I think the night nappy is for when they get bigger and sleep longer stretches. You don't want to be waking them up to change their nappy if they are happy to sleep for 12 hours straight! When they are little though, it is fine to change them when they wake if you are happy to do that. I must admit, when mine were little, I didn't change them overnight unless they pooed or had very full nappies because it would take longer to resettle.