Living in a country that has twice as long winter than most others, I know quite a bit about SAD, or Seasonal Affective Disorder.
What happens, in layman's terms, is that the body has a reaction to the lack of sunlight, which causes us to feel inexplicably tired and drained of energy. We feel sad and restless, and left untreated these symptoms can develop into a serious and dangerous depression.
Instead of feeling like weirdoes for having "shameful" psychological problems, we should recognize this for what it is; our bodies' natural physical reaction to the changes in the environment. Sort of like when we breathe in fumes from a car, and the carbon dioxide causes us to cough.
Just like we protect our health by avoiding to have a pick nick next to a running car, we need to protect our health against the lack-of-light depression.
In those cases where the symptoms are very strong, some doctors prescribe medicine and light-therapy, where the patient is exposed to hours of strong lights. People who have strong feelings of depression should most definitely seek medical care. This is a serious physical, not mental disease!
Those who are only having a mild case of The Blues, however, may try doing a little health treatment at home.
Since you're feeling low due to your body not getting enough light, the first step is to bring more light into your home. Lots of light! Turn on every lamp you have! Buy small, decorative lamps and pack the windows and bookcases with them. OK, it may not be all that environmental friendly, but you can always make up for that in the summer, by not using any lamps at all.
You could also light candles – a very popular thing to do in Sweden in November – January. But be careful – don't EVER leave candles burning unattended!
If you look through the window and all you see is another concrete apartment building, smoking factory chimneys, or a messy, slushy street, then that sight's going to make you feel even more gloomy. Shut it out!
Cover your windows with curtains or Venetian blinds in bright, cheerful colours, and focus on the inside of your home instead. Try to see it as a cosy cave, where you're kept safe and warm from the cold and the darkness outside.