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Even after the researchers controlled for other potentially slimming factors such as caloric intake, sleep duration/timing and the possibility that people getting early light exposure might be more active, light exposure still seemed to account for 20 percent of BMI, according to the researchers.

Does that mean people who live in sunnier climates would be thinner than their colder counterparts? Maybe, said Zee, though they didn't include such a comparison in the current study. "People tend to lose more weight in the summer, when you're getting more light earlier in the day," she added.

Zee recommended between 20 minutes and 30 minutes of bright morning light between 8 a.m. and noon.

"Whenever possible, be exposed to early light," she said. "Walk to work if you can. Bright, outdoor light will be way above the 500 lux. If you can't get outside, work near a window. If you can't get near a window, at least make sure your work environment is well-lit," Zee added.

Jaclyn London is senior clinical dietician at Mount Sinai Hospital in New York City. She said that the "researchers raise an interesting question, but the study is very small and relied on self-reporting of diet and weight. There was no study intervention either, so they weren't able to prove causality," London noted.

"However, I do think there's some evidence here to suggest that the impact of light may influence metabolic changes and possibly play a role in BMI. And, with 67 percent of Americans being overweight or obese, it's certainly an intervention worth looking at," London said.

The study was published April 2 in the journal PLOS One.

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