A 40 watt incandescent bulb will not add a significant amount of heat to the coup, and it will only be on for a short time, so any effect, if any would be minimal (in my opinion).
We have our timer set to come on in the evening as the hens are waiting at the door at sun up, and neither one of us want to go out in the dark to let them out. We also have a lot more activity in the evening to ward off predators, in the morning everything is pretty quiet, so we like to wait until the sun is up to let them have the run of the place.
My Beautiful Bride read somewhere you want 14 hours of light, so that's what we try to do.