Early morning rays the secret to staying lean: study

Early morning sunshine may be the secret to staying lean, a new study from Northwestern University suggests.

For the first time, light exposure has been linked to weight loss, with those exposed to even moderately bright light in the morning logging significantly lower body mass indices (BMI) than those who had most of their light exposure later in the day.

"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," study senior author Dr. Phyllis C. Zee said in a press release.

The optimal time to bask in the sun is between 8 a.m. and 12 p.m., with just 20 to 30 minutes enough to affect BMI, according to researchers, who note that light accounts for 20 percent of a person's BMI.

The study, published in the journal PLOS ONE, included 54 male and female participants with an average age of 30. A wrist actigraphy measured their light exposure and sleep parameters for a week and they recorded their caloric intake using food logs.

The findings emphasize the importance of "circadian health" in which exposure to light and dark is synchronized with your internal body clock.

"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," Giovanni Santostasi, co-lead author, said.

In the study, 500 lux was the minimum amount of light needed to have a lower BMI. To put this in perspective, most Americans work in an indoor environment with 200 to 300 lux of light. Even on a cloudy day, outdoor light is more than 1,000 lux of brightness.

"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."