Well, I am probably anthropomorphizing here, but maybe during the day there are more things going on to distract him from the discomfort in his bladder. But at night, the symptoms may be harder for him to ignore. It could be a mild infection. Or it could be something else entirely, but getting him cleared medically will let you know for sure.
Niko: American Showline GSD 5 years old
Rosa: American Muppet Dog (GSD/Border Collie mix) 5 years old