I would recommend either fleece or wool. They breathe better overnight, and thats a good thing.

My fleece ones don't go over my night nappies - so I just use them during the day. (Mind you, I use night nappies that are really really high in the rise cos my kids are belly sleepers)

So most of the time I use either soakers, shorties or longies - depending on the weather and what the kids are wearing to bed