i had a similar problem with my 6 week old, in that , this week, when i was changing hi in the night, (as before he always needed changin) at his 4 am feed, , i noticed he was had a completly dry clean nappy, but was having plenty of wet and dirty nappies in the day, i spoke to the health visitor and pead about it and they said not to worry, he was probably just learning to hold it overnight, when he is in a deep sleep....and to not disturn him to change anymore, unless he obv dirty, as it is all helpinig him for the future for when he needs to hold it...

as i say he only 6 weeks old, but she said some babies just start early being able to hold it sometimes...