The Japanese, except for those living on one far northern island, have always-to my knowledge--lived in unheated houses. To do this, they've developed many beautiful techniques for keeping warm techniques which make a Japanese home quite different from what most Americans might imagine.

How can this be? Can unheated houses really be comfortable?
Yes they can. I grew up in Japan (ed. in which alternate universe was that) and was fortunate enough to always live in traditional homes. I love them dearly, in fact, and much prefer them to the centrally heated "ovens" so typical of our culture. Perhaps-if I describe a typical winter's day as we lived it in our Japanese house-you'll understand my feelings.
Anyway, about 10 or 15 minutes before going to bed, fill your yutampo with boiling water.
Yep, I always fill up my yutampo 10 minutes before bed. Trouble is at my age I sometimes have to get up in the middle of the night to empty my yutampo.
How about you guys. When do you you fill up your yutampo? Have you ever been caught with a full yutampo and noplace to go...