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A nightlight may keep those monsters under the bed. But it may also open the door to the blues¹. Because a new study reveals that animals exposed to light all night long show signs of clinical depression.

If you have access to electricity, you no doubt switch on a lamp, maybe even watch a little TV, after the sun goes down. But our bodies use cues about lightness and dark to regulate our hormones² and of course our sleep cycles. So what might these extra photons be doing to our health?

To find out, scientists housed mice in a room where the lights were always on. After three weeks, they found that mice who lived in the spotlight³ showed symptoms of depression, more so than mice who enjoyed eight hours of darkness at night. Interestingly, mice who could escape the light by ducking into a dark tube also escaped the worst of the depression. The findings were presented on October 21st at the Society for Neuroscience meeting in Chicago, and they’ll be published in the journal Behavioural Brain Research in December.

So flip⁴ that light switch at your own risk. Because the artificial brightness that helps keep us up could also bring us down.

—Karen Hopkin