That's a good point. There's also a bunch of other confounding factors which affect sleep. E.g. circadian rhythm, variance of sleep/wake times, how physically tired you are, how much recent REM/deep and whether there is a deficit, how stressed you are, screen usage before bed, eating close to bed time, exercise close to bedtime, etc... Maybe the Vitamin D3 taken close to bedtime is enough to tip some people over the edge to some kind of insomnia, given the presence of other melatonin inhibitors.