I don't see why you'd want to do it, because it's not natural for the gecko, and I've never tried it (nor had any desire), but, based on what I know about human sleep, which comes from reading sleep literature as a parent of young kids, theoretically this is what you'd do:
keep the gecko in a room where there is no natural lighting. Make sure that all the lighting they get is from an artificial source. Start by providing light that mimics the normal time for daylight, so at first, the gecko will still be "nocturnal". Gradually make "sunset" and "sunrise" both later. Eventually, you will have them flipped and theoretically you will have a diurnal leo, although the leo will still think it's nocturnal. The only thing is, if you've done that and then sell it or change your mind, it's kind of too bad for the leo as far as adjusting.
One of mine sleeps mostly @ night - he's awake during the late afternoon into the early night. Personally I don't mind it but I think his mate is getting sick of his shenanigans. :main_rolleyes:
I would think that geckos dont necisarriy go by time of day ("Oh no, its 7:30 I better get up, and eat some mealies!" lol), but rather by light and darkness. I figure they come out at night, when they are eveloved to hunt better... I wonder if your "experiment" would work, but honestly I doubt it. My leos are comfortable with me, and they will lounge around in the open during the day... they still sleep, but they dont feel the need to be in hiding while doing so. I just dont see the point in trying to change their natural activity periods... especialy if it is just for the benifit of humans veiwing them.