Mine is on a timer to go on in the early AM and off after sunrise. I don't like the idea of it suddenly going dark on them, I worry they will get caught off the roost, but that's probably silly as I find them on the roost an hour before sunset.
I've read a number of times that a regular light all night disturbs their rest. That's why people use red heat lamps; supposedly the chickens can't see the red light. Don't know whether I believe that or not, as red food ant water dish rims are common, they will peck a bloody spot on a chicken, and they don't like red clothing. Maybe some day someone will explain all this to me.