I don't think that you would have much success with Christmas lights. Almost all lights of that type are incandescent, which make plenty of visible light but almost none of it is usable to a plant. Unless you had some of those fancy new LED xmas lights I'm not sure you would be producing an appreciable amount of PARs. Under incandescent string lights your seedlings would be very strechy with disappointingly narrow stems. They would gain height very rapidly until they fell over from a lack of support. Shop lamps have much much better performance, just make sure you've got them lowered over the plants as closely as possible. A 1-2 inch gap between the tops of the plants and the bulb is ideal.

I've seen some experiments where people are using LED Christmas lights (red and blue) and they are having good success so far. I don't see them having the intensity to grow plants to maturity but they seem to be fine for seedlings.

Mike

On the subject of OUTDOOR Christmas lights: I use them strung among the base of plants to prevent a hard freeze when there are frost warnings. They give off just enough heat to keep things from turning to stone!

they are using the lights to heat the seedlings ...not to use overhead!!