Could sunshine help you slim down?

According to a new Northwestern Medicine® study, people who had most of their daily exposure to bright light in the morning had a significantly lower body mass index (BMI) than those who had most of their light exposure later in the day.

"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 power of sunshine

"Light is the most potent agent to synchronise 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.

The influence of morning light exposure on body weight was independent of an individual's physical activity level, caloric intake, sleep timing, age or season. It accounted for about 20 percent of a person's 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," Dr Zee said. The exact mechanism of how light affects body fat requires further research, she noted.

What's the magic light number for a low BMI?

In the study, 500 lux was "the magic number" or minimum threshold for having a lower BMI.

Even on a cloudy day, outdoor light is more than 1 000 lux of brightness. It is difficult to achieve this light level with usual indoor lighting, the scientists noted.

The study included 54 participants (26 males, 28 females), with 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, you should get appropriate exposure to light. When indoors, try working near a window and go outside for lunch or breaks.

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