Samer Hattar of The Johns Hopkins University's Krieger School of Arts and Sciences said for most of history, humans rose with the sun and slept when it set, and the 24-hour light of the 21st century may have a serious cost.
"Basically, what we found is that chronic exposure to bright light -- even the kind of light you experience in your own living room at home or in the workplace at night if you are a shift worker -- elevates levels of a certain stress hormone in the body, which results in depression and lowers cognitive function," Hattar said in a statement.
The study, published online in advance of the print issue of the journal Nature, found mice demonstrated special cells in the eye -- intrinsically photosensitive retinal ganglion cells -- reactivated by bright light, affected the brain's center for mood, memory and learning.
"Mice and humans are actually very much alike in many ways, and one is that they have these intrinsically photosensitive retinal ganglion cells in their eyes, which affect them the same way," Hattar said.
The researchers exposed laboratory rodents to a cycle of 3.5 hours of light and then 3.5 hours of darkness -- shown in previous studies not to disrupt the mice's sleep cycles.
The study found it caused the animals to develop depression-like behaviors -- a lack of interest in sugar, pleasure seeking, remembering, learning or physical activity as the mice on a regular light-darkness cycle schedule.
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