I've kept three wild caught bimacs. They were all shy at first, hiding and not accepting food offered on a stick. In the wild it doesn't pay to walk around in bright sunlight, so if your lights are bright, maybe you can try keeping them off all the time, with just ambient light during the day, until your bimac learns new habits. I think the best way to get diurnal behavior is to train your octopus to associate you with food, and to feed on a regular schedule, at about the same time of day (every 2nd, 3rd, or 4th day). If you only offer food on a stick, your bimac will quickly learn that it pays to get your attention, not to hide from you. Just leaving food in the tank allows them to rely on the rules they learned in the wild: stay in the shadows, and hide from anything bigger than themselves. If the new rules are: Dance on the glass when my human is in the room (and I might get fed), then they'll quickly learn to live by the new rules.
I think they are diurnal in the same way that humans are. If the pay is good, many people are willing to work at night and sleep during the day. If the day job pays more (or is safer, or more convenient), then they'll take the day job.