Losing weight is linked to the time, intensity and length of exposure to daylight, says new research out of Northwestern University, and for that, morning sun is best.

People who had most of their daily exposure to even moderately bright light in the morning had a significantly lower body mass index -- which is ratio calculated from a person's weight and height -- than those who had most of their light exposure later in the day, the study found.

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

The research found the effect of morning light on body weight was independent of an individual's physical activity level, caloric intake, sleep timing, age or season -- but, regardless, accounted for about 20 percent of 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," said Phyllis C. Zee, a medical doctor and senior study author. "The message is that you should get more bright light between 8 a.m. and noon."

The study found 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. However, the exact mechanism behind how light affects body fat requires further research.

Many people fail to get enough natural light in the morning, Zee said, because the American lifestyle is predominantly focused on indoors activities.

The study included 54 participants, 26 males and 28 females, who were an average age of 30. They wore wrist monitors 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.

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

Even though duration and timing of sleep was not linked to the results, "owl" chronotypes, people who stay up later and sleep later, would be a population affected by later light exposure. But even "larks," those who wake early, would be affected by lack of early light if they stayed inside in the morning.

While the study wasn't designed to examine how light exposure affects body fat, previous research at Northwestern and elsewhere showed how light plays a role in regulating metabolism, hunger and satiety.