I live in the third cloudiest US city during winter months, so in addition to the decreased daylight hours, the clouds obscure the sun. I use a light box, but it's had to tell if it does any good at all. I still get depressed in the winter/early-spring. I sometimes wonder how much of light boxes' effectiveness is due to the power of suggestion. Despite my doubts, I still use my light box. I've never had an episode of mania while using it.

I wish you continued good luck with your light box. If it works, or there's even a shred of a doubt, keep it. It can't hurt.