It sometimes amazes me how much the weather affects my mood. On a day like today, when it's overcast and has been raining intermittently, I find I'm lethargic and also feel like I'm crawling out of my skin. I have to have the lights on even though I don't really need them, just to keep myself from feeling like I'm going crazy. If it's sunny, then I feel great and am perfectly happy. I also enjoy thunderstorms, though, so I often get excited if I actually hear thunder (not all that common here).
I don't have seasonal affective disorder, though. Even though there's a big discrepancy in the length of the days here in summer and winter (roughly 7.5 hours in winter and 17 hours in summer, just going by sunrise & sunset times), I don't get depressed in winter. I just like sunshine, whatever the temperature.