updated 11/29/2010 12:46:21 PM ET 2010-11-29T17:46:21

Basking in the glow of your TV, smart phone or living room lights late into the night may put you at risk for depression, suggests a new study.

The research, which involved hamsters, adds to growing evidence in both animals and people that exposure to even dim lights at night can lead to all sorts of negative health consequences, including breast cancer, sleep disorders and weight gain.

"We've set up a link between exposure to light at night with depression in these animals," said Tracy Bedrosian, a doctoral student in neuroscience at The Ohio State University in Columbus. "If it does apply to humans, people might want to think about getting dark shades, not leaving the TV on all night long, and making sure to give themselves darkness when they go to sleep."

Major depression has grown more common in recent decades, Bedrosian said. And while there is probably no single reason for the trend, researchers suspect that light disturbances may play a part.

That suspicion is based, in part, on the simple observation that people today are exposed to far more sources of artificial light at night than they were 100 years ago. More people have computers in their bedrooms. More people fall asleep with the TV on.

Studies have also found that people who work night shifts have higher rates of mood disorders compared to people who sleep when their bodies are supposed to sleep.

To test the link between light and depression, Bedrosian and colleagues split a group of 16 hamsters into two groups. All of the animals spent 16 hours a day under bright lights. During the rest of the time, eight of the hamsters were given true darkness. The other eight were exposed to dim lights, at a level similar to the glow of a TV in a dark room.

Eight weeks later, the hamsters that never experienced true darkness scored significantly lower on a series of mood tests, Bedrosian reported at the annual meeting of the Society for Neuroscience in San Diego.

The darkness-deprived hamsters drank 20 percent less sugar water than the other group, for example, suggesting that they weren't getting the same enjoyment out of activities that they used to find pleasurable. The depressed group also gave up far sooner on a swimming activity.

When the researchers looked at the brains of the animals, they found major differences between the groups in a region called the hippocampus.

In the dimly lit and more depressed group, there were fewer hair-like growths, called dendritic spines, in this part of the brain. That suggests that there was less communication between nerve cells there. Lowered density of the hippocampus has also been observed in people with major depression.

To explain how light exposure at night might affect the moods of both hamsters and people, the researchers turn to a hormone called melatonin. Our bodies start churning out the hormone as soon as we sense darkness, and its influence is broad. Among other roles, melatonin acts as an antioxidant. It regulates our circadian rhythms. It helps us fall asleep, and it controls the release of other hormones.

With even a small amount of ambient light at night, the body might release the wrong amount of melatonin, or melatonin might get produced at the wrong time, leading to any number of problems, said neurologist Phyllis Zee, director of the Sleep Disorders Program at Northwestern University in Chicago. Studies have already implicated problems in the melatonin system with mood disorders, diabetes and sleep disturbances.

"They're all somehow related," Zee said, "and perhaps melatonin helps explain why there is this very strong relationship between depression, sleep, and circadian rhythms, as well as obesity and metabolism."

"Light affects so many biological systems," she added. "Light is a very powerful drug for the brain."

© 2012 Discovery Channel


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