Unless you're trying to match what the sun is doing in your area, the 12 on/12 off cycle should be fine, but as pointed out, you may have it... offset a bit. I have a bearded dragon that's doing the same thing—going to sleep a little earlier than his lights go off. I noticed today that they're on a little longer than the sun is up, and it's time to shorten them back to 12 & 12, so I think that will likely solve the problem.
There's also a chance mine is beginning to—or thinking about(?)—brumating. Time will tell.
How's his appetite & digestion (poop)?