I commented on someone else saying this, but I’ll say it here also. There’s no way to isolate the variable of a noctural lifestyle. There are many confounding factors that can’t be controlled. Is it that being awake at night that isn’t healthy or are people who feel like being awake at night already typically less healthy? I’d bet on the latter.

We can take vitamin D supplements, so that isn’t the issue. However, a lot about your lifestyle must change with this, as you say. Is this a cause? Are nocturnal favoring people associated with mental health issues? Do nocturnal people eat less healthy? (Fewer options to healthy eating open?)

Studies involving humans are flawed. We can’t control every factor of someone’s life, so usually it’s self reported and also not forcing lifestyle changes on people —at least not for long term studies. They’re still useful, but people often look at studies that say “nocturnal lifestyle associated with worse long-term health outcomes” and read “nocturnal lifestyle causes worse long-term health outcomes” which is a very different thing.