Of course, the Americans had to have a fitting acronym for it. It's called Seasonal Affective Dysfunction, and is a result of lack of light during the winter seasons. If it is combined with bad weather (three months of continuos rain and storms should do it), the likelyhood for developing depressive symptoms just from the regular change of seasons is high.
In Norwegian we say we become "værsjuk" - literally: Sick of the weather. Whether or not we actually get sick of the lack of light is still up for discussion. After all: a large chunk of Norway does not have sunlight at all in large parts of the year, and if it is a normal reaction to sleep more, become less physically and mentally active and to need more social reinforcement under such conditions, that does not mean half of the Norwegian population should be medicated. There are things to do, like going out despite the bad weather, use what little daylight there is, and be physically as well as socially active. Which is, probably, the reason amateur choirs, bands, dancing groups and just about any other social indoor activity you can think of flourishes in Norway during the winter months.
But sometimes it's just too much. I was listening to the storm tonight, third named storm in two weeks (they only name them when it gets serious, the kind of "don't go out unless you have to evacuate your building" type storm). Mornings like today I understand the motivation of writers such as Olav Duun, Amalie Skram and August Strindberg or more recent Scandinavian social-realistic and pretty depressive writer Jon Fosse.
These are writers I do not wish to understand. I have no desire to understand about guilt, gloom, obsession with self, the slowly developing contempt and hatred of others developing from too close familiarity, desperation born of unchangeable circumstances and tragedies rising from being trapped in the same small community, same tracks, same rooms month after month after month.
And after this happy little rant I started writing about the options: of how Scandinavians go to sunny paradises abroad in order to get a shot of warmth and light and get back on the feet. Not a good thought this winter. Sometimes sleeping 12 hours a day and spending the "days" trying to function while the brain is always on the low gear is the good option.