- Completely Different Title: The French title is "Dans les Bras de Morphée" ("In Morpheus's Arms" after Morpheus, the god of dreams in Classical Mythology).
- Production Posse: Mark Gatiss' The League of Gentlemen collaborator Reece Shearsmith guest stars as Rassmussen.
- What Could Have Been: Showrunner Steven Moffat commissioned a follow-up story from Mark Gatiss before this one even aired, but it was ultimately replaced by "Empress of Mars" when Series 10 was actually produced.
- You Look Familiar:
Trivia / Doctor Who S35 E9 "Sleep No More"