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Apparently D3 inhibits production of melatonin. Can't remember where I read this so not a hugely useful comment but I promise I didn't just make it up! Anecodtally, I've also experienced similar issues taking 1000 IU D3 before bed and having trouble sleeping. I moved the dose to the morning and Boom!, good sleep.

Edit: Here is the study. https://pubmed.ncbi.nlm.nih.gov/23665342/ - only a single study but still interesting

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I take 2,000 IU of Vitamin D3 nightly right before bed and have no trouble sleeping.

The difference is probably the people who fall asleep before it kicks in vs. those who normally take 15+ minutes to fall asleep and then can't.

Damn, 15 minutes to fall asleep must be godly!

That's a good point. There's also a bunch of other confounding factors which affect sleep. E.g. circadian rhythm, variance of sleep/wake times, how physically tired you are, how much recent REM/deep and whether there is a deficit, how stressed you are, screen usage before bed, eating close to bed time, exercise close to bedtime, etc... Maybe the Vitamin D3 taken close to bedtime is enough to tip some people over the edge to some kind of insomnia, given the presence of other melatonin inhibitors.

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