The princess in Hans Christian Andersen’s The Princess and the Pea might have had fibromyalgia syndrome, according to two researchers in Stockholm. The chronic sleep disturbance and musculoskeletal pain common in FMS patients aligns with the restless nights that the fairy tale attributes to royal hypersensitivity. A princess is so delicate, the story goes, that even a little nub under a mountain of eiderdown would keep her awake. But the Swedish researchers say she wasn’t just a spoiled royal, she was suffering.

Ours is the era of medical reasoning. Academic and neuroscientific publications have recently turned their gaze to the creative geniuses of the past, attempting to explain (or explain away) their rare insights. You might have heard that Nietzsche had stomach problems—irrit...