One of the things your body does is produce wake-up hormones in the light and sleepy hormones in the dark; unfortunately, light is very light: outside on a sunny day, rather than inside with only a 15 W fluorescent tube. Seasonal affected disorder is more widespread than people think, because they are too tired to notice that they have the Winter blues, and because you don’t have to be clinically depressed to be affected. I do actually feel tired and listless if I sit in a dark room, and often feel claustrophobic in a room with no windows (althougth I can do lifts and bathrooms). This is why our househas lots of lights in it and is always flood lit. It’s also why I work in a glass conservatory, but that is just my good luck, rather than an actual plan.