The debilitating headaches can come with nausea, shortness of breath, anxiety, and more—and new research shows nerve cells in the eye may be to blame.

For people who get migraines, bright lights can often make the debilitating headaches even worse. And it’s not just pain that makes them so miserable: Many sufferers also report other reactions—both physical and emotional—when exposed to the sun or sources of light. In a new study, researchers detail these reactions for the first time, and offer a new explanation for how they occur.

Writing in Proceedings of the National Academy of Sciences, the study authors describe a previously unknown connection by which light-sensitive nerve cells in the eye transmit information to brain regions involved in mood and involuntary processes such as heart rate, breathing, and fatigue. This explains why people with migraines tend to feel more uncomfortable in the light, say the authors, even if their actual pain level remains unchanged.

For the study, researchers at Beth Israel Deaconess Medical Center showed different colored lights to 81 people who frequently experienced migraines—once during an attack and once between attacks. They also showed the same lights to 17 people who had never had a migraine.

Among people in the migraine group, lights of all colors triggered unpleasant physical sensations—including tightness in the chest or throat, shortness of breath, light-headedness and nausea—both during and between attacks. And every color except for green elicited intense emotional responses as well, such as anger, nervousness, hopelessness, sadness, depression, anxiety, and fear. "Green light actually gave rise to positive emotions, with patients using words like soothing and calm and relaxed to describe what they felt," says lead author Rami Burstein, PhD, a professor of anesthesia at Harvard Medical School.

People in the non-migraine group, on the other hand, reported no physical symptoms under any of the colored-light conditions—and they reported that all of the colors evoked positive emotions, not negative ones.

The researchers also found, in an experiment on rats, that nerve cells in the retina (the back of the eye where light is detected) send signals to regions of the brain that regulate physiological, autonomic, endocrine, and emotional responses to changes in the external environment.

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It's still unknown why people with and without migraines react so differently to light, say the authors. But they say their findings do show how light can trigger such a wide variety of responses, and that its relationship with migraines is not just about making headache pain worse. Their research also helps explain earlier findings that even blind migraine sufferers can be bothered by light, as long as their vision is affected by damage to light receptors in the eyes and not to the optic nerve itself.

Burstein says this information may lead to treatments to counteract these negative effects. Some promising research involves green light, which Burstein has also found to be the only color of light that actually decreases migraine pain severity, rather than increase it. Green light appears to activate neurons in the retina and the brain to a lesser extent than blue, red, amber, and white light, he says.

“We are in the process of developing lenses that will be able to filter out all but the green light, as well as light bulbs that will emit just the right wavelength, which will allow migraine patients to function more normally,” he says. While green-tinted bulbs may work well in homes and private offices, he adds, special glasses may be more practical when people have to work or spend time in spaces with other people.