If I don't wear a hat, especially (but not exclusively) during the winter, I get headaches. It was mostly a coincidence that let me discover that the periodic headaches I often got went away when I wore a hat. Sometimes, I wonder, of all the people I know who get headaches, how many of them even tried a hat? Did a doctor ever suggest it as a treatment? Maybe there's lots of people with this "odd" circumstance who never find out relief is that easy.
Long ago, wearing a hat was the normal way of things. After all, the world was cold. Houses were typically heated with a single source of heat, a stove or fireplace, and the farther from it you were, the more cold you had to deal with. Clothes were the first and best protection. We lose a fair amount of heat from the head, and so a hat was as essential as any other garment. Yes, people wore hats indoors.
But rich people, the aristocracy, sometimes were able to afford so much luxury that they could heat every room in their houses. As with so many other things rich people did, not wearing a hat was a ridiculously impractical thing that they took up as a means of flaunting their wealth. I'm so rich I can be warm without a hat! Soon everyone else who was rich had to do it, too. Amongst the wealthy, not wearing a hat became a status symbol. But when they went outdoors, it was still cold, so the social protocol became that you wore hats only outdoors.
So many of these dumb bits of intentional impracticality end up filtering down to everyone else by becoming "a matter of courtesy", and not wearing hats indoors is a perfect example. As progress brought us to where everyone could keep their homes heated better and better, more and more people adopted the habits of those wealthier than them, as a status symbol again; and the "don't wear hats indoors" rule became widespread. Ask someone who believes in it why you don't wear hats indoors and they will invariably have no better answer than "just because that's how it's done!" but this is really where it comes from.
For the first time in a thousand years, we are finally in a position to look back on what peasants used to do and ask ourselves, did that make more sense? In particular, we are realizing that even if our industry and standard of living can have us all living literally like princes (the average middle-class American lives far better than a medieval prince did, by almost any standard you choose), our impact on the environment can't sustain it. We don't want to start living the "nasty, brutish, and short" lives of a serf, sure, but we're just starting to also reject some of the more impractical things that we got from the aristocrats. Maybe we should be thinking more about, instead of keeping our houses at 75°F all winter so we can dress like it's summer, maybe we should be letting the house get down to 65°F (still well warmer than the winter temperature of the house of anyone other than a rich person, any time before central heating) and putting on a sweater.
And a hat, maybe. A hat probably will do more than a sweater to make you feel warmer in a chilly house. Maybe it's time we put that rule about hats indoors back into the dustbin of history where it came from.
(But maybe not tophats. Those are just silly.)