I checked the ingredients. That is because it contains glycerin. Which is a great and safe supplement to take for anyone with sleeping issues. But will cause very vivid dreams at the start. D3 will not by itself have a huge effect on dreams.
This is such a weird fact that I googled it and sure enough it is widely noted!
my bottle of Nature Made D3 also contains glycerin
> Glycerin
Do you mean glycine? I've not heard of glycerine having a positive effect on sleep, although glycine is often recommended.
Glycine is an amino, glycerin is a triol.
When I tried to search specifically for "glycerin" and sleep I just get a couple of reddit threads, but no real sources.