I have the same problem =(
Because of Spocks bum - she has what is called Jaquets Errosive Nappy Disorder. basically the only thing stopping it from breaking down again is disposable over night (for the 12 hours) or putting her in MCN and my changing her ever 3 -4 hours over night (bascially exactly like the day). but we are trying to encourage longer stretches of sleep. so i have painfully accepted this is just the way it has to be. I've tried a couple of different brands of night nappies too...it seems to be the duration the nappy is on rather than the material etc..