Seasonal Affective Disorder (SAD) is caused by lack of light and can be treated using light boxes. You get a very bright light and you stand in front of it for about twenty minutes first thing in the morning. Supposedly it helps.
The symptoms of SAD are that you get depressed. You have a hard time getting out of bed. You are gloomy. Life can seem difficult to take. It’s hard to get anything done. You have no motivation. Greater anxiety.
Some of us were not meant to live so close to the poles, I guess. We need to live where the sunlight is steadier year around. There is a reason why suicide happens at higher rates in Northern climes. You would think this would be true in Southern climes, too, but I think the weirdness of our planet’s orbit makes it so that SAD is not such a big thing during Southern winters. Even though the Southern end of the planet is tipped away, the planet is closer to the sun than during the rest of the orbit, and so things don’t get so bad.
In the North, when it is winter, we are both tipped away from the sun and farther from the sun due to the point in our orbit. So the incidence of SAD is higher. That’s my theory, anyway. Hopefully it is backed up by evidence, too.