Morning heat problem

Good morning. First timer here. I have 2 zone gas heat. We're in NY and are having -5 temps. I have my thermostat set to a constant 73. The reason it's so high is that we have an open floor plan and our main living room is a four seasons ,all glass sun room. It has baseboards, but is always colder than the rest of the house. Anyway, why is the temp at 67 this morning? Shouldn't the temp be 73 all day and night?
Thanks for the help and stay warm!