I could be wrong in saying this, but I don't think it is UT, because even if he takes a shorter nap, it still happens. But honestly, I have no idea. With it being summer, he is so busy and so tired during the day, that I am hesitant to cut any sleep. He is also getting his molars and gets so cranky when he doesn't nap well during the day. However, yesterday he had a crib party and didn't sleep at all for the 2 hours he was in it. As you can imagine, he was in rare form at bedtime. I don't know - we will see I guess.