That article wasn't much help for the question at hand: how to supplement with artificial light. I'm in a similar position, in the middle of a gloomy winter, with no more than 6 hours of useful light daily, and very little sunshine. I want to add lights, but I'd like to know how to go about it. Do I turn them on late morning and keep them on a couple of hours past sundown? Do I extend the light period by, say, 5 minutes every week? Do I wind it down in the spring and confuse the heck out of the plants?

My plants are in a south facing window, so when it's sunny, extra light is the last thing they need. But when it's overcast 18 days out of 20, like this winter, they're not doing well.