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From the article:
While hamsters exposed to light at night for four weeks showed evidence of depressive symptoms, those symptoms essentially disappeared after about two weeks if they returned to normal lighting conditions.
Even changes in the brain that occurred after hamsters lived with chronic light at night reversed themselves after returning to a more normal light cycle.
These findings add to the growing evidence that suggest chronic exposure to artificial light at night may play some role in the rising rates of depression in humans during the past 50 years, said Tracy Bedrosian, lead author of the study and doctoral student in neuroscience at Ohio State University.
“The results we found in hamsters are consistent with what we know about depression in humans,” Bedrosian said.
Most importantly, hamsters that lived in dim light showed increased expression of the gene that produces tumor necrosis factor.
They found that blocking effects of that protein, called tumor necrosis factor, prevented the development of depressive-like symptoms in hamsters even when they were exposed to light at night.
However, hamsters that were returned to a standard light-dark cycle after four weeks of dim light at night saw their TNF levels and even their density of dendritic spines return essentially to normal.