is off.
Overnight, depending on age and insulation, the internal temperature will drop. It might go to 15C or to 12C. You and your dogs won't mind because you are in bed.
So in cold, frosty weather, if you want to leave the heating on, set the thermostat to 12C - 15C, and it will be about the same as usual. It won't be stiflingly hot, and it won't cost you a fortune in energy. If the weather changes outside, the boiler will run as much, or as little, as necessary to maintain your chosen overnight temperature.
That' the beauty of programmable stats; once you've decided your preferred daytime, evening, morning and overnight times and temperatures, you can forget about them and they will take care of it.
BTW 12C is often recommended (by insurers) for houses, 24 hours a day, while you are away. It will prevent freezing and damp, and a lot of the time it will not even need to turn itself on, because outside weather and solar gain will keep it around that mark. During summer, it will probably never come on.