The finding came from a study of mice at Johns Hopkins University and was published in the journal Nature. It explains how our ability in the 21st century to use light throughout the night may actually cause us a great deal of harm.
Samer Hattar, a biology professor in the Johns Hopkins University's Krieger School of Arts and Sciences, explained:
"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."
The research on mice revealed how exposure to bright light activates certain cells in the eye, known as intrinsically photosensitive retinal ganglion cells (ipRGCs). This impacts the brain's center for mood, learning, and memory.
Hattar explained why this study on mice paves the way towards research into human therapies. "Mice and humans are actually very much alike in many ways, and one is that they have these ipRGCs in their eyes, which affect them the same way," he said. "In addition, in this study, we make reference to previous studies on humans, which show that light does, indeed, impact the human brain's limbic system. And the same pathways are in place in mice."
It is known that some people develop "seasonal affective disorder" in the winter because of shorter days and less light, and that they can be treated with "light therapy" - when a person is consistently exposed to bright light.
The team Hattar worked with, which was led by Tara LeGates and Cara Altimus, hypothesized that mice would have the same reaction to the lack of light. In order to prove this, they exposed rodents to a rotation of light for 3.5 hours and then darkness for 4.5 hours.
Although prior research demonstrated that this rotation did not disrupt the mice's sleep, the scientists in this trial found that the animals began to show depression-like behaviors.
The authors wrote:
"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. 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."
The results also showed that the animals' cortisol levels (a stress hormone associated with learning problems) were increased. The symptoms were alleviated with Prozac, a common anti-depressant, which allowed the mice to return to their normal levels of learning and healthy moods. This showed that their ability to learn was harmed by depression.
The finding implies that people need to be aware of their lengthened exposure to bright light at night because it can impact their mood for the worse and cause them to have difficulty learning.
"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," Hattar concluded.
Written by Sarah Glynn