Overexposure to bright light not only keeps you up at night, but animal studies show it may be linked to depressive symptoms and learning problems.
That’s what Samer Hattar, a biology professor at Johns Hopkins University and his team found while studying mice exposed to repeated cycles of bright light. Previous studies hinted at the connection between continued exposure to light and depression in animals, but Hattar also found that the bright lights contributed to poorer learning as well.
In the study, published in the journal Nature, the researchers exposed lab mice to 3.5 hours of light followed by 3.5 hours of darkness, which previous work shows does not disrupt sleep cycles in the animals. After two weeks of the light exposure, the mice exhibited symptoms of depression and learning deficits as well as higher levels of the stress hormone cortisol.
“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,” said Hattar in a statement. “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.”
At the end of the experiment, the mice were treated with the anti-depressant Prozac, and their normal behaviors returned, which further suggests that their mood and learning issues were linked to depression.
According to Hattar and his team, the study results may have relevance to humans since we share light-reactive cells in the eyes called intrinsically photosensitive retinal ganglion cells (ipRGCs) with mice. These cells react to bright light and activate the brain‘s limbic system–which is responsible for memory and emotion. At night, the system is designed to slow down, but when exposed to light, it becomes active again, essentially working overtime, when it should be resting. “We still don’t know the exact brain regions that are impacted, but we suspect that some regions that are supposed to be settled at night must be activated and disclose some changes in the hormonal levels, specifically in corticosterone. Corticosterone in turn leads to depression and learning deficits,” says Hattar.
Previous studies hinted that bright light exposure at a night can decrease melatonin levels and lead to poor sleep, and scientists suspect that the continued light exposure can cause disruptions in hormones that may up the risk of heart disease, obesity, diabetes and even cancer. (It’s worth noting, however, that these studies looked at over-exposure to light at night; the body’s wake-sleep cycle depends on a certain amount of light during the day. When the days get shorter, such as during winter months in the northern hemisphere, lack of light exposure — and the resulting disruption in levels of hormones like melatonin — can also trigger depressive symptoms known as seasonal affective disorder.)
So what do the findings mean for people who can’t turn out the lights at night — night shift workers, those in the extreme northern pole where at times daylight stretches for 24 hours, or even those of us who remain glued to the television or computer screen into the wee hours? Apparently, Hattar says, going completely dark isn’t necessary to keep the mood system responding within normal bounds. It’s primarily the blue light in devices or light fixtures that keeps those photoreceptors in the eyes alert. And, he says “there are applications that can make the screen dimmer and take away the blue light. If you dim the screen to the lowest you can, it will help you avoid the negative effects of the blue light at night.”
For shift workers, Hattar recommends a similar dimming strategy, such as using the minimum amount of light that still allows for comfortable sight. “See how much you can lower the intensity [of the light] so you can still see comfortably, but you are not exposed to incredibly bright light,” says Dr. Hattar. “Get more bright light during the day and limit it at night. It is really actually quite simple. The discovery is quite interesting, but the remedy is quite simple.” And thankfully so, since it’s not likely we’ll be going dark at night any time soon.