Alex Chernavsky, who has commented as follows (emphasis added):
For what it’s worth, I’ve taken Vitamin D at different times of the day, and I’ve never noticed any effect on my sleep. Of course, my sleep is already pretty good, in the sense that I fall asleep quickly and don’t usually wake up during the night. (My sleep is not good in the sense that I don’t get enough of it.)
By email, I learned that Alex is now taking Vitamin D3 — this particular product, which is vegan (“plant-source”) — at 5000 IU every other day. On weekdays, he takes it at about 8:00 am, on weekends, 9:30-10:00 am.
What might explain Alex’s failure to notice better sleep?
1. Not enough D3. I found that 2000 IU/day had no noticeable effect, whereas 4000 IU/day did produce noticeable benefit. Alex is getting 2500 IU/day — or less, if he takes it too late on the weekends.
2. His source of D3.
3. Individual differences large enough to matter. If you do sensitive psychology experiments, you will learn there are individual differences in everything.
4. Ceiling effect. Alex’s sleep is too good to notice improvement.
Those are the just the obvious possibilities.