Suddenly swollen eyes mainly refer to swelling of the subcutaneous tissues of the eyelids. The subcutaneous tissue of the eyelid is loosely structured and various factors that cause edema of the subcutaneous tissue can cause a sudden swelling of the eye. There are different reasons for sudden swelling of the eyes. One may be due to a lack of sleep, excessive fatigue, slow blood circulation, stagnation of venous blood, and fluid leakage from the blood vessels causing edema of the eyelid tissue, resulting in sudden swelling of the eyes. Second, it may be due to contact with certain substances that cause allergy to the eyelid skin and edema of the tissues, resulting in sudden swelling of the eyes. Third, it may be due to external forces, such as trauma or eye rubbing, which can cause eyelid tissue damage and edema, resulting in sudden eye swelling. Fourth, it could be inflammation of the eyelid area caused by various factors, such as viral or bacterial dermatitis, blepharitis, or eczema of the eyelid skin, which can cause sudden swelling of the eye and may usually be accompanied by red, itchy, painful skin. Fifth, it may be caused by various pathogenic factors such as conjunctivitis and keratitis, which can also cause sudden swelling of the eye and may be accompanied by eye symptoms such as redness and pain in the eye in addition to eyelid symptoms. Sixth, it may be a foreign body in the conjunctiva or cornea that causes a local inflammatory reaction, resulting in a sudden swelling of the eye. To sum up, there are more reasons for the sudden swelling of the eyes, and it is recommended to go to the hospital for further examination of what is going on.