You may want to reconsider your plans to fall asleep to the strains of Netflix from the laptop resting on your chest tonight. A new study of mice suggests that watching glowing screens late at night can raise the risk of depression and learning disorders.

While lack of sleep is certainly one drawback to burning the midnight oil, Johns Hopkins University researcher Samer Hattar focused on the effects of late-night exposure to bright light. In a paper appearing Wednesday in the journal Nature, he and his colleagues describe how exposing mice to a disrupted light cycle – three and a half hours of light, followed by three and a half hours of darkness -- changed their behavior.

"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," Hattar said in a statement Wednesday.

The reason Hattar used this particular light cycle technique, rather than just a much longer light exposure, is that this method has been previously shown not to disrupt the mouse's sleep cycle. This way, the scientists can separate light exposure-specific effects from the effects of disturbed rest.

Mice that were exposed to the aberrant light-dark cycle showed depression-like behaviors. They were less interested in drinking sugar water and when placed in a container filled with water, and were quicker to give up swimming than normal mice. They were also slower to learn.

The mice also had increased levels of the stress hormone cortisol. When the animals were treated with the anti-depressant Prozac, they stopped exhibiting depression-like and learning disorder symptoms.

The suspected physiological link between light and behavior is a kind of photoreceptor in the eye, called intrinsically photosensitive retinal ganglion cells, or ipRGCs. They react differently to light than the classic photoreceptor cells – rods and cones – do, and are thought to play a role in mediating circadian rhythms by relaying information about day and night cycles to the brain.

In future studies, Hattar says he wants to investigate which areas of the brain region may be responsible for the light-mediated behaviors in mice.

For now, though, he suggests that if you must use your laptop or mobile device at night, to turn down the brightness on the screen.

“If you need to do something at night, you don't have to be in complete darkness; you just need to lower the light,” Hattar said.

SOURCE: LeGates et al. “Aberrant light directly impairs mood and learning through melanopsin-expressing neurons.” Nature published online 14 November 2012.

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