A Northwestern University study finds a link between a person’s weight and the timing and duration of bright-light exposure.

Are you a morning person who soaks up the early-day’s rays? According to a Northwestern University study, people who are exposed to bright morning sunlight have a lower body mass index (BMI) than those who are not.

The study linked a person’s weight to the timing and duration of bright-light exposure. People who were exposed to even moderately bright light in the morning had lower BMIs than those who were exposed to light later in the day. The study was published in PLOS ONE and was performed on 54 adults who wore actigraphy monitors to track their levels of rest and activity.

The subjects with the lowest BMIs were those who were exposed to light earlier in the day, said Dr. Phyllis C. Zee, senior study author and director of the Circadian Rhythms and Sleep Research Program at Northwestern’s Feinberg School of Medicine. She recommends soaking up bright light from 8 a.m. to noon, and said that 20 to 30 minutes of morning sun is enough to affect a person’s BMI.

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Morning-light exposure accounts for about 20 percent of a person’s BMI, according to the researchers. Other factors that affect BMI are sleep timing, age, season of the year, and—most importantly—physical-activity level and caloric intake.

Zee said that lack of exposure to light at the right time of day can desynchronize the body’s internal clock, causing weight gain and affecting metabolism. How, precisely, light exposure affects body fat is a topic that needs more research, Zee added.

Zee said that many people work in poorly lit settings that have about 200 to 300 lux (a measure of luminosity), but they need about 500 lux to lower their BMI. Cloudy skies are just as good for getting sun exposure—they can offer more than 1,000 lux of brightness. Outdoor light is best; it’s tough to replicate the same results with indoor lighting.

“Outdoor light is the brightest. Natural light through a window is also good,” Zee said, adding that spending time outdoors or having a workspace with windows—or even bright artificial lights—can enhance daytime light exposure.

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Giovanni Santostasi, a research fellow in neurology at Northwestern and co-lead author of the study, came up with a way to use the timing, duration, and intensity of light exposure to calculate a figure known as mean light timing (MLiT).

When Santostasi combined those three factors, he was able to identify strong correlations between light and BMI.

“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 in a statement.

Zee was careful to note that you don’t want to decrease the amount of sleep you get just to fit in more sun exposure. Night owls who sleep through the morning are exposed to sun later in the day, but those who are up during the early-morning hours won’t get all the benefits of decreased BMI unless they still get adequate sleep.

“Exposure to light in the morning can help advance the timing of circadian rhythms, which will make it easier to wake up in the morning, and also improve sleep quality and metabolism,” she said.

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