Japan boasts a tropical climate, and with the exception of the northern prefectures and Hokkaido, most places in Japan actually stays pretty warm all year round in comparison to other countries on the same longitudinal coordinates. Tokyo’s coldest month tends to be February, but even then the temperature rarely falls into the negatives. There is modest snow only once every few years if lucky.
However, that is not to say that winters in Tokyo are not unbearable. Even though Japan is much warmer than my hometown New York, in-home heating is absolutely horrible.
The walls of buildings are thin, drafts slip in from outside from the slits in the mailbox, and cold air seeps in through the large windows. There is no internal heating system that turns on automatically like it does in apartment buildings in New York. In short, your home will be as cold as the outside.
Even if you turn on the AC on its heat setting, the floor will never become warm enough. I find that my feet are always cold even if the AC is blasting at 30 degrees Celcius. Many Japanese homes are made out of wood and are prone to catching fire, so many places prohibit space heaters, which pose a significant fire hazard. Most people get through the winter with compromises such as hot packs (called hokkairo), hot water bottles that are placed in beds underneath blankets (yutanpo), and kotatsu (low-end tables with a built-in heater and blanket placed to trap the heat).
Nevertheless, I find it hard to live and sleep comfortably in my own home during the winter, and the cold often makes me wake up several times throughout the night, decreasing my quality of sleep.
Nowadays, modern mansions and high-end apartment buildings are building rooms with floor heating, but it’s not a popular trend for average people who cannot afford such a luxury. My biggest advice for people living in older homes is to cover those cold wooden floors with carpeting, which does not become cold and keeps the feet warmer. Also invest in some good socks!