I believe that Seasonal Affective Disorder somehow has an impact on every person. For somebody it can be a slight decline of mood which will be overcome within a day, but sometimes people can fall into such a deep depression which can be too hard to be dealt with. Namely for this reason, plenty of medical solutions were developed.
My personal treatment is sweets or something delicious. It always cheers me up. I don’t even worry about excessive weight. On the contrary, it’s a good motivation to go in for sports.
Some specialists advise to keep record of mood changes with obvious reasons or, if there are not such, with circumstances under which you lost your high spirits. A so-called diary. My friend though, who is rather influenced by SAD, says it’s not useful. But I guess that every method should be tried. Who knows which one will be suitable for you?