A Northwestern Medicine study reports the timing, intensity and duration of light exposure during the day is linked to your weight -- the first time this has been shown, though take population correlations with a grain of sunshine.

People who had most of their daily exposure to even moderately 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 study found. BMI is a ratio calculated from a person's weight and height. 

"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 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.

"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., professor at Northwestern University. "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, Zee says.

"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.


Representative log linear light plots from three individual participants. Individual light data (lux) for up to 7 days is plotted by time of day (clock time in hours). The smoothed average is shown as a bold line while the MLiT500 is shown as a star, the open circles represent individual illuminance data points and each day is represented as a different color. The 500 lux threshold is shown as a continuous horizontal line. Panels A–C represents light profiles with a range of MLiT500. Areas where there are no data points (night time in Panel A and C) are where the light level readings were zero since the log of 0 is not a number. Panel A, represents an individual with a MLiT500 at 13∶52 hrs; panel B represents an individual with an MLiT500 at 10∶02 hrs and panel C represents an individual with an MLiT500 at 14∶35 hrs. 

What's the Magic Number for Low BMI?

Many people do not get enough natural light in the morning, Zee said, because the American lifestyle is predominantly indoors. We also work in poorly lit environments, usually about 200 to 300 lux. 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.

Light May Be Next Frontier for Weight Loss

"Light is a modifiable factor with the potential to be used in weight management programs," 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."

Santostasi, a physicist by training, developed a new measure for the study that integrates the timing, duration and intensity of light exposure into a single number called mean light timing or MLiT.

He searched for a correlation between light exposure timing, duration or intensity in the study raw data, but none of those factors individually were associated with BMI. It was only when he began combining parameters, that he saw "the strong signal" when all three were examined together.

"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," Santostasi said.

The study included 54 participants (26 males, 28 females), 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.

Befriend Your Body Clock – Get Morning Light

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.

As part of a healthy lifestyle, people should be encouraged to get more appropriate exposure to light. Workplaces and schools should have windows. Employees should be encouraged to go outside for lunch or breaks, and indoor lighting should be improved in the school and workplace "This is something we could institute early on in our schools to prevent obesity on a larger scale," Zee said.

While duration and timing of sleep was not linked to the results, "owl" chronotypes, 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 shows light plays a role in regulating metabolism, hunger and satiety.

Citation: Reid KJ, Santostasi G, Baron KG, Wilson J, Kang J, et al. (2014) Timing and Intensity of Light Correlate with Body Weight in Adults. PLoS ONE 9(4): e92251. doi:10.1371/journal.pone.0092251. Source: Northwestern University