I haven’t read it, but is it plausible form the data and controls that outdoor lights = denser city -> city lifestyle (nightlife, processed foods, stress/noise, etc) -> diabetes

If so, it seems less likely to me that this is some actual light impact and far more likely to be just a marker of lifestyle.

Edit: it said one of the controls was adjusting for “urban/rural area“. I have no idea what that means, but that is at least part of above. I’m unsure how a binary urban/rural is measurable. In any rural environment one that is more lit is going to be more densely populated. I think the same is probably true for urban areas.

Seems like maybe a control for population density might be more useful.