Scientists have solved the mystery of why migraine sufferers shun the light.
Light intensifies migraine headaches because of a particular group of retina cells at the back of the eye, research has shown.
The photoreceptors send signals to the brain via the optic nerve which stimulate migraine pain neurons.
Even small amounts of light are enough to affect the nerve pathway, sending victims running for the shadows.
Migraine is a one-sided, throbbing headache associated with symptoms that can include nausea, vomiting and fatigue.
The pain is believed to develop when the meninges -- the system of membranes that surround the brain -- becomes irritated.
Nearly 85pc of migraine sufferers are highly sensitive to light but until now no-one understood why.
Scientists, whose findings are published on-line today in the journal Nature Neuroscience, suspected that a group of recently discovered retinal cells which play a role in the body's night and day cycle were involved.
They contain melanopsin photoreceptors, which help control biological functions such as sleep and wakefulness.
Laboratory animal tests confirmed the theory, revealing a direct link between melanopsin retinal cells and brain neurons that become electrically active during migraine attacks.
"When small electrodes were inserted into these 'migraine neurons', we discovered that light was triggering a flow of electrical signals that was converging on these very cells," said Prof Rami Burstein, of Boston, US. "This increased their activity within seconds."
The neurons remained activated for a time even after the light was taken away, explaining why migraine sufferers might have to hide in the dark for 20 to 30 minutes before their headaches improve.
"Clinically, this research sets the stage for identifying ways to block the pathway so that migraine patients can endure light without pain," said Prof Burstein.