It gets down to about four or five in bedrooms, six in the room I work in, 8 to 10 in rooms closer to the kitchen. Been doing this for years, no ill effects.
I love the cold and winter. I'll run outside when it is -30F with the proper clothing. But damn, we keep our house at 60F in the winter and I often wear a winter hat inside. 39F in the house is nuts. Not to mention you have to be careful not to freeze your pipes.
I would rather just burn the gas and keep the house at 17 so I can walk around wearing my preferred house garb of just an old housecoat with no belt. My extremities excluding my hands and feet get too cold below 17.
If your garb is JUST an unbelted housecoat there is indeed a risk of cold extremities and not just hands and feet. Now pardon me while I clean the "mental images" screen. I should probably have caveated my remarks with "if appropriately dressed"!
Technically I said extremities excluding my hands and feet because my hands and feet don't get cold. My nose does. What did you think I was talking about?