Sylvie, This is JMO.
If you can, wait to see what effect, if any, the light has to do on the behaviour of the chickens. I am not sure the light you are describing would be bright enough to do anything. But if you sneak down after dark (at least an hour after dark) and listen to them, it should be quiet, not noisy.
If it is not quiet, then you should shutter the window or cover it with something to darken it. With the window darkened you will probably want to put a red light in there with a timer on it. You need the timer so the light will go out completely at a certain time. I wouldnt want a light in my face all night would you? If you are doing it for egg laying then this would apply to that also.
If it is quiet, then I would just watch the egg production. If you want to extend the hours so they get 12 hours of daylight then I would do the red light on a timer.
This post is going to be updated by me everyday. I am curious wether the egg laying is affected by the red light. Someone mentioned that it would decrease it.....so I am watching, LOL.
My hens started laying (if I remember correctly) a week before they were 6 months. I posted it on here, you could search for the post by author I believe. I hope this helped.