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First off, let me say you should see a doctor just to be safe.
But, my mom has that problem with one of her eyes- when her eye gets "tired", the pupil dialates to a very large size and remains that way- has nothing to do with light and dark- it doesn't change size.
She went to the doc and then on to a neurologist who ran multiple tests and could not find anything wrong.
However, I did find this on the net:
The pupil is nothing more than the opening created by a group of muscles in the eye. When light is shone into the eye, the pupil size becomes smaller and vice versa when light is taken away. That is the normal process. Some people naturally have a large pupil. If you are taking any type of medication or have allergies, this may be another reason that the pupils are larger. Forms of adrenalin in medication or the adrenalin released in your body's response to stress, may cause an enlarged pupil appearance. Too much caffeine, sugar, or any stimulant, are other causes, while a deficiency in B vitamins, particularly choline, is also a possibility. It's always a good idea to have your eyes checked for pathologies by a professional. Until then, keep your sunglasses handy. A larger pupil makes the eye slightly more susceptible to patholgies due to ultraviolet rays. Take notice of any of your blood related family members' pupils. This could just be a genetic trait.
interesting?