I always say that it's an X file to get DS's sleep sorted!!
It wasn't really an 11.5h day, though, not the way I see it, well one of my theories
He slept 2.5h, so his total A time for the day is 9h. It doesn't mean that he can do 9h stretch in one go. It is to find that magic balance that he's tired enough to take the nap, refreshed by the nap, to carry on for the rest of the day AND enough total A for the long night sleep.
The 12h night sleep was after 12h day with 2h nap, so 10h total A. Today he slept almost 3h, a bit broken but i don't think it matters as the wake up time is important. He was up 6:50, asleep at 7:25pm. 9.5h total A. I'm not sure if this theory applies to him yet, or he needs fixed body clock sleep schedule, so we shall see!