Some of us have melatonin-related conditions that mean we have to sleep with the light on. Turning off one's neighbor's light is a selfish act which can make the person feel nauseous and ill for up to 12 hours or more. Perhaps those who do this don't mind having their neighbors vomit all over them..... eyemasks are your friend, but the person with the condition has no easy solution.
One learns something new about illness every day, but I don't believe there is any condition that compels someone to vomit all over a person sitting adjacent to them.
If I had the condition described above, I would certainly explain it to my seatmate, and to the FA to avoid the inadvertent de-illumination of my overhead light. I've seen FAs turn off those lights above sleeping pax many times, but fortunately, I've never seen the subsequent projectile vomiting.