I read a warning on my amytriptaline to avoid prolonged exposure to direct and artificial sunlight. why?

Photosensitivity : Photosensitivity is a side effect commonly of amitriptyline. This is a sensitivity to light and you may sunburn easily. A rare side effect is hyperthermia and heat stroke. Amitriptyline boost norepinephrine levels which are involved in body temperature regulation. I would just wear sunscreen and stay hydrated if you have to be outside for long periods in a shady spot.