We are only a couple days from the solstice - it's Tuesday the 22nd this year. After that, the days will start gradually getting longer until summer, and the hens' brains ought to notice for sure next weekend.What kind of light? I have seen many people suggest this. Will a simple lamp on be enough? What about a florancent he lamp
That said, I've read it doesn't take much - even just a 40w bulb will do it, but you'll either need a timer or get up at zero-dark-stupid to turn it on in the wee hours of the morning. Don't extend in the evening, as they will need to go to roost normally.