People who had most of their sunshine exposure in the morning, starting at 8 a.m., had a significantly lower body mass index (BMI) than did those who got most of their light exposure later in the day, researchers found. BMI is an estimate of body fat based on height and weight.
"The earlier this light exposure occurred during the day, the lower individuals' body mass index," said coauthor Kathryn Reid, research associate professor of neurology at Northwestern University's Feinberg School of Medicine. "The later the hour of moderately bright light exposure, the higher a person's BMI."
We know what you're thinking: The slimmer people probably woke up early and walked or exercised outside, and that's why they're leaner. But that's not it, said Reid. "Physical activity wasn't different for the early risers" in the study, compared with the late risers, she said in an interview.
More likely, it has to do with the type of light that occurs early in the morning versus after noon. Early-morning light waves have a higher amount of blue light (a shorter wavelength); this has been shown to have the strongest affect on the body's circadian rhythms, which affect energy and metabolism, the researchers wrote.
The "magic number" for light exposure is a minimum of 500 lux, about twice the brightness of a typically lit indoor room. Those who were exposed to at least 500 lux in the morning were the slimmest, Reid said.
The study found that, on average, for every hour that passed after 8 a.m. before a person was exposed to 500 lux, BMI increased by 1.28 points, or about 5 to 10 pounds.
The study, published in the journal Plos One, was a small one - just 54 adults, average age 30, who wore monitors to track light exposure, activity and sleep - but Reid said the results were still "very exciting."
She continued: "This is the first time it's been shown that light plays this role" in affecting weight.
What's the best way to soak up some of those early rays? Study senior author Phyllis Zee, M.D., professor of neurology and director of Northwestern medical school's Sleep and Circadian Rhythms Research Program, recommended getting between 20 and 30 minutes of exposure to bright morning light between 8 a.m. and noon.
"Whenever possible, be exposed to early light," she told HealthDay. "Walk to work if you can. Bright, outdoor light will be way above the 500 lux. If you can't get outside, work near a window. If you can't get near a window, at least make sure your work environment is well lit."
Americans typically don't get enough exposure to natural light and work in poorly lit environments, Zee added in a statement. An indoor room, on average, has 200 to 300 lux of light, while "even on a cloudy day, outdoor light is more than 1,000 lux of brightness."
One easy way to help improve the country's health and obesity problem might just be by improving lighting in offices and schools, she said.
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