Reading on your iPad, surfing the web on your laptop and watching TV late into the night might not only be bad for your sleep -- it could also be bad for your mental health, according to a new study in mice.
Researchers from Johns Hopkins University found that being exposed to bright lights for long periods of time -- whether it's from a screen, or even from just having the lights on in your bedroom or in the office if you're doing shift work -- is linked with higher levels of the stress hormone cortisol, which could eventually lead to problems with brain functioning and even depression.
The study, published in the journal Nature, was conducted in mice, but researchers said that the findings still apply to humans because mice and humans are similar in important ways -- one being that both have light-activated cells (called intrinsically photosensitive retinal ganglion cells) in the eyes, which are known to impact the brain regions responsible for mood and memory.
"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," study researcher Samer Hattar, a biology professor at Johns Hopkins, said in a statement.
The study involved exposing mice to a cycle of 3.5 hours of light, and then 3.5 hours of dark. Past studies showed that this sort of light-dark cycling doesn't affect the mice's sleep -- but the new study did show that it resulted in behaviors indicative of depression.
"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," Hattar said in the 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."
A similar study, conducted by Ohio State University researchers and published earlier this year in the journal Molecular Psychiatry, also showed a link between light at night and symptoms of depression in hamsters.
Light at night has been linked with other health problems in past research. The American Medical Association has even officially recognized light at night's link with cancer, disrupted sleep and dangerous driving, MyHealthNewsDaily reported.
The World Health Organization has identified shift work -- where the body's natural circadian rhythm is disrupted -- as a "probable carcinogen."