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Why do hazel eyes change color in the sun?

Hazel eyes are one of the most unique and fascinating eye colors. Unlike blue, brown, or green eyes that tend to stay the same shade, hazel eyes can appear to change color depending on the lighting conditions. This is especially noticeable in natural sunlight, where hazel eyes can shift between various shades of green, brown, and gold. In this article, we’ll explore why hazel eyes appear to change color in the sun and other bright lighting. We’ll look at the structural and pigmentation factors that cause this optical phenomenon and examine some common theories that have been proposed over the years. Whether you have hazel eyes yourself or are just curious about them, read on to uncover the interesting science behind these mystical, shape-shifting peepers!

What Are Hazel Eyes?

Hazel eyes are defined as eyes that contain a mixture of melanin pigments – specifically a combination of brown and amber. This results in an eye color that has no definite boundary and can range from light brown to dark golden-greenish hues.

In hazel eyes, the concentration and distribution of melanin isn’t uniform as compared to other eye colors. Areas of the iris contain more melanin while other areas contain less. This mosaic pattern and blending of brown and gold is what sets hazel eyes apart.

It’s estimated that about 5-10% of the world’s population have hazel eyes. They are most prevalent in people of Celtic, Arabic, Brazilian, and multi-ethnic descent.

The Structure of the Human Eye

Before we look at why hazel eyes can appear to shift color, it helps to understand some basics about the anatomy and function of the human eye:

The iris – This is the colored part of the eye and it functions like the aperture of a camera, controlling the amount of light that enters the eye by adjusting the size of the pupil. The color of the iris comes from the amount and type of melanin pigment present.

The pupil – This is the black circular opening in the center of the iris which allows light to enter the inner eye. It constricts and dilates to alter how much light passes to the back of the eye.

The sclera – Also known as the “whites” of the eyes, the sclera is the opaque, fibrous outer layer of the eye containing collagen and elastic fibers.

The retina – This inner light-sensitive layer of tissue contains photoreceptor cells called rods and cones. These convert the light into neural signals that are sent to the brain which enables us to see.

Melanin pigment – This is the primary pigment responsible for eye color. It is produced by melanocytes within the iris. Higher amounts of melanin in the iris results in darker colors (brown/black) while lower amounts cause lighter colors like blue and green eyes.

Melanin Concentration in Hazel Eyes

As mentioned earlier, hazel eyes contain a combination of brown melanin (eumelanin) and amber/yellow melanin (pheomelanin). The concentrations and spatial distribution of these melanins are not uniform which is why hazel eyes can display such a wide array of colors and mixtures.

Some key things to note about melanin patterns in hazel eyes:

– Compared to brown eyes, hazel eyes have less total melanin content overall. But they have more melanin than blue or green eyes.

– The melanin concentration can vary significantly within different areas of the iris, with some patches containing more eumelanin while other lack melanin.

– The outer border of the iris tends to have more melanin than the inner area around the pupil.

– Eumelanin is concentrated mainly around the outer rim while pheomelanin is dispersed throughout the inner region.

– The area directly surrounding the pupil often contains the least melanin, allowing the amber and gold hues to show through.

How Lighting Conditions Affect Appearance

The way hazel eyes can shift in color and brightness largely comes down to how light interacts with the unique melanin patterns. Different lighting conditions can make the various shades and hues more or less prominent. Here’s a look at how external light impacts what we perceive:

Dim and dark conditions – In low light, the pupil dilates significantly to allow more light in. This makes the darker brown/black melanin around the outer iris more visible and dominant. The hazel eyes will appear dark brown or black.

Indoor ambient lighting – With the pupils moderately dilated under indoor room lighting, there is interplay between the brown and amber hues. This brings out the classic hazel pattern and multi-colored appearance.

Bright outdoor light – Intense natural lighting causes the pupils to constrict. This makes the concentration of melanin around the outer rim less visible. Under these conditions, the golden-green pheomelanin hues shine through and hazel eyes look more green-yellow.

Sunlight and UV rays – Sunlight contains higher concentrations of blue/UV light compared to artificial light. These shorter wavelengths lead to more Rayleigh scattering in the thinner inner iris. This makes hazel eyes look even lighter and brighter when in direct sunlight.

