Vitamin D is found in milk and warm milk has been known to help folks get to sleep.
I don't think Vitamin D has anything to do with why you are getting more sleep.
Vitamin D is good for you bones and it is good for reducing depression.
I know that Magnesium is very good for making you less anxious and feeling nervous it also helps to relax your muscles and makes you sleep easier during the night.
And there are others
I took high doses of vitamin D for awhile...didn't improve my sleep any. Wish it did.