Northwestern University researchers have discovered the easiest way to shed weight, and maintain a lean and a healthy body. They recommend people should get up early and engage in some outdoor physical activity.

In a latest study, the researchers found that people who had maximum daily exposure to morning sunlight had a significantly lower body mass index (BMI) than those who were mostly exposed to afternoon light.

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

This is the first study to determine a link between the timing, intensity or duration of daytime light exposure and body weight. Researchers did not consider factors like physical activity level, caloric intake, sleep timing, age or season for the study.

Phyllis C. Zee, M.D., and study senior author said that morning light exposure, between 8:00 a.m. and noon, for a maximum 30 minutes makes a positive difference to a person's BMI.

"Light is the most potent agent to synchronize your internal body clock that regulates circadian rhythms, which in turn also regulate energy balance. 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.

Zee said that American lifestyle is to blame for people receiving insufficient amount of natural light in the morning. Everyday lives of Americans are largely indoors and in poorly lit environments, usually about 200 to 300 lux.

In the study, the researchers found 500 lux was 'the magic number' for managing a lower BMI. Even on a gloomy day, natural light is estimated to be more than 1,000 lux of brightness. Researchers said that it is difficult to achieve this light level indoors. Therefore, exposure to morning light is important for a healthy BMI.  

Zee said that further research is required to find out how light affects body fat.

The finding will be published April 2 in the journal PLOS ONE.