Your eyes are getting more sensitive to light. Here's the biological reason.

COMT is an enzyme that breaks down dopamine and norepinephrine in your prefrontal cortex. This is your brain’s primary mechanism for controlling how reactive you are to sensory input. When COMT works properly, dopamine clears at the right pace, and your sensory threshold stays steady. Too much dopamine hanging around and everything feels too intense, too loud, too bright.

The Val158Met variant is carried by roughly 25% of people of European ancestry in the homozygous slow form. People with the slow variant have dopamine that clears more slowly from their prefrontal cortex. This means your sensory neurons stay in a heightened state of reactivity, and light registers as much more intense than it does for people with fast COMT. The effect compounds over time as your nervous system becomes sensitized.

For you, this means that sensory input that seems mild to others feels overwhelming. Bright light doesn’t just feel bright, it feels aggressive. You start avoiding situations with bright light. Your sensory sensitivity to light, sound, and smell often increases together. You may notice you’re also more emotionally reactive to stress.

SLC6A4 encodes the serotonin transporter, the protein that recycles serotonin back into nerve cells after it’s been released. Serotonin is your brain’s primary brake on sensory overload. When serotonin recycling is working well, your sensory gates close down appropriately, and you can tolerate stimuli without becoming overwhelmed. When the brake is weak, everything feels too intense.

The 5-HTTLPR short allele is carried by roughly 40% of the population in at least one copy. People with the short variant have reduced serotonin transporter expression, meaning less efficient serotonin recycling. This leaves serotonin depleted in your synapses, and your amygdala and sensory cortex become hyperreactive to stimuli. Light, sound, and visual complexity all feel more intense. Over weeks and months, this creates a sensitization effect: your nervous system learns to be more reactive.

You experience this as progressive sensory sensitivity. Bright light that was tolerable three months ago now causes you to squint and feel irritated. You’re also likely more emotionally reactive, especially to stressful social situations. You may notice you’re more prone to overstimulation in general.

MTHFR is an enzyme that converts dietary folate into methylfolate, the active form your cells use for methylation reactions throughout the body. One of the most important of these reactions is the synthesis of tetrahydrofolate (BH4), which is essential for nitric oxide production. Nitric oxide controls blood vessel dilation and vascular function throughout your retina. When MTHFR works well, your photoreceptors get steady, rich blood supply.

The C677T variant is present in roughly 40% of people of European ancestry. This mutation reduces MTHFR enzyme activity by 40-70%. Reduced MTHFR function means lower BH4 production, which impairs nitric oxide synthesis, which reduces blood flow to your photoreceptors. Photoreceptors are metabolically greedy cells; they demand constant oxygen and nutrients. When blood flow drops, they become hypoxic and stressed, and they become more reactive to light. Meanwhile, toxic homocysteine accumulates, which amplifies oxidative stress.

You experience this as progressively worsening light sensitivity that develops over weeks to months. Your eyes feel strained even at moderate light levels. You may also notice brain fog, fatigue, or a sense that your eyes are working harder than they should be. If you’ve been deficient in B vitamins, this becomes much worse.

BDNF (brain-derived neurotrophic factor) is a protein that supports the survival of existing neurons and encourages growth of new neurons and synapses. It’s essential for neuroplasticity, which is your brain’s ability to rewire itself in response to experience. BDNF is also a key regulator of stress resilience. When BDNF is working well, your nervous system adapts quickly to stress and recovers when stress ends. When BDNF is reduced, your nervous system stays sensitized longer.

The Val66Met variant is carried by roughly 30% of the population in at least one copy. People with the Met allele have reduced BDNF activity, especially under stress. This impairs your nervous system’s ability to adapt to sensory overload and recover after stressful situations, meaning light sensitivity does not improve on its own and may progressively worsen. Your sensory systems stay in a heightened state. Once light sensitivity develops, your brain has less ability to habituate or downregulate the response.

For you, this means that the progressive worsening of light sensitivity you’ve experienced is not just a symptom; it’s a sign that your nervous system is becoming increasingly sensitized without adequate recovery. You may also notice that stress, poor sleep, or intense sensory days lead to worse light sensitivity for the next several days. Your eyes feel fragile.

VDR is the vitamin D receptor, a protein that sits on cell membranes and in the nucleus and allows your cells to respond to vitamin D. Once vitamin D binds to VDR, it triggers calcium signaling cascades throughout your body, including in your retina. Calcium signaling in photoreceptors is essential for normal light detection and for managing oxidative stress. When VDR function is normal, your photoreceptors can maintain proper calcium balance and respond appropriately to light.

The BsmI and FokI polymorphisms together affect VDR expression. Roughly 30-50% of people carry variants that reduce VDR responsiveness to vitamin D. This means your photoreceptors cannot efficiently activate vitamin D signaling, even if your blood vitamin D levels are adequate, leaving your retinal cells undersupported and more vulnerable to light-induced oxidative stress. Your retina becomes progressively more reactive to light because your cells cannot mount an adequate protective response.

You experience this as light sensitivity that worsens despite getting adequate sun exposure or maintaining normal vitamin D blood levels. Your eyes feel photosensitive even in dim indoor lighting. You may also notice that standard vitamin D supplementation doesn’t seem to help, or that you feel worse in bright conditions regardless of season.

SOD2 is superoxide dismutase 2, an antioxidant enzyme that sits inside mitochondria and neutralizes superoxide, one of the most damaging reactive oxygen species. Photoreceptors are packed with mitochondria because they’re constantly converting light energy into electrical signals. Every photon that hits your retina creates free radicals. SOD2 is your first line of defense. When SOD2 is robust, your photoreceptors can handle this oxidative load. When SOD2 is weak, free radicals accumulate and damage the cell.

The Val16Ala variant is present in roughly 40% of the population in the homozygous variant form. The Ala16 allele results in lower SOD2 activity in mitochondria. This means your photoreceptors are less able to neutralize the oxidative stress from light exposure, so they accumulate damage and become progressively more reactive and inflamed. Over weeks and months, this sensitization becomes a self-reinforcing cycle: damaged photoreceptors produce more reactive oxygen, which causes more damage, which causes more light sensitivity.

You experience this as progressive light sensitivity that worsens with time and with exposure to bright light. Your eyes feel tired and strained after even moderate light exposure. You may notice that your light sensitivity is worse on days when you’ve had more screen time or sun exposure. Your eyes may feel irritated or inflamed, even though they look normal to an eye doctor.