Light at Night Harms Health, Even While You're Fast Sleep

If you tend to fall asleep with the TV or a bedside light on, add it to the list of ways you are probably harming your health without even trying. In fact, per CNN—citing a recent study led by Dr. Phyllis Zee, chief of sleep medicine at Northwestern—exposure to very dim light for just one night can raise your heart rate during sleep and lead to elevated blood sugar the next morning. Prior studies have linked artificial light to health issues, but the connection between light and sleep is less understood. According to NPR, Zee was surprised to discover that physiological changes occurred even though participants reported getting good sleep. "It's almost like the brain and the heart knew that the lights were on," she said.

In an interview with Medical News Today, Zee gave three possible explanations. First, light "can cause awakening or arousal," as one might expect, but the minor changes noted didn't sync up with the insulin resistance observed. Effects on the body's circadian clock and melatonin secretion were ruled out based on blood tests of the 20 study participants that found no melatonin decrease. That leaves her hypothesis: that even dim lighting through closed eyelids (it's thought no more than 10% of the light gets through to the eye, and the light participants were exposed to was akin to a street light filtering through a window) "activates brain regions that regulate the autonomic nervous system."

Zee's study was small and only provides a glimpse into the issue, but long-term effects are likely, according to Dr. Charles Czeisler of Harvard. "People think as long as they fall asleep and are unconscious that it's not having physiological effects, but that's simply not true," he told NPR. "If you're chronically exposed to this type of thing, that's going to increase the risk of chronic diseases." Northwestern Now shares some tips from Dr. Zee: Notably, if you must have light, go with amber or red/orange instead of blue or white, keep it close to the floor, and consider a sleep mask. (More sleep stories.)