Pupils are the black-colored circles in your eyes that allow light to enter. The pupil is surrounded by the colored part of the eye called the iris. The iris is responsible for adjusting the diameter of the pupil to let more or less light in. The pupil is round in shape and appears black because the light striking it's absorbed into deeper parts of the eye.
The size of your pupil keeps changing throughout the day according to the lighting conditions around you. If you're in a bright environment, the pupil will shrink in size to allow less light to enter the eye. In dark surroundings, the pupil expands to let more light in.
Your pupil's size can also change depending on whether you're looking at nearby or far-away objects. If the pupil doesn't vary in size in response to changes in lighting and gaze, there might be something wrong.
In an adult, the pupil's diameter usually varies between 2 and 4 millimeters in bright light and between 4 and 8 millimeters in dark.
The maximum pupil size also varies significantly among different age groups. For example, the pupil is the widest around age 15, after which it begins narrowing in an inconsistent fashion after age 25.
Why Does Pupil Size Change?
Pupil size can change dramatically in response to not only light but also your emotions, periods of intense concentration, recently eaten foods, prescription and recreational drugs, and underlying diseases or disorders. Past physical injuries to the eye or the head can also have a long-lasting impact on the average pupil size.
Response to light. Variations in lighting conditions are the most common reason for a change in pupil size. The pupils in both eyes respond independently to bright or dim light, so it's possible for one pupil to expand or contract while the other remains stable. Pupils also make small adjustments in size to help you focus better on a close or distant object.
Emotions. Processing or experiencing emotions leads to changing pupil sizes too. The pupils slightly expand or dilate every time you feel excited or nervous. Seeing strong emotional cues like a person laughing or crying could also cause differences in pupil size. In general, pupils tend to grow wider if you vividly feel a positive or negative emotion. The pupils can also widen if you're concentrating hard on a particular task like solving a math problem or retrieving a memory.
Medications and drugs. Both prescription and over-the-counter drugs can cause your pupils to expand or contract. Your ophthalmologist or eye doctor might give you eye drops that widen or dilate your pupils. This allows medical professionals to take a better look at your inner eye. Illegal or recreational drugs can also produce noticeable changes in pupil size. The most commonly abused drugs that affect pupil size include cocaine, LSD, MDMA, heroin, methamphetamines, and ketamine.
Health Problems That Can Affect the Pupil
Flaws in the pupil could be the result of a disorder of the eye or the pathway connecting the brain and the eye. It's also possible for the pupil in only one eye to be affected. Symptoms of an abnormality in pupil function include headache, light sensitivity, double vision, and drooping eyelids. Here are some of the most common pupil disorders.
Anisocoria. Essential or functional anisocoria simply means you have unequal pupil sizes in the two eyes. About 20% of healthy people have some degree of anisocoria. It's usually harmless and doesn't require treatment unless it interferes with your normal vision.
Horner’s syndrome. Horner's syndrome is a rare disease caused when the nerve pathway from your brain to one side of your face is disturbed and the eye on that side of your face is affected. This problem may happen after a stroke and will often be accompanied by difficulties in balance, swallowing, and hearing.
Adie’s tonic pupil. Adie’s tonic pupil is a rare neurological disorder and usually only affects one eye. In this condition, your pupil stays expanded and shows very slight changes in size if any. It also pupil doesn't respond to changes in the light or other stimulants.
Third cranial nerve palsy. A complete third eye palsy results in one eyelid being completely closed, with the eye bulging outward and downward. The condition can exist in varying degrees and is generally seen in only one eye.
Argyll Robertson — or AR — pupil. AR pupils are small, and they don't contract when exposed to bright light. But they show a normal change in size when focusing on nearby objects. AR pupils are extremely uncommon and could be a sign of nerve damage due to syphilis or diabetes.