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Addition...
What's the point of giving a reward for staying dry at night if the child has no conscious bladder control while they sleep? I continue to assert that there is no point of making night wetting an issue. Use pull-ups or diapers as long as you feel the need, don't make a big deal when they wet themselves at night, and just accept that it could be a long time before you have no more wet nights. No point discussing, rewarding, etc., if we believe they have no control over it. And that is what I believe. I wet the bed into my early elementary years (didn't have the luxury of the big kid pull-ups to save the humiliation, nor was it widely accepted at that time that this could be "normal"), so I can speak from experience. It wouldn't have mattered if you gave me $100 bill every time I stayed dry through the night. I had no control over the times I wet the bed.
I apologize for the soapbox, I know you just asked a simple question about which type of pull-up or diaper works best at night, and this got way off track.
OK, ONE LAST THING...
The reason we use pull-ups at night vs. diapers, is that my son can and does put them on and off himself. He dresses himself after his nighttime bath, puts his pull-up on, and will pull it down to pee before he goes to bed. Although the idea behind pull-ups is for training purposes, and we aren't "training" through the night, they still provide a greater sense of independence. Pull-ups certainly do more closely resemble underpants. If my son were in a diaper, I would have to put it on him, take it off if he needed to pee, etc. I don't think that at age 3 or 4 he would have liked being treated more like a baby wearing a diaper. There is a small element of being treated like the growing boy he is, not like a baby. He still feels like a big boy in his pull up because he is in control of it, just like his underwear during the day.
OK - I swear I'm done.
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I have to respectfully disagree with the idea that you should wake yourself up in the middle of the night to wake your son up in the middle of the night, if he is otherwise sleeping well. As most of the posters seem to agree, nighttime dryness is probably a way off yet for your son, so why make an issue of it and wake him up?
We used the nighttime Huggies Pull-Ups, but honestly, I don't see an awful lot of difference between those and the regular Pull-Ups. Our experience has been that on occasion, THEY ALL LEAK. It depends on the position they're lying in when they pee, how much the pull-up may have moved or slid down a little during sleep, etc. We recently decided to try Target brand pull-ups (didn't realize they made them until recently), and they seem to do as good a job as any others, and they are cheaper. Plus my son loves the robot and dog on the front of them. :) I'd say to get the water proof mattress cover (but you probably already have one), and just keep with whatever pull-up makes you happy. Like I said, I think they all do about the same job, give or take a little.