I was going to say what Traveller just said. A friend of mine's little boy was waking with a wet bed but dry nappy, and after watching him a few nights she worked out that he was not really waking properly when he got the urge to wee. The little guy was still mostly asleep but probably thought he was doing the right thing by pulling his nappy down to wee IYKWIM - just was too much asleep to actually get up to the toilet. She put his pjs on backwards, but also sewed the shirt to the pants so he couldn't just pull the pants down. Worked a treat.

Good luck.