on not so sunny days....hmm I would jus leave them as long as they arent freezing they should be fine. Carrying them inside for some cfl lighting isn't very practical, maybe if you had some HID's. Again I don't know what your workin with so it's tough to help you. If it's turning seasons on you and getting really cold and shitty, then maybe you could construct a small greehouse (very very cheap to make) that way the plants stay safe, still get light and you can control the environment alot better.