Glaring light is the answer.
This is why your natural reaction is to squint when you spend a long time in a dark space and then quickly enter a bright one. To prevent eye injury, the eye automatically responds by limiting the quantity of light that enters the eye. As a result, the pupil gets smaller and the iris has time to react. Your eye adjusts to the intense light and the squinting disappears when the iris relaxes and shrinks the pupil.
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