Light takes at least two weeks to start working and only a few days to stop working if taken away. One night with a burned out lamp can mess up the whole program. Some say even a few hours of the light burned out can mess things up.
You need enough light to read a newspaper in all parts of the coop easily but it doesn't need to be a bright light. You should select a light with all of the "warm" parts of the spectrum, not a "cool" lightbulb. I use a single, fluorescent bulb of the warm spectrum on a timer that turns on at sunset since I don't want my rooster crowing at 3 in the morning. I use a timer that senses dusk so that it automatically changes with day length, which is really handy. I can just set the amount of hours after dusk that I want it to stay on and every so often, as the days get longer, reset the timer to adjust for the longer hours in the spring.
I light my chickens. They are livestock for me,not pets. Plus, in AZ, they take a break in summer from the heat, they don't need another break in winter because of light.
I learned quite a lot about lighting and animals not only from my college classes in Animal Science but also from showing horses. We use lights to keep them from growing winter coats and to bring broodmares into heat. One night with the lights messed up can spoil months of work, making the animal think it's the beginning of winter and resetting their internal clock. Many animals are sensitive to lights. Sheep, horses, chickens. I'm not sure about cattle, i suspect they are. Humans are also effected, which is why we can become more depressed in the winter. I suspect there are more things that happen in our bodies related to daylight hours that researchers haven't found, yet, since endocrinology is a fairly young and complicated field.
The part of the brain that's effected is the pituitary.
