As others have mentioned, cut the heat back a little at a time. Chickens can certainly handle your weather without any heat *once they have been acclimated*. Since your outdoor temps are fluctuating, it might be helpful to put a thermometer inside the coop. This way you can see the effect on the temperature when you change out the bulb. It might not be as simple as just progressively going to a lower watt bulb since the outdoor temps are fluctuating too. Your chicks may very well be scared of the dark at first, however in terms of their development and eventual egg laying, it will be beneficial to have complete darkness at times. I know I am more restless at night if there is a light on myself.
