When I was studying physiological psychology, I was taught the term "homeostasis". This term has several applications, both human and otherwise, but in my studies it was used to indicate that our bodies and minds like things to stay the same and the introduction of change, particularly a rapid and extreme one, stresses a person. When your body is stressed, your immune system has more difficulty coping with the bombardment of nasty things it encounters and is more likely to fail in its mission of keeping you healthy.
A very good way to avoid catching a cold is to minimize powerful fluctuations in temperature. Given that there is no central heating in most Japanese domiciles, this would seem to be pretty easy, but the truth is that the use of space heaters and kotatsu tend to increase the chances a person will overheat himself or herself by sitting close to the source of heat in a cold room rather than sit in a spot distant from the heating source in a lightly heated room which is being warmed by central heating.
Before anyone gets the wrong idea and believes that I'm suggesting that central heating is somehow superior in any way (because there are some people who love nothing more than inferring something so they can take me to task for what I didn't say), that's not what I'm saying. What I am saying is that most people who live in cold weather set the thermostat low (65-68 degrees F./19-20 degrees C) and wear a sweater so the rooms they are sitting in aren't really hot. When your room is really cold, there's nothing more satisfying than practically sitting on top of the only heat source (space heater/kotatsu) and getting nice and toasty. Unfortunately, this is usually followed by getting up and walking into a much cooler area of the apartment or going outside into the frigid air. This is forcing your body to adjust pretty frequently to rapid changes and increases the chances you'll get sick.
Keeping the aforementioned and other points in mind, I've got some tips for keeping well at this time of year for those of us in Tokyo (though these can apply to other folks as well):
- Use your space heater (or kotatsu) at a lower temperature setting if you tend to sit close to the source and try not to warm up too quickly. Wear several layers of clothing to keep your body heat in rather than rely so heavily on being externally warmed. Maintaining your body temperature with clothes will minimize the frequency and intensity of temperature fluctuations.
- Avail yourself of the cheap and plentiful Japanese oranges (mikan) at this time of year and eat one or two every day and try to incorporate more vegetables into your diet, especially tomatoes and leafy green vegetables.
- Sleep with a knitted cap and socks on. Most of the heat leaves your body through your head and feet. Wearing a cap in particular is something people don't tend to do, but it will seriously help you stay warm in bed, especially if you tend not to use your space heater at all through the night for safety reasons.
- Invest in a good comforter or blanket. A down comforter is light and warm (though some people are allergic) and can serve your very well. A lot of the blankets in Japan are pretty thin or not well insulated.
- Make it a priority to exercise regularly. You will find that you're less likely to get sick if you are stronger overall and that your circulation will be better if you are getting some aerobic exercise.
- Drink as much water as you can to help cope with the dry winter air (and dry air from heating). This is something you have to make a priority, not just wait until you feel thirsty or dry. If you suffer from sore throats in the morning, it may also be a good idea to invest in a humidifier for your bedroom though be careful if you buy the kind which puts out hot steam. I still have a scar on my wrist from getting it too close to a hot humidifier positioned on my nightstand.
- Get yourself a pair of fleece-lined slippers for winter which are warmer than the average Japanese house slippers.
- Wash your hands every time you come inside after being outdoors and be mindful of touching your eyes, nose, or mouth while you are out so you don't transfer any germs you get on your hands to your mucous membranes. If you can't wash your hands easily, carry the sort of hand wipes which are treated with alcohol to clean your hands.
- While drinking hot beverages would seem to logically make you feel warmer, it actually causes your body to try and adjust to the internal heat by making your body colder. It's better not to drink very hot beverages and try for something that is warm or room temperature.
- If you can get your hands on them, wear high quality long underwear as part of your layers of clothing. Land's End has mail order that ships to Japan and is reasonably priced, has western sizes, and is good quality. They carry silk weave long underwear which should keep you from being too hot in heated rooms and warmer in cold weather.
Another problem is the fact that most offices are kept at inferno-level temperatures during the winter so there is the inevitable shock to the system when going from indoors to outside. The former president of my former company used to justify roasting us to death in summer by saying it was unhealthy to be too cold then go out into the summer heat, but didn't believe there was anything wrong with setting the heat such that it was 85 degrees F. (29 degrees C.) inside in the winter when it was 40 degrees F. (4 degrees C.) outside. If at all possible, do the best you can to spare your body these types of extreme transitions.
For many foreigners, not getting sick is more important than it is for a Japanese person. Part of the reason for this is that some of us work under conditions where we are not paid for sick leave (this is the case for both my husband and I). Also, the truth is that a lot of the time a foreign person is "blamed" for being sick or disbelieved. Every time my boss or I became ill (which wasn't all that often), the president would say that it was our responsibility to take care of ourselves and that we were failing in our jobs if we allowed ourselves to take ill. While the Japanese staff took days off for sniffles and low fevers, we soldiered on with raging colds and the flu (which is when we inevitably were lectured about how getting sick was our own fault). Of course, this was the same president who used to take a half day off whenever he got a headache. :-p
While I'm not offering myself up as an authority on maintaining health, I can say that I haven't caught a cold in about a year and a half, and, at the very least, I doubt my advice will make you less healthy.