You don’t have to be racist to notice that dwarves spend a lot of time deep underground where the sun don’t shine, so would derive no benefit from heavy melatonin.
On the other hand, I seem to remember something from the final battle in The Return of the King, where Tolkien mentions the Haradrim of the Southlands, dark and proud.
The melatonin of dwarves? What a weird thing to obsess on while watching a show about magic and dragons.