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Why are my eyes brown under sunlight?

Why are my eyes brown under sunlight?

The color of our eyes is determined by melanin, a pigment that is produced in certain cells called melanocytes. Melanin comes in two forms: eumelanin which produces brown/black pigment, and pheomelanin which produces reddish/yellow pigment. The specific ratio and distribution of eumelanin and pheomelanin in the iris of the eye produces the various eye colors we see.

Brown eyes contain a large amount of eumelanin, which helps protect the eyes from ultraviolet (UV) damage. When brown-eyed people are exposed to sunlight, the excess production of melanin caused by UV exposure leads to increased eumelanin pigment in the iris, making the eyes appear even darker brown. This is an evolutionary protective mechanism against the harmful effects of the sun’s rays.

The Role of Melanin in Eye Color

Melanin is produced by melanocytes, specialized cells in the eye’s iris that contain melanosomes – tiny packets of the pigment. The melanosomes are transferred from the melanocytes into other cells called keratinocytes. The specific ratio and arrangement of eumelanin and pheomelanin melanosomes in the iris determines eye color:

– Brown eyes have a high concentration of eumelanin in the iris
– Green/hazel eyes have a moderate amount of eumelanin
– Blue eyes have a low concentration of eumelanin
– Gray eyes have mostly pheomelanin
– Amber/yellow eyes have a small amount of eumelanin and more pheomelanin

The total amount and type of melanin present is genetically determined. However, melanin production can increase in response to sun exposure and other factors, causing the eyes to darken temporarily.

Why Brown Eyes Darken in the Sun

Brown eyes naturally contain a high concentration of the dark brown/black pigment eumelanin in the iris. This pigment helps absorb and scatter light, reducing glare and protecting the eye from sun damage by absorbing UV radiation.

When exposed to UV rays from sunlight, the body ramps up melanin production as a defense mechanism. More melanin is generated and deposited into melanocytes in the iris, increasing the eumelanin levels beyond the regular genetically-determined baseline concentrations.

This excess eumelanin further darkens brown eyes, temporarily making them appear almost black in bright sunlight. The extra melanin helps filter more UV light and shield the interior eye structures from damage.

Other Factors Affecting Temporary Eye Color Darkening

While sun exposure is the most common cause of temporary darkening of brown eyes, other factors can also contribute by stimulating melanin production:

Pregnancy – Hormonal changes and increased pigmentation can darken brown eyes during pregnancy. Melanocyte stimulating hormone is elevated, enhancing melanin synthesis.

Medications – Certain medications used to treat conditions like hypothyroidism and Parkinson’s disease can increase melanin as a side effect.

Inflammation – Inflammation in the eye from conditions like iritis can stimulate pigment production, darkening brown eyes.

Diet – Consuming certain carotenoid antioxidants like beta-carotene from fruits and vegetables may contribute to increased melanin synthesis.

Ethnic origin – Those of African, Hispanic/Latino, or Asian descent tend to have darker brown eyes due to higher baseline melanin levels.

Is the Darkening Permanent?

In most cases, the darkening of brown eyes in the sun is a temporary effect that subsides once UV exposure is reduced. The extra melanin generated in the iris in response to sunlight is eventually broken down and removed.

However, there are a few scenarios in which the color change may become permanent over time:

– Cumulative sun damage over many years can lead to permanent darkening as melanocytes continuously ramp up melanin output. Wearing UV-blocking sunglasses can help prevent this.

– The natural aging process often includes gradual darkening of brown eyes as melanin levels increase with age.

– Certain medications, inflammatory conditions, and pigment disorders may trigger a persistent increase in melanin.

– Physical trauma causing bleeding in the eye can sometimes leave behind permanent melanin deposits.

But for the most part, the sunlight-induced darkening of brown eyes is a reversible adaptive process that protects the eyes from UV damage with a temporary boost in melanin. The effect fades when sunlight exposure is reduced.

Eye Color Melanin Type Melanin Amount
Brown Mostly eumelanin High
Green/Hazel Moderate eumelanin Medium
Blue Low eumelanin Low
Gray Mostly pheomelanin Low
Amber/Yellow Low eumelanin, more pheomelanin Low


In summary, brown eyes contain large amounts of the melanin pigment eumelanin which gives them their distinctive color. When exposed to UV light from the sun, the body ramps up its production of melanin in the iris as a protective mechanism. This excess melanin generated in response to sunlight exposure makes brown eyes appear darker temporarily. The effect is usually reversible once UV exposure is reduced as the excess melanin is broken down. Wearing sunglasses can help prevent permanent darkening over time from chronic sunlight exposure. So the temporary darkening of brown eyes in the sun is an adaptive process to filter extra UV light and shield the delicate interior eye structures from damage.