Multiple realities
(covers information from several alternate timelines)
"You're sensitive to light."
"Only compared to a Human from your universe. It's the singular biological difference between our two races.

Photosensitivity was a medical condition in which a lifeform was sensitive to light.

After a symbiotic lifeform captured several members of the Enterprise NX-01 crew in 2152, Doctor Phlox was able to determine that the creature was highly photosensitive, resulting in a security team using EM emitters to weaken the creature. (ENT: "Vox Sola")

The sole biological difference between the otherwise identical prime universe Humans and mirror universe Terrans was that the latter were highly sensitive to bright light. Being Terran, Gabriel Lorca had severe photosensitivity that was exacerbated by the prime universe's different light, and to mask such an ailment, he claimed it was the result of eye damage from destroying the USS Buran during the 2256 war with the Klingons to spare his crew a slow death on Qo'noS. He refused treatment, supposedly to maintain a reminder of the incident but in reality because he knew analysis of his eyes would reveal nothing wrong and thus expose him as an impostor. Ultimately, Lorca's true nature was only discovered when Michael Burnham saw Philippa Georgiou being dazzled by the ISS Charon's super-mycelial reactor. (DIS: "Choose Your Pain", "Vaulting Ambition")

The neural parasites encountered by the USS Enterprise in 2266 were found to be extremely photosensitive. Visible and ultraviolet light were both fatal to the organisms. (TOS: "Operation -- Annihilate!")

Due to having evolved on the dark side of their Tidally locked homeworld Remus, Remans prefer darker enviroments and are easily blinded by lights. (Star Trek Nemesis)

In 2367, Susanna Leijten developed an extreme sensitivity to light during the early stages of her exposure to the Tarchannen III parasite. (TNG: "Identity Crisis")

The Night Aliens were an extremely photosensitive species. Native to the Void region of the Delta Quadrant, the species' physiology evolved to survive in complete darkness. When one of the species was being treated in the sickbay on board the USS Voyager, The Doctor needed to dim the lights in order to accommodate the needs of the alien. (VOY: "Night")

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