Exposure to outdoor light in the morning may play a key role in achieving or maintaining a healthy weight, according to a new study conducted by researchers from Northwestern University in Chicago and published in the journal PLOS ONE on April 2.
“Light is a modifiable factor with the potential to be used in weight management programs,” co-lead author Kathryn Reid said. “Just like people are trying to get more sleep to help them lose weight, perhaps manipulating light is another way to lose weight.”
The researchers found that people who received most of their exposure to moderately bright or brighter light before noon had a significantly lower body mass index (a measure of weight relative to height, used to estimate obesity) than people who got most of their light exposure after noon.
“The earlier this light exposure occurred during the day, the lower individuals’ body mass index,” Reid said. “The later the hour of moderately bright light exposure, the higher a person’s BMI.”
The study was funded by grants from the National Heart, Lung and Blood Institute, the National Institute on Aging and the Office of Research on Women’s Health.
Not just how much light, but when
The body’s circadian rhythms — fluctuations in hormone levels and other biological systems depending on the time of day — are known to play an important role in metabolism and weight regulation. But this study marks the first time that scientists have shown the importance of the connection between the timing, intensity and duration of light exposure.
The researchers had 26 men and 28 women of average age 30 wear wrist actigraphy monitors to measure their light exposure and sleep patterns for seven days. Participants maintained their normal daily routines and recorded their daily dietary intake with food logs.
When the researchers looked independently for a connection between BMI and timing, intensity or duration of light exposure, they found no relationship. Then, co-lead author Giovanni Santostasi developed a measure for combining these three factors into a single number, mean light timing (MLiT). This number alone accounted for 20 percent of BMI.
“I saw that what seemed to be most associated with body mass index was not just how much light you receive but when you get it and for how long,” Santostasi said.
Just 20 minutes a day
The study adds new evidence that the modern lifestyle, spent mostly indoors, can have serious health effects. Senior author Phyllis C. Zee noted that the typical U.S. work environment is lit at only 200 to 300 lux, but the minimum brightness needed to lower BMI in the study was 500 lux.
Outdoor light on a cloudy day exceeds 1,000 lux.
It is important that workplaces and schools be redesigned to incorporate more windows, and that people be encouraged to spend more time outdoors in the morning, Zee said.
“We focus on how too much light at night is bad; it’s also bad not to get enough light at the appropriate time during the day,” Zee said.
The researchers noted that it’s not just “night owls” who are at risk of not getting exposure to sufficient bright light at the right time. Even an early riser who spends most of the morning indoors under artificial light would suffer the same negative metabolic effects.
“Light is the most potent agent to synchronize your internal body clock that regulates circadian rhythms, which in turn also regulate energy balance,” Zee said. “The message is that you should get more bright light between 8 a.m. and noon.”
It takes only 20 to 30 minutes of exposure to outdoor morning light to have an impact on your BMI, the study found.
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