The theory is that vitamin D intake helps set the circadian clock because it has been correlated with exposure to sunlight for hundreds of millenia. I think Gwern did a single blind experiment on himself and found some correlation to lower sleep quality if it's taken at night.

I'm actually just completing a high-strength vitamin D tablet course and the advice for taking it in the instructions is to take it with your evening meal, so sounds like a load of hogwash.

Here, for example, are the instructions that came with my tablets:

https://www.sunvitd3.co.uk/site/wp-content/uploads/2019/01/S...

Note the:

"Please Note: SunVit-D3® tablets should be swallowed whole with water and taken in the evening with a main meal."