If your child is often wetting the bed at night, he is likely not ready to be sleeping without a diaper or pull-ups. From PubMed Health (US National Library of Medicine):
Nighttime bladder and bowel control develops somewhat more slowly. So,
even once your child is dry during the daytime, it can take a while
before they notice in their sleep that they need to go to the toilet.
By the age of five years, about 8 out of every 10 children can also
control their bladder and bowel at night.
So at first it can still make sense to continue using diapers at night
even if a child no longer needs them during the day. If the diapers
stay dry for several nights in a row, your child might well also be
able to stay dry at night. This is a good time to see what happens
without a diaper.
Further information from the NCBI (National Center for Biotechnology Information) states that nighttime bedwetting up to the age of five is perfectly normal, and suggests
- if your child has been day-trained for at least 6 months, give him a
2-day trial to see if he can stay dry at night. If he does not, wait
a few months and try again for a couple of nights. Eventually he will
spontaneously manage it, when the difficult learning of managing his
toileting is ingrained enough that he can do it in his sleep.
- if your child wakes at night on his own, take him to the toilet as
this will help him get used to paying attention to his toileting
needs in a sleepy state.
While a reward system may be successful and while it might be nice to be through with the expense and nuisance of diapers, if you allow your child to come to this naturally, you don't have to make this a battle at all. If your child does manage some dry nights in diapers, make positive comments that address the accomplishment without suggestion that to be wet is in any way bad ("You stayed dry last night! It's really hard to learn how to do that!").
Also, note that
- if your child suffers from constipation it can contribute to bedwetting (NCBI)
- boys are developmentally slower at this (NCBI)
I remember listening to a radio show about toilet-training once where an anxious parent called in to speak to the expert, and the expert said, "Ma'am, do you know any 17-year-olds who can't manage toileting?" She said No, and he assured her that everybody gets it eventually in their own time - so not to stress!