[globular-toast]

I'm highly sceptical of it being anything but a placebo for most people. I live in the UK so I'm a good candidate for deficiency but I don't actually feel anything is wrong with me. Everyone thinks they are tired all the time and their sex drive isn't as high as it could be. If I supplement vitamin D the only thing it does is give me constipation. I'm sure there are some people with actual deficiency, but if you're white and go outside sometimes you're probably fine.

[beagle3]

There's a difference between sickness-causing deficiency and optimal level. Most people are somewhere in between. If you tried and it didn't make a difference, then you're probably around the optimal level.

However, the specifics do matter; see e.g. https://www.gwern.net/zeo/Vitamin-D - this is n=1 but consistent with many other n=1 self-experiments I'm familiar with; personally, my sleep is improved with increased protein consumption AND 10,000 IU D3 before 10AM (Both contribute; I've not gone about 10,000 IU, but 5,000 UI has a much lesser effect)

I have not find a peer reviewed time-of-taking-D3 paper when I looked, but D3 is recognized as a timing mechanism ("Zeitgebber") in many papers I've found.

[nicoburns]

Maybe you're not actually deficient, even though in theory you could be. I used to get regular seasonal depression, to the point that I'd struggle to get out of bed at times. And I'd crave sunshine. Of ot was sunny. I'd obviously tried a lot of things. Changing diet, exercising, and also SAD lights which helped a tiny bit, but not really. Regular supplementation with vitamin D has made this almost entirely go away. I'm a completely different person.

So while you not everyone who is feeling tired or depressed will have a vitamin D deficiency, I'd say that it absolutely can cause those symtoms.