Exposure to light at night – even from a tablet or laptop computer – may cause depression, according to a new study.
Researchers have found that exposure to bright light elevates levels of a stress hormone in the body which triggers depression and reduces the ability to learn.
Up until the invention of electricity, humans rose with the sun and slept when it set. However, people can now continue to work into the early hours.
The new study of mice found this may come at a serious cost. When people routinely work late, they risk suffering depression and learning problems, and not only because of lack of sleep. The culprit could also be exposure to light at night from lamps, computers and even iPads.
Samer Hattar, a biology professor at Johns Hopkins University in the United States, said: “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 study demonstrates how cells in the eye – called intrinsically photosensitive retinal ganglion cells – are activated by bright light, affecting the brain’s centre for mood, memory and learning.
Prof Hattar added: “Mice and humans are actually very much alike in many ways, and one similarity is that they have these cells in their eyes which affect them the same way.”
The scientists knew that shorter days in the winter cause some people to develop a form of depression known as seasonal affective disorder, or Sad, and that some patients with the mood disorder benefit from light therapy, which is regular exposure to bright light.
Prof Hattar’s team believed that mice would react in the same way, and tested their theory by exposing 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 the team found that it did cause the animals to develop depression-like behaviours.
Prof Hattar said: “Of course, you can’t ask mice how they feel, but we did see an increase in depression-like behaviours, 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.”
He said the animals also had increased levels of cortisol, a stress hormone that has been linked in numerous previous studies with learning issues.
Treatment with Prozac, a commonly prescribed anti-depressant, mitigated the symptoms, restoring the mice to their previous healthy moods and levels of learning, and bolstering the evidence that their learning issues were caused by depression.
Prof Hattar said the results indicate that humans should be wary of the kind of prolonged, regular exposure to bright light at night.
“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,” he said.
The findings were published in the journal Nature.