We’ll look at when and why your pupils change size. First, the range of “normal” pupil sizes, or, more accurately, what the average is.

Pupils tend to become bigger (dilate) in low-light situations. This allows more light into the eyes, making it easier to see. When there’s a lot of bright light, your pupils will become smaller (constrict).

A fully dilated pupil is typically in the 4 to 8 millimeters in size, while a constricted pupil is in the 2 to 4 mm range.

According to the American Academy of Ophthalmology, pupils generally range in size from 2 to 8 mm.

Accommodative response

Pupil size also changes based on whether you are looking at something close or far away. When you’re focusing on an object that’s near, your pupils become smaller. When the object is far away, your pupils widen.

The size of your pupils isn’t something you can consciously control. And if you have a dilated pupil, you won’t necessarily feel it (although some people say they feel a tightening in the eye).

Chances are what you’ll notice first are changes in your vision. Dilated pupils tend to be sensitive to bright light, such as sunlight, and can cause blurry vision. If you’ve ever had your pupils dilated with drops during a visit to the eye doctor, you know the feeling.

Pupils are the black center of the eye. Their function is to let in light and focus it on the retina (the nerve cells at the back of the eye) so you can see. Muscles located in your iris (the colored part of your eye) control each pupil.

While your two pupils will usually be roughly the same size, pupil size overall can fluctuate. Factors that cause your pupils to become bigger or smaller are light (or the lack of it), certain medications and disease, and even how mentally interesting or taxing you find something.

A variety of factors can influence pupil size, and not all of them have to do with light and distance. Some of these other factors include:

  • your health
  • medicines and drugs
  • your emotions

Concussion

A concussion is a brain injury that results from the brain smacking against the hard skull during a fall, a hit to the head, or a fast impact involving the whole body. One symptom is bigger-than-normal pupils. In some cases, one pupil may be bigger and the other smaller (asymmetrical).

Anisocoria

Anisocoria is a condition in which one pupil is wider than other. While it can be a natural occurrence, affecting about 20 percent of people, it can also signal a nerve problem or infection.

Cluster headache

This is an intensely painful headache that usually affects one side of the face, directly behind the eye. As the name implies, it occurs in clusters (sometimes as many as eight headaches a day), and can then disappear for weeks or months at a time.

Because this type of headache affects nerves in the face, the pupil on the affected side can become abnormally small (called miosis) during the headaches.

Iritis

This is an inflammation of the iris of the eye that can be caused by infection, trauma, and autoimmune diseases (diseases in which your body attacks its own immune system).

Since the iris controls the pupil, it’s not common to see abnormally shaped pupils in cases of iritis. According to research in the , the pupil is typically smaller than normal.

Horner’s syndrome

Horner’s syndrome is a condition that occurs when nerve pathways that run from the brain to the face become injured. That injury can cause pupils to become smaller. Some causes include:

  • stroke
  • trauma
  • tumors
  • certain cancers

Horner’s syndrome can also occur if you’ve had an injury to the carotid arteries (blood vessels in the neck that carry blood and oxygen to the face and brain) or the jugular vein (vein in the neck that carries blood from the brain and face back to the heart).

Certain drugs can dilate pupils while others constrict them. Some drugs that affect pupil size include:

  • Anticholinergics. These are drugs commonly used to treat things like an overactive bladder, Parkinson’s disease, diarrhea or stomach cramps. According to the Kellogg Eye Center at the University of Michigan, they can slightly dilate pupils.
  • Sedatives, including alcohol and antihistamines. In one small 2006 , the antihistamine diphenhydramine caused pupils to become smaller.
  • Opiates. These are powerful drugs used to treat pain. Both legal opioids (like prescription oxycodone) and illegal (heroin) can constrict pupils.

Parts of the brain that help us feel and decode emotion as well as mentally focus can make pupils widen.

  • One small 2003 study showed that when people listened to emotionally charged sounds (a baby laughing or crying) versus sounds that were considered neutral (routine office noise), their pupils became bigger.
  • When you look at others with dilated pupils, your pupils tend to dilate as well. This is called “” and is most likely to occur when you look at someone you trust or who’s known to you.
  • Researchers have found that when we have to think very hard because a task is difficult or new to us, our pupils dilate — and the harder the task, the more they dilate.

Visit your doctor if you notice changes in your pupil size that are unrelated to light and viewing distance or if you’re having any changes or problems with your vision.

How often you get your vision checked depends on your age and certain health factors. But overall, most adults should have their vision checked every couple of years.

Most people have pupils that are only a couple of millimeters wide and symmetrical (meaning both eyes have the same size pupil). A small subset, however, naturally have one pupil that’s bigger than the other. But pupils aren’t static.

Under certain conditions — including those that are environmental, psychological, and medical — it’s completely normal for your pupils to change size, getting either smaller or bigger depending on the circumstance. You need healthy pupils to see properly.