Theories on Why Sunlight Changes Hazel Eye Color

With this understanding of melanin patterns and lighting effects, the ability of hazel eyes to shift color is no longer mysterious. Still, over the years several interesting theories have been put forward to explain why sunlight in particular seems to cause hazel eyes to lighten dramatically:

Chromatic aberration theory – Isaac Newton first noted that the shorter wavelengths of sunlight are preferentially focused in a slightly different place than longer wavelengths when passing through the lens of the eye. This chromatic aberration leads blue light to be concentrated in the center area of the iris, essentially illuminating the lighter area around the pupil.

UV-stimulated fluorescence theory – The shorter UV wavelengths present in strong sunlight could stimulate re-emission of light by certain substances in the iris, making the eyes appear brighter. This fluorescent glow effect is likely very minor though.

Pupillary response theory – Hazel eyes may contain low levels of cryptochromes – photoreceptor proteins found in plants and some animals. These are sensitive to blue light and could trigger pupillary constriction in response to sunlight, revealing more of the inner iris.

Dispersion of light theory – Passing through the fibrous matrix of the iris could cause dispersion where sunlight separates into its composite wavelengths. This would concentrate shorter blue light around the pupil, making hazel eyes seem brighter and more golden.

While these theories may have some merit, the predominant scientific view is that simple melanin patterns combined with pupillary constriction in sunlight is enough to account for hazel eyes changing color.

Other Factors Influencing Eye Color

Although melanin content and lighting conditions have the biggest impact on eye color, there are some other factors that can subtly affect the appearance of hazel eyes:

Age – Melanin concentration and composition slowly changes over the years. Hazel eyes typically darken somewhat with age as the ratio of eumelanin increases.

Mood – Emotions can lead to slight dilating or constricting of the pupil which affects iris visibility. Stress might make hazel eyes look darker for example.

Clothing and background colors – Contrasting colors around the eyes can make them seem lighter or darker. Green clothing tends to enhance the green hues in hazel eyes for instance.

Alcohol consumption – Acts as a pupillary dilator. This can lead to hazel eyes appearing darker for several hours after drinking.

Medications – Various prescription eye drops and neurological medications can lead to subtle temporary changes in melanin distribution and pupil size.


Hazel eyes are prized for their dazzling ability to shift color seemingly magically in different lighting. As we’ve learned however, there are sound scientific reasons why hazel eyes change from shades of brown to green to gold when transitioning from indoors to bright sunlight. The unique blend of melanin pigments within the iris, and how their concentration varies across regions, essentially creates an optical illusion. The pupils serve as a movable window that reveals the different melanin patterns under varying conditions. Sunlight simply provides the best contrast that lets us appreciate the full chameleon-like beauty of hazel eyes. So the next time your hazel-eyed friend complains about their eyes changing too dramatically outdoors, assure them that it’s perfectly normal and just a result of the exquisite iris biology they’ve been blessed with!

Eye Color Total Melanin Content Melanin Types Present Lighting Conditions When Most Apparent
Blue Low Pheomelanin traces Bright sunlight
Green Low-moderate Low eumelanin Indoor and natural light
Hazel Moderate Mix of eumelanin and pheomelanin Bright sunlight
Brown High Mainly eumelanin Dim light

The Takeaway on Hazel Eyes

– Hazel eyes have a unique blend of brown and amber melanin pigments that are not evenly distributed across the iris
– Higher melanin concentration around the outer part makes the eye appear darker in dim light when the pupil is dilated
– With the pupil constricted in bright sunlight, the areas with less melanin around the pupil appear lighter and more golden
– This melanin mosaic effect combined with changes in pupil size makes hazel eyes seem to shift dramatically between various colors
– Direct sunlight provides the best contrast for seeing the full spectrum of hues and patterns in hazel irises

So in summary, hazel eyes are not actually changing color per se, but rather we are seeing different parts of the iris better under different lighting. Their visual magic is down to the special interaction between the relative levels and distribution of melanins combined with optical effects of pupil size. Appreciate and embrace the chameleon-like beauty of your hazel eyes!