Good Day Sunshine: Could Morning Light Help Keep Us Lean?

Exposure to morning light, whether it's pure sunlight or bright indoor lighting, is associated with leaner body weights, researchers say.

The findings fit with a growing body of evidence that suggests keeping our internal body clocks synchronized with the natural light-dark cycle is beneficial to our health and our waistlines.

To better understand this relationship, researchers at Northwestern University recruited