Light therapy used for people who suffer from SAD helps them overcome their depression.
As to why is it bad? I assume light therapy does not have cancer causing rays but if it does, that would be bad.
I also assume it would also be helpful if people just got out and exercised more in the winter time rather than relying on artificial light.
I have a tendency to get depressed in the winter time myself and try to go outside for walks whenever I'm feeling low.