US scientists have discovered that mice exposed to bright, unnatural light at night show signs of "depression".
In their study, the mice had higher levels of stress hormone cortisol and were less likely to move around and explore new objects in their cages.
"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, professor of biology at Johns Hopkins University in Baltimore.
Professor Hattar and his colleagues found the bright light particularly affects eye cells called intrinsically photosensitive retinal ganglion cells, which are linked to the part of the brain that manages mood.
He warned that mice are similar to humans in many ways, so people should be careful how much artificial light they expose themselves to 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.
"Basically, only use what you need to see."
Another spokesman at John Hopkins University told the UK's Telegraph that staring at iPad screens could also impact our moods.
"When people routinely burn the midnight oil, they risk suffering depression and learning issues, and not only because of lack of sleep," they said.
"The culprit could also be exposure to bright light at night from lamps, computers and even iPads."
Professor Michael Berk, the chair of psychiatry at Deakin University, told ninemsn the study is very interesting and adds to earlier work into why depression levels appear to be increasing in the western world.
"Despite the fact we are healthier and wealthier and less plagued by issues facing other countries, we are not happier," he said.
"We are interested in what the factors are in the modern environment that might be increasing vulnerability to depression. Changes in sleep is a very valid possibility as to why the prevalence of depression might be increasing."
Professor Berk said it appears blue light that is omitted from TV and computer screens could interfere with our circadian rhythms and our mood hormones.
"We know the body's circadian clock is set by light –– you need bright light in the morning to tell your brain to wake up and you need darkness at night to tell you to shut down the components of your circadian biology that tell you to sleep," he said.
"This study is interesting and valuable and it certainly does suggest that disruption in sleep cycles may play a role in risk for depression and it suggests that exposure to night time light might be one of the pathways whereby sleep is dis-regulated."
Professor Berk conceded it's difficult to avoid light at night in modern society, and said more research is needed to determine whether people who do not expose themselves to light at night are less likely to suffer depression.
The study was published in the journal Nature.