I couldn't find a common consensus or Word of God (director or writer interview) that explains why Eleven from Stranger Things get nosebleeds. She gets nosebleeds quite frequently when she uses her powers. Some articles have said it is due to overexertion, but she gets them sometimes when she is just trying to see people from far away (thus no lifting heavy weights). Other articles say it is because her brain isn't made for the telekinesis, and still others say that it is because of her powers source coming from Vecna. Why conclusively does Eleven get nosebleeds when she uses her powers? I would specifically like a definitive or authoritative source, preferably the director or writer.
I can't find an interview where anyone associated with the show specifically calls out why her nose bleeds. However, two relevant pieces of information:
- As @valorum shared from TVTropes, nosebleed is a common indicator of psychic effort in TV and movies. An early use of the trope was 1984's Firestarter, "in which it's used in place of the original book's far-less-visible 'tiny cerebral hemorrhages'."
- Firestarter was one of the influences cited by the creators for the character Eleven, as seen in the image associated with her character on page 17 of the show's original story bible from when the project was titled 'Montauk.'
From these details, I think it's safe to say that her nose bleeds because it is a common trope from sci fi movies of the 80s that Stranger Things uses for its inspiration, as well as being a specific allusion to 1984's 'Firestarter.'