A new animal study reported in the Journal of Neuroscience suggests that nocturnal light exposure may induce depressive responses and alter neuronal structure.

In the study involving Siberian hamsters (Phodopus sungorus), senior author Prof Randy Nelson from the Ohio State University, first author Dr Tracy Bedrosian of the Salk Institute, and their colleagues, found that blue light had the worst effects on mood-related measures, followed closely by white light. But hamsters exposed to red light at night had significantly less evidence of depressive-like symptoms and changes in the brain linked to depression, compared to those that experienced blue or white light.

“The findings may have important implications for humans, particularly those whose work on night shifts makes them susceptible to mood disorders,” Prof Nelson said.

“Our findings suggest that if we could use red light when appropriate for night-shift workers, it may not have some of the negative effects on their health that white light does.”

The study examined the role of specialized photosensitive cells in the retina called ipRGCs that don’t have a major role in vision, but detect light and send messages to a part of the brain that helps regulate the body’s circadian clock. This is the body’s master clock that helps determine when people feel sleepy and awake.

What people experience as different colors of light are actually lights of different wavelengths. The ipRGCs don’t appear to react to light of different wavelengths in the same way.

In one experiment, Prof Nelson’s team exposed adult female Siberian hamsters to 4 weeks each of nighttime conditions with no light, dim red light, dim white light or dim blue light. They then did several tests with the hamsters that are used to check for depressive-like symptoms. For example, if the hamsters drink less-than-normal amounts of sugar water that is seen as evidence of a mood problem.

The results showed that hamsters that were kept in the dark at night drank the most sugar water, followed closely by those exposed to red light. Those that lived with dim white or blue light at night drank significantly less of the sugar water than the others.

The scientists then examined the hippocampus regions of the brains of the hamsters. Hamsters that spent the night in dim blue or white light had a significantly reduced density of dendritic spines compared to those that lived in total darkness or that were exposed to only red light. Dendritic spines are hairlike growths on brain cells that are used to send chemical messages from one cell to another.

“A lowered density of these dendritic spines has been linked to depression. The behavior tests and changes in brain structure in hamsters both suggest that the color of lights may play a key role in mood,” Prof Nelson said.

“In nearly every measure we had, hamsters exposed to blue light were the worst off, followed by those exposed to white light. While total darkness was best, red light was not nearly as bad as the other wavelengths we studied.”

The scientists believe these results may be applicable to humans. In addition to shift workers, others may benefit from limiting their light at night from computers, televisions and other electronic devices, they said. And, if light is needed, the color may matter.

“If you need a night light in the bathroom or bedroom, it may be better to have one that gives off red light rather than white light,” Dr Bedrosian said.

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Bibliographic information: Tracy A. Bedrosian et al. 2013. Nocturnal Light Exposure Impairs Affective Responses in a Wavelength-Dependent Manner. The Journal of Neuroscience, 33 (32): 13081-13087; doi: 10.1523/JNEUROSCI.5734-12.2013