Nobody knows yet, but sunlight exposure causes the body to produce other things than Vitamin D, like nitric oxide (or similar correct me if I'm wrong here). It

Nobody knows yet, but sunlight exposure causes the body to produce other things than Vitamin D, like nitric oxide (or similar correct me if I'm wrong here). It also affects circadian rhythms, so maybe people who get out more sleep better, maybe they are in better health from being more active, etc etc.