NOEL KING, HOST:
Today is the winter solstice. It's the shortest day of the year, and this is also a time of year when a lot of people feel down or depressed. Now scientists think they have new information on why short days lead to such dark moods. NPR's Jon Hamilton has the story.
JON HAMILTON, BYLINE: About 1 in 5 adults gets the winter blues or a more serious problem called seasonal affective disorder. But scientists haven't been sure whether a lack of daylight is really to blame. Samer Hattar is a researcher at the National Institute of Mental Health.
SAMER HATTAR: People have always wondered if even the effect of light and depression are true.
HAMILTON: Skeptics wanted to know how could light coming in through the eyes mess with brain circuits involved in mood. Hattar says there hasn't been a good answer until now.
HATTAR: It's completely novel. It's completely new, and people have completely missed it.
HAMILTON: The answer involves research that began in the early 2000s when Hattar was at Johns Hopkins University. Back then, most scientists thought that when light struck the retina, only two kinds of cells responded - rods and cones. But Hattar's team was pretty sure there were other light-sensitive cells.
HATTAR: People used to laugh at us if we'd say there are other photoreceptors distinct from rods and cones in the retina.
HAMILTON: The skeptics stopped laughing when the team discovered a third kind of photoreceptor. These cells responded to light, but they didn't seem to help us see. Their most obvious function was keeping the brain's internal clock in sync with changes in daylight. And Hattar says many scientists assumed that this function also explained seasonal depression.
HATTAR: People thought that the only reason you get mood problems is because your clock is misaligned.
HAMILTON: Hattar was pretty sure there was a better reason. And after years of searching, he found one. In September, Hattar's team published a study of mice suggesting a direct pathway between the third kind of photoreceptor and brain areas that affect mood. When the cells were present, artificially shortened days caused a mouse version of depression. On the other hand...
JEROME SANES: If you remove those cells with genetic tools, the depression-like symptoms go away.
HAMILTON: That's Jerome Sanes, a researcher at Brown University. Sanes wasn't involved in Hattar's study, but he was so intrigued by the result that he decided to look for a similar brain circuit in people.
SANES: We studied young adults. We put them into an MRI machine and measured functional MRI signals.
HAMILTON: All while the participants were exposed to different levels of light. This allowed the team to identify brain areas receiving signals from the retina's special photoreceptors. Sanes says two of these areas were way up in the front of the brain.
SANES: It's interesting because these areas seem to be the areas that have been shown in many studies to be involved in depression and other affective disorders.
HAMILTON: Sanes' team presented their research at the Society for Neuroscience meeting in November. And Samer Hattar says the finding appears to be the last piece in a complex puzzle.
HATTAR: Now you have a circuit - that you know your eye is influencing your brain to affect mood.
HAMILTON: If so, it would explain how shorter days make people sad. Now Hattar says he has a new question. Why would evolution produce a brain that works this way?
HATTAR: You will understand why you would need light to see. But why do you need light to make you happy? We're not plants. Why do we need light to make us happy, and why do we need high-intensity light to make us happy?
HAMILTON: Hattar has no idea, but he hopes to find out. Jon Hamilton, NPR News.