I can't speak to the average temperature there in 1000 AD, but in Barrow, Alaska the summer averages above freezing for a high four months of the year. During the coldest part of the winter the lows average around -20 (lowest record is -56). Even at -20, with no windchill (assuming they're indoors) it takes about 30 minutes to get frostbite. While it may not be comfortable to disrobe in extreme climates, sheltered by a fire would keep it from being unsafe in short periods. Dampness caused by sweat can be more dangerous than the changing process, so likely that will be the biggest incentive to change out the under garments.