context: The black circle in the center of your eye is your pupil. It changes size thousands of times a day. When you're in dim light, it gets bigger to let more light in. When you're in bright light, it shrinks to protect your eye and keep light out. When your pupil shrinks (constricts), it's called miosis. If your pupils stay small even in dim light, it can be a sign that things in your eye aren't working the way they should. This is called abnormal miosis, and it can happen in one or both of your eyes. Age: It's normal for a newborn's pupils to stay small for about 2 weeks so her eyes have extra protection from bright light. Your pupils tend to get smaller as you get older, too. The muscles that work your pupils can get weak and have a tough time opening them. This can make it harder for you to see at night. Inflammation : Swelling inside your eye can make it hard for your pupils to get bigger. Sometimes this happens if you've injured your eye. It may also be because of a condition called uveitis, which is swelling in your iris -- the part that gives your eye its color -- and the tissues around it. Side effect of a medication: Certain anxiety, muscle spasm, and seizure medications like diazepam ( Valium) or antihistamines like diphenhydramine ( Benadryl) can make your pupils shrink. So can narcotics, either prescribed or illicit. Genes: Being born without the muscle that controls your pupils or with pupil muscles that aren't formed correctly is called congenital miosis or microcoria. You get it when one or both of your parents pass down a problem gene to you. It can happen in one eye or both eyes. If you have it, you may also be nearsighted and have trouble seeing things far away. Or you may have glaucoma, which means there's too much pressure inside your eyeball. Horner's syndrome: This rare condition affects the way your brain "talks" to one side of your face, including one of your eyes. It can make one of your pupils smaller than the other. You can inherit it from your parents, or it can happen after a neck injury or neck surgery. You can also get it if your chest, neck, or brain doesn't form correctly. Sometimes kids get it if they have a rare type of cancer called neuroblastoma or a tumor in another part of their body. Horner's syndrome may cause no other symptoms, or you could have issues like: Droopy upper eyelid (ptosis) Raised lower eyelid Lighter eye color in the eye with miosis (heterochromia) Less sweat on the side of your face with the miotic eye Some other causes of miosis include: Neurosyphilis (a bacterial infection in your brain that comes from untreated syphilis, a sexually transmitted disease) Severe lack of vitamin D To find out if you have abnormal miosis, your doctor will take a close look at your eyes in a dark room. She'll ask you to look at a faraway object. Then she'll check: The size and shape of your pupils The size of your eyelid opening Whether your pupils are equal in size The position of your pupils How your pupils react to bright light Normal pupils are 2 to 4 millimeters in bright light and 4 to 8 millimeters in the dark. Your doctor can measure your pupils in both eyes to see how well they shrink and grow. Sometimes, your doctor may use drops that are supposed to make your pupils big to see how yours react. Or she may order images of your chest, brain, or neck to rule out signs of Horner's syndrome. Your doctor's recommendations will depend on what's causing your abnormal miosis. If a medication is to blame, she may be able to find a different option that solves the issue. If your pupils are small because of inflammation in your eye, she can give you long-lasting dilating drops (atropine or homatropine) that make your pupils wider. These are a lot like the drops your eye doctor uses to dilate your eyes during an exam, but they can last up to 2 weeks. If Horner's syndrome is causing it, she may need to do several tests to figure out how best to treat it.