I suffer it a bit too. Its brain chemistry...
You have something called serotonin in your body. This is basically the stuff that keeps you on your toes during the day, helps you manage your anger, influences a bit of your mood etc. etc. People who suffer from phobias often have significantly lower serotonin levels, which is among other reasons why they dont respond to certain situations the same way other people do.
One of the reasons you get groggy if you sit in a dark area/room for a long time, is because serotonin production/activity decreases, and melatonin production/activity increases. This is a sort of biological clock, a roughly 24 hour period called the circadian rythm that biological organisms have.
So to sum up, since our days are so much shorter now, meaning less daylight (if you even get alot of it with all the hazy grayness), you are producing less serotonin. So you're not as alert, more easily irritated/annoyed, not particularly feeling 'good' (doesnt have to mean bad either), etc. etc.
