As someone who suffers from Seasonal Affective Disorder, or SAD, I thought I'd help shed some light on the subject (pun intended, sorry).
Seasonal Affective Disorder, despite the godawful cutesy acronym (GCA), really is sad. It is classified as a mood disorder that is biochemical in nature. SAD is thought to be caused by the the body's reaction to decreased sunlight, specifically in a decrease in serotonin production, although no reproducible clinical evidence has been able to satisfactorily pinpoint the cause of the disorder. Symptoms include hypersomnia (sleeping a lot), hyperphagia (eating a lot), and weight gain; given the similar symptoms, it is easy to see why the disorder can be misdiagnosed as depression.
Treatment typically involves daily application of full-spectrum light. This comes in the form of light boxes (overhead lights that are installed on a wall), desk lamps (well, really big desk lamps), or visors that are worn just above the eyes. The exact treatment varies from patient to patient, but generally involves twenty-minute applications of about 10,000 Lux of light (Lux is used as a measurement of intensity; indoor lighting ranges from about 200 to about 700 Lux, a sunny spring day is about 10,000 Lux, a cloudless summer day is about 50,000 Lux), although treatments of only 60 Lux have been effective when used in visors. Ultra-violet light has been shown to have no effect on the SAD patient, and most full-spectrum light sources include a UV filter of some sort.
That's all the clinical stuff. Here's the bit about what it's like.
Imagine your energy draining away, day by day, so slowly that you can't even tell it's disappearing. Every now and again, you begin to feel that something is wrong, that you're in trouble for something, that there is a terrible doom of some sort that is about to happen. You start to have unreasonable, irrational, paranoid thoughts that your friends are excluding you, or that your co-workers don't respect you. Eventually, you stop caring about things, you don't want to do anything but stay in bed and kill time.
This is the worst case, of course. It begins subtly a week or so after the autumnal equinox. It gets worse and worse, and hits bottom about three or four weeks after the winter solstice. It's a devious thing, and most SAD sufferers don't even realize what's happening. The depression gnaws away at the personality, reducing a person to an obnoxious, disturbed, and unpleasant individual.
I didn't know what was going on until I was 22 (or 23, I'm not certain; ironically, my birthday is about three or four weeks after the winter solstice), when I consciously noticed my mood problems and decided to go for help. As a child, my parents had sent me to therapy, but usually in the late Spring, when I was getting better anyway, so it looked like the therapy worked and I didn't need to be there any more. The therapist I went to listened to my story, and suggested that SAD may be at the root of my problem. I agreed, but then she said that a diagnosis couldn't be made unless there was four consecutive years of evidence. I was terrified that I was stuck for four years, but before total desperation settled in I remembered that my grades throughout my primary education tended to form an upside-down bell curve when taken as a function of time over the course of the school year; this saved my proverbial emotional butt.
I was loaned a light box. It was an obnoxious size, too big for one person but too small for two to carry comfortably. A roommate helped me get it home and I plopped down in front of it for twenty minutes. I didn't notice any effect, but my excitement about getting treatment was enough to carry me for the day. A few days later, I did notice an effect: I was starting to be me again. No angst, no depression, energy up, no disgust with myself or the world. It was my electric teddy bear, my happy light.
Nowadays, I have a desk-lamp version, which pokes up over my computer and keeps me company during the morning email. Now that I know what to look for, I can notice when my mood starts shifting, and start the therapy then. During the longest nights of the year, I usually have to do another twenty-minute session in the evening, just to keep me from going to bed around six in the evening. On advice from a doctor, I've also tried St. John's wort, and found it to be a mildly effective stop-gap for when I'm away from my light, although not a good alternative.
SAD is a weird thing, although I'm sure that the cause and cure will someday be figured out by people that know a lot more about this thing than me. It's kinda spooky to know that something in your body can subvert your mind and personality, your soul, if you prefer. But at least I get a bright morning every day, well, for twenty minutes, at least.