My DS does the rocking on hands and knees, it started when he was about 11 months and he is still doing it at 3 yrs. He is in a single bed with rails both sides, so that keeps him enclosed. I asked our nanny what she thought about the rocking motion and she said that is just some children's way of putting themselves back to sleep. It does seem to have calmed down a bit but he does it now and again.