I have a similar situation—enclosure is within view of a picture window (and cham loves looking out it).
When I set things up, I looked up sunup/sundown data for where I live. Coincidentally, it came out that noon really is noon, and sunup & sundown are—on average—equidistant either way all/most of the year.
So I set the timers for 6am to 6pm. The real beauty is, I don't have to do anything when the clocks change. The clocks may change, but the sun doesn't.

So I think the windows
DO play a part in it. Your cham will be up with the sun, and probably prefer to go to sleep accordingly. OTOH, if you moved him to a place where he couldn't see the sun, there may be less influence—IDK.