Seasonal Affective Disorder is really only the beginning. You don't have to be crippled badly enough by the season alone to have it really affect you negatively. "Winter blues" leads to a lack of energy and a desire to eat more in all of us to some extent, and if you're otherwise depressed this can compound your symptoms.
The metaphor of darkness and light works well with depression because darkness and light have very significant physiological effects upon us. Diurnal is the standard behavior cycle for most living things, and even though we humans can act against type, we are generally responsive to having light in our lives. Darkness is a time of hibernation, introspection, preparing for the light to return. Sunlight encourages us to think, to act, to experience outwardly rather than inwardly. Both are perfectly normal and healthy and we need a balance between focusing on within versus focusing on without. With depression, though, darkness takes precedence, and the darkness that is created within magnifies the darkness in the world until it's unbearable.
You might be eating more and feel guilt over the weight you're gaining, or think you're gaining. Or you're less motivated to exercise. Or when the sun sets at 4:30 in the afternoon it feels like all the light in your life is vanishing with it.
The light box described in the link is probably not necessary for someone who is depressed but not specifically prone to SAD. However, daily walks in the daylight will get your blood flowing, expose you to sunlight, and get you the exercise you need anyway. You don't want to exercise if you're depressed (since depression discourages you from doing anything that might benefit you), and this is just compounded by the time of year.
The sunlight is going to be diminishing for a few more weeks. Make an extra effort to get outside, even if it's cold out. Buy a full-spectrum bulb or two if you can. Set your alarm for the same time every day and get up, no matter how much you don't want to. It's going to turn around soon, and before long you'll see that the light creeps in a little earlier and lingers a bit longer than it had been. Don't be afraid to make a note of the time so you recognize the increased light more quickly. The light always returns. That's a promise.