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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."

The reason for that recommendation is that Vitamin D absorbs better in a meal with fat, and dinner is the biggest meal for most people.

See my other comment in reply to parent. There might be lower quality sleep because of the timing.

https://www.gwern.net/zeo/Vitamin-D