Seth was day trained from 2.5, but wore pull ups at night until the day of his 4th birthday. He was dry at night probably 75% off the time, but he insisted that when he turned 4 he wanted to wear big boy pants in bed! It has been just over 3 weeks now, and I think he has had 4 or 5 accidents, but he was having a large drink of water when he was in bed. We've since said no drinks for about 30 mins-hour before bedtime, and (touch wood) since then he's been consistently dry. He'll probably wet the bed tonight now I've said that!
I've always believed that it's hormonal, so you're never going to get them dry at night if they're not ready physically, but I do also think there is a psychological element to it. I think that sometimes Seth was just too lazy to go for a wee because he knew full well that he had a nappy on and it doesn't matter.
I was in no rush to get him dry at night, but he made the decision to try himself, and he's doing really well.