my oldest had this issue and the doctor laied it out so fast I almost thought it should have been common knowledge. While potty training him, we would have him to pee every 2-3 hours. After a while it was like clockwork. Even if I was on an errand, I would have to find a bathroom for him, like he had a stopwatch on. At night it was terrible becuase he would soak his pull ups to the braking point each night. The doc explained to me that holding urine is something that is trained. If all day, day in and day out, his bladder is trained to go pee every 2-3 hours, that usually a kid in a deep sleep will just go with no knowledge on that same schedule. Being that said I stopped making him go when told. We had a few accidents during the day where he would play instead of go, but nothing serious. He started holding it longer and HE would tell me when he needed to go instead of ME telling him. It only took a couple weeks and the peeing at night stopped. I will state, I would have to remind him as soon as his eyes were open in the morning to go potty or else he would go in the pullup, but again, that only lasted a few weeks. It took time, but I think re-training his bladder to expand more and empty less frequently was just like training any other musscle in the body. It worked. Start to finish, it took about 2 months and I've not bought a pull up since...well, till lately with the youngest. :-)
Good Luck either way!