Sleep Math (follow-up to our napless weekend)

I am like Jill, I just don't get the math behind sleep patterns. Monday Doug was a no napper and went to bed at normal time (with whining at dinner). Then Tuesday he was normal and yesterday he had a really late nap. So of course after a really late nap he had trouble falling asleep. He stayed in his room until Jo was going to bed (she complains for a couple minutes and then passes out...but that gives Doug the opening to say that her crying woke him up or to say that I need to check on her because she is crying). So about ten minutes after Jo quit crying and went to sleep I walk down the hallway to discover Doug on the floor hanging out of the door of the boys' room. So he got to "help" me with evening activities. He put all the silverware in the basket in the dishwasher. He helped me get the laundry going and pull the dry stuff out of the dryer. He helped carry trash bags and was like a little shadow.
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You would think that going to bed so late (it was 10:30ish) would make him tired the next morning...but no, Andy and Doug burst out of their room as happy as can be, and earlier than normal, before the clock hit the 6am mark.