My son, who was trained just as he turned three, was day and night trained at the same time. But he never peed at night, even when he was in diapers, after about 18 months or so. My daughter, who is three and not yet even remotely interested in training, doesn't pee at night either. I have no idea why...but it does make life easier! This sort of thing seems to vary wildly from child to child, and a normal kid could be anywhere on the spectrum.