Bright Light at Night Could Harm Your Metabolism, According to Science

Researchers at Northwestern University compared the impact of three hours of morning versus evening exposure to bright and dim lighting on hunger and metabolism and found bright light was associated with higher blood sugar and increased insulin resistance, which are risks for weight gain. Moreover, evening exposure to light had the greatest impact on blood glucose levels.

“We typically recommend avoiding bright light in the evening due to the effects it has on the circadian rhythm,” explains Kathryn Reid, PhD, a research professor of neurology at Northwestern University Feinberg School of Medicine. “More research is needed because the impact of light exposure later in the evening on metabolism might be a different mechanism.”

An earlier study published in JAMA Internal Medicine found people who had most of their exposure to bright light in the morning had significantly lower BMIs than those who had most of their exposure to bright light in the evenings. Rates of obesity were 33% higher among those who were exposed to artificial light (from sources like incandescent light bulbs or television screens) than those who slept in the dark.

Researcher Yong-Moon Park, MD, PhD, a postdoctoral fellow at the National Institutes of Health, suspects nighttime exposure to light alters the circadian rhythm, triggering changes in hormones that regulate appetite and metabolism.

“It’s also possible that [being exposed to] light at night simply disrupts sleep,” says Moon. “There is increasing evidence that not getting enough sleep is associated with weight gain.”

It’s not just exposure to light at certain times of day that appears to affect metabolism; the lack of sunlight during the winter months could also impact the number on the scale. Research published in Scientific Reports found fat cells tend to shrink when exposed to sunlight. In contrast, the lack of sunlight during winter appears to slow metabolism and promote fat storage, which could be one of the reasons for winter weight gain.

You don’t have to descend into darkness or live in a warm-weather climate to avoid gaining weight. In fact, Reid notes, “Light exposure during the day has many positive effects on health, mood, sleep and well-being, and has acute effects on alertness levels. My advice would be to maximize bright light exposure during the daytime and avoid bright light at night, particularly in the hours prior to bedtime and during sleep.” Going for a daily walk is a great place to start to get more daytime light, even if you need to bundle up in the winter.

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