I doubt I'm any help KM, but wanted to just jump in and say DS understood the lights on a timer after several days (I did need to stay in his room with him sometimes, and also PU as he was getting upset with waiting in the dark and me telling him it was still night time) when he was 17 months. I made the lights very bright so there is a big difference between the dark room and light room (energy saving bulbs come on reasonably slowly and brighten up which actually helps with the transition from dark to light too).
I did find though that we had SO many things go on (canines, virus, asthma, developmental leaps bringing a good dose of SA...the list goes on) that I could not possibly expect him to stay in bed until the lights came on. If he woke ill or teething at 5am it would have been cruel to make him wait, even with me there with him, until 6am or later. I had to ditch the lights on a timer and there's been no opportunity to reintroduce yet because these illnesses and teething have been on and off (mainly on) for almost 3 months!
Anyway - point being if it is purely habitual I think B will understand the lights in a short space of time. Like pps have said the lights are easier to understand, although B is young he'll get it.
xx
