December 8th, 2014
By turning on lights, we can continue our daily activities after sunset. However, new research suggests that this is not without consequences: a regular overload to bright light at night may cause depression and learning issues, even though you get enough hours of sleep.
“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,” said Samer Hattar, a biology professor in the Johns Hopkins University’s Krieger School of Arts and Sciences, Washington.
To explore this effect, Hattar and his team studied mice. The rodents, like humans, have special cells in the eye activated by bright light, affecting the brain’s center for mood, memory and learning. These cells are called ipRGCs, and affect mice and humans in the same way.
The research team exposed laboratory rodents to a cycle consisting of 3.5 hours of light and then 3.5 hours of darkness. Previous studies using this cycle showed that it did not disrupt the mice’s sleep cycles, but Hattar’s team found that it did cause the animals to develop depression-like behaviors.
“Of course, you can’t ask mice how they feel, but we did see an increase in depression-like behaviors, including a lack of interest in sugar or pleasure seeking, and the study mice moved around far less during some of the tests we did,” he said. “They also clearly did not learn as quickly or remember tasks as well. They were not as interested in novel objects as were mice on a regular light-darkness cycle schedule.”
Furthermore they found that the mice had higher levels of the stress hormone cortisol, known to be related to learning problems.
The animals were treated with Prozac, an anti-depressant, restoring both their mood and their learning abilities, and supporting the evidence that their learning issues were caused by depression.
Hattar points out that people should be aware of the negative effects of the kind of prolonged, regular exposure to bright light at night that is routine in our lives.
“I’m not saying we have to sit in complete darkness at night, but I do recommend that we should switch on fewer lamps, and stick to less-intense light bulbs: Basically, only use what you need to see. That won’t likely be enough to activate those ipRGCs that affect mood,” he advises.
LeGates, T., Altimus, C., Wang, H., Lee, H., Yang, S., Zhao, H., Kirkwood, A., Weber, E., & Hattar, S. (2012). Aberrant light directly impairs mood and learning through melanopsin-expressing neurons Nature DOI: 10.1038/nature11673