Using a small night light wasn’t associated with weight gain, and the association was modest when women slept with light coming in from outside of the room.
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While poor sleep is associated with obesity and weight gain, study author Dale Sandler said this “did not explain the associations between exposure to artificial light while sleeping and weight.”
"Evolutionarily we are supposed to be sleeping at night, in a dark place," Sandler told NBC News. "It's much more important than people realize for a whole variety of health reasons."
Co-author Chandra Jackson wonders how urban environments, where street lights and storefronts can be bright all night, might get in the way of one’s circadian rhythms. In cities with ample artificial light at night, there’s increased opportunity for photons to disrupt those rhythms.
"Humans are genetically adapted to a natural environment consisting of sunlight during the day and darkness at night," Jackson said in a statement. For example, the body automatically produces a hormone called melatonin at nighttime. This hormone is known to help regulate the body's biologic clock. But "exposure to artificial light at night may alter hormones and other biological processes in ways that raise the risk of health conditions like obesity."
Previous research on exposure to room light before bedtime has found disruptions in melatonin signaling, specifically as melatonin levels drop in the presence of artificial or natural light, could potentially impact sleep, thermoregulation, blood pressure, and glucose homeostasis.
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The new study has some limitations, the authors noted, including the fact that it does not include men. Researchers also acknowledged that other confounding factors may explain the associations between artificial light and weight gain, but their findings remained unchanged even when controlling for factors like age, race, socioeconomic status, physical activity and so on.
Ultimately, the research suggests high-calorie diets and sedentary behaviors aren’t the only factors contributing to the rise in obesity in women, according to lead author Yong-Moon Park. The findings also highlight “the importance of artificial light at night and gives women who sleep with lights or the television on a way to improve their health.”
Read the full study at jamanetwork.com.