This Nighttime Habit May Make You Gain Weight, New Study Finds

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Researchers at the National Institute of Environmental Health Sciences (NIEHS) just discovered a dark side to sleeping with the light on.

According to their new study in JAMA Internal Medicine, snoozing with the lights and television on is "significantly associated" with weight gain and obesity.

The NIEHS team is the first to spot this kind of connection between artificial light and weight gain. Their findings are based on questionnaire data from 43,722 American women spanning the years 2003 through 2009. Participants were not shift workers, daytime sleepers, or pregnant when the study began (and no men were included, so we don't know if these findings apply to them too).

By analyzing self-reported fluctuations in weight, height, waist/hip circumference, and body mass index over the five-year period, they found that artificial light does indeed affect weight—but not all light is created equal. Sleeping with a small night light on, for example, wasn't associated with weight gain, but sleeping with the television on was. Light coming in from the outside didn't appear to have significant effects on weight, while bedroom light did.

Those who slept with the lights or TV on in the bedroom were up to 17% more likely to gain 11 pounds or more over the course of the study. For some clues as to why, we can look to existing research on sleep, hormone regulation, and appetite: We know that disrupted sleep can decrease our levels of leptin, a hormone that suppresses appetite, and make us hungrier during the day—especially for high-fat, high-carb treats. Poor sleep—even for just one night—has also been shown to mess with metabolism. It isn't hard to believe that those who fall asleep with the lights on are getting less quality shut-eye than those who keep a dark bedroom, so this could be part of the problem.

Study co-author Chandra Jackson, Ph.D., also says that our circadian rhythms, or internal clocks, could be partially to blame: "Humans are genetically adapted to a natural environment consisting of sunlight during the day and darkness at night," she writes. "Exposure to artificial light at night may alter hormones and other biological processes in ways that raise the risk of health conditions like obesity."

Lesson learned: Lights out before bed! If you've been known to pass out with the lights on because you're too tired to walk to the switch, attaching your lights to a voice-activated device like an Amazon Echo could help. And consider swapping out that pre-bed TV habit with a great book instead. Your eyes (and maybe even your waistline) will thank you.

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