L.,
There could be any number of reasons from "just habit" to PTSD, but obviously, you know your husband best.
Just some random thoughts to ponder.......
Is the problem more that the light keeps you awake, or that he won't talk to you about it? Could this "lights on" issue have become symbolic of larger issues in your relationship? (For examp, is it just this he has trouble talking about with you, or are there other things going on?) Have you broached the subject with him, maybe conversationally, and/or with humor, during calm times? You probably have, given it's been 10 yrs, but I know I get "stuck in a rut" sometimes.
Obviously, we can't change others' behavior....is it possible the two of you could come up with a solution together? "Honey, the issue of the light at night obviously is upsetting to you. Is there some way we can discuss it without it causing a fight? The light keeps me awake, and I feel bad that I don't understand why you need it on. Let's come up with a plan to figure out a compromise." or whatever........
Good luck!