By Traci Pedersen Associate News Editor
Reviewed by John M. Grohol, Psy.D.on November 23, 2010
The research shows that female Siberian hamsters who were exposed nightly to dim light for eight weeks experienced considerable physical changes in the hippocampus. This is the first study to definitively show that light at night, by itself, may be linked to changes in the hippocampus.
“Even dim light at night is sufficient to provoke depressive-like behaviors in hamsters, which may be explained by the changes we saw in their brains after eight weeks of exposure,” said Tracy Bedrosian, doctoral student in neuroscience at Ohio State University and co-author of the study.
Most importantly, the night-time light used in the study was only 5 lux—the same as having a television on in a dark room, said Randy Nelson, co-author of the study and professor of neuroscience and psychology at Ohio State.
“You would expect to see an impact if we were blasting these hamsters with bright lights, but this was a very low level, something that most people could easily encounter every night,” said Nelson, who is also a member of Ohio State’s Institute for Behavioral Medicine Research.
For the study, half of the hamsters lived in 16 hours of daylight at 150 lux and eight hours of dim light at 5 lux; the other half lived in a regular light-dark cycle consisting of 16 hours of light at 150 lux and eight hours of complete darkness.
Once the eight weeks were over, the hamsters were tested for depressive behavior. One of these tests measured how much sugar water the hamsters would drink. Normally, hamsters enjoy sweetened water; however, depressed hamsters will not drink as much. Scientists assume this occurs because they’re not getting as much pleasure from normally enjoyable activities.
Final results showed that hamsters housed with a dim night light displayed more depressive behaviors compared to hamsters who lived under a standard light-dark cycle.
Furthermore, when the experiment was over, researchers found that the dendritic spines in the hippocampi of hamsters that had been exposed to the dim night light had far less density.
“The hippocampus plays a key role in depressive disorders, so finding changes there is significant,” Bedrosian said.
Notably, there were no differences in the levels of the stress hormone cortisol between the two groups of hamsters. This finding is important because cortisol has been associated with changes in the hippocampus.
“To the best of our knowledge, this is the first study to document that light at night is a sufficient stimulus to induce changes in the hippocampus, without changes in cortisol levels,” Nelson said.
The researchers believe that these brain changes are linked to the production of the hormone melatonin. Having a light on at night stifles the emission of melatonin, a hormone which helps the body know that it is nighttime. Bedrosian says that the lower levels of melatonin at night may be the reason for the low density in the dendritic spines.
Source: Ohio State University
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