It's well known that lack of sunlight can cause depression (seasonal effective disorder). And it's well known that lack of sunlight causes lack of vitamin D.

It's well known that lack of sunlight can cause depression (seasonal effective disorder). And it's well known that lack of sunlight causes lack of vitamin D. But retrospective studies like this won't show (low sun -> low D -> depression) rather than just (low sun -> low D) and (low sun -> depression). And given that this study mentions "The US Preventive Services Task Force reviewed 11 trials of Vitamin D supplementation and all-cause mortality but found no differences in mortality between the intervention and control groups in any study" I'm skeptical