Can Sun Exposure Help You Lose Weight?

By Jane Farrell

Exposure to light in the morning rather than later in the day may be a good way to manage weight, new research shows.

The study, from Northwestern Medicine, found that the timing, intensity and duration of light exposure is linked to Body Mass Index (BMI).

“The earlier this light exposure occurred during the day, the lower individuals’ body mass index,” said co-lead author Kathryn Reid, research associate professor of neurology at Northwestern University Feinberg School of Medicine. “The later the hour of moderately bright light exposure, the higher a person’s BMI.”

The study took into account a person’s activity level, caloric intake, sleep timing, their age and the season in which they got the exposure.

“Light is the most potent agent to synchronize your internal body clock that regulates circadian rhythms, which in turn also regulate energy balance,” said study senior author Phyllis C. Zee, M.D. “The message is that you should get more bright light between 8 a.m. and noon.” About 20 to 30 minutes of morning light is enough to affect BMI.

“If a person doesn’t get sufficient light at the appropriate time of day, it could de-synchronize your internal body clock, which is known to alter metabolism and can lead to weight gain,” Zee said. The exact mechanism of how light affects body fat requires further research, she noted. 

The study was published in the journal PLOS ONE.

 Zee noted that the American lifestyle is primarily an indoor one, where we usually have about 200 to 300 “lux” of brightness. In the study, researchers found that 500 lux was the minimum required to make a difference. Outdoor light is 1,000 even on a cloudy day.

The investigators studied 26 men and 28 women, with an average age of 30. The participants wore a wrist monitor that measured their light exposure. They also kept track of their caloric intake via food logs.

an average age of 30. They wore a wrist actigraphy monitor that measured their light exposure and sleep parameters for seven days in normal-living conditions. Their caloric intake was determined from seven days of food logs.

As part of a healthy lifestyle, researchers said, people should be encouraged to get more exposure to light – whether through windows in the workplace or going outside for lunch or breaks. They also said that indoor lighting should be improved in schools and workplaces.

 “This is something we could institute early on in our schools to prevent obesity on a larger scale,” Zee said.