My almost 17 month old DD hasn't wee'd overnight in I don't even know how long. We've managed to not need a nappy change overnight since she was a newborn (the wet nappy didn't bother her) and I'm sure it was by the 6 month mark that I realised she was waking up dry. We do have to change her shortly after waking though cause the nappy ends up quite soaked once she does wee.