The color of a person’s eyes is not always static. While eye color typically stabilizes after early childhood, various factors can lead to apparent or actual shifts in hue throughout a person’s life. These changes involve the underlying biology of eye color, how external conditions influence perception, and the role of developmental processes, medical conditions, and certain medications.
The Science Behind Eye Color
Eye color primarily results from the amount and type of melanin within the iris. Specialized cells called melanocytes produce melanin, which comes in two main types: brown-black eumelanin and red-yellow pheomelanin. The concentration and distribution of these pigments determine whether eyes appear brown, blue, green, or other shades.
Brown eyes contain the highest melanin concentration, absorbing most light. Conversely, blue eyes have very low melanin in the front layer of the iris; their blue appearance is due to light scattering by collagen fibers in the iris’s stroma. Green and hazel eyes have intermediate melanin levels. Green eyes also contain a yellowish pigment called lipochrome, and hazel eyes feature a mix of melanin that can reflect both brown and green tones.
How Perception and Environment Influence Eye Appearance
Many perceived changes in eye color are not actual alterations in iris pigment but optical illusions or temporary physiological responses. Surrounding lighting conditions, whether natural or artificial, can significantly affect how eye color is perceived, making the iris appear to shift in shade.
Reflections from clothing, makeup, or the immediate environment can also influence the apparent hue of the eyes. Wearing certain colors might enhance or diminish the contrast with the iris, creating the impression of a color change. Additionally, physiological responses like pupil dilation can alter how the iris appears. When the pupil widens, less of the iris is visible, which can make the eye appear darker, while a constricted pupil might make the eye seem lighter.
Developmental and Age-Related Eye Color Shifts
Biological changes in eye color can occur, particularly during early development. Many babies are born with lighter eye colors, often blue or gray, because their melanocytes have not yet produced a full amount of melanin. Over the first few months or years of life, as melanocytes continue to produce melanin, the eyes can gradually darken, commonly transitioning to green, hazel, or brown. This process typically stabilizes by three years of age.
Subtle changes can also happen in adulthood, though dramatic shifts are rare. Prolonged exposure to sunlight can lead to a slight darkening of the iris over time due to increased melanin production. Conversely, some individuals, particularly those with lighter eyes, may experience a slight lightening as they age, possibly due to pigment degradation.
Medical Conditions and Medications That Affect Eye Color
Permanent changes in eye color can indicate underlying medical conditions or medication side effects. Heterochromia, where a person has different colored irises or multiple colors within one iris, can be present from birth (congenital) or acquired later in life. Acquired heterochromia may result from various factors, including eye injury or inflammation.
Conditions such as Horner’s syndrome, a rare neurological disorder, can cause one eye to become lighter. Fuchs’ heterochromic iridocyclitis, an inflammatory condition, can lead to a loss of pigment in the iris, resulting in a color change. Pigmentary glaucoma occurs when pigment flakes off the iris, which can lighten parts of the iris. Uveitis, an eye inflammation, can also cause noticeable changes in eye color if it affects the iris.
Certain medications, particularly prostaglandin analogs used to treat glaucoma, are known to cause a gradual darkening of the iris. Latanoprost, for example, can cause increased pigmentation, especially in mixed-color irises like green-brown or blue/gray-brown. This darkening is usually permanent. Any sudden or unexplained change in eye color warrants a consultation with an eye care professional, as it could signal a health issue.