I suffer with Reverse SAD - it is a much rarer and less understood form but it does exist, there is ample info online. In fact I just came to the Depression Forum now because it always starts around this time of year for me, like clockwork. The spring tends to be my lowest and it lifts somewhat around June, but I never really feel my best until early September. Then no mood issues during the fall and winter, that's when I am most even tempered, happiest and most productive.

There isn't nearly as much known about Reverse SAD as the more common variant, but there are studies showing that some individuals who were born in the winter are more prone to Reverse SAD. I was born at the end of February. I also apparently experienced some trauma during the first few months of my life that I have no conscious memory of and only know from my father's report. It was about my mom's intense anxiety around not knowing how to care for a newborn baby (myself) and apparently it took her a few months and help from a nanny to learn and become more comfortable. My dad described it to me as quite erratic.

Other than these, I have no idea about the source. It's gotten more pronounced with age (after my mid-30's). Typically first it starts not with depression but a short hypomanic-like spell, followed with a few months of a very typical Reverse SAD episode that's described in professional literature. I do intensely dislike hot weather and humidity and am not crazy for long bright sunny days. Never had issues with colder, cloudy weather and milder climates (when I lived in such areas).