Like any condition, it is difficult - if not impossible - to pinpoint just one cause for this condition. However, we DO know that the effects of light are heavily implicated in SAD. Exposure to light therapy seems to improve symptoms, as does the arrival of a bright, sunny day. (If you are interested in learning more, this article has more information). There are even specific lamps that one can buy to stimulate 'light therapy' and help with symptoms.
Let's delve deeper into this. Sometimes, we all feel down when there isn't any sun around. Does this mean we all have SAD? No, of course not! There are specific criteria to diagnose SAD, along with protocols or methods for treatment. But what is really interesting is the actual effects of the weather, in terms of light/sunshine, on mood.
Think about a rainy day: rain is beautiful! It gives flowers and grass the water they need to grow, it cleans the streets, etc. But sometimes, the rain makes us feel groggy, irritated, and tired (perhaps because the lack of light means that we feel its time to sleep...?). There are times when I don't mind the rain, but there are other times when I REALLY hate it and wish there was some sunshine!
Now that we are entering spring-like weather, it is great to see more sunshine, and to feel the warm weather settling in. What a nice change from the unpredictable Canadian winter we have had this year! One resolution I've made to myself is to try to enjoy the sun as much as possible. There is something extremely empowering and motivating about waking up to the sun filling the room. In fact, there are many studies that show that the sun actually stimulates parts of our limbic system, the part of the brain that controls our emotions and mood (again, see the article above for a start!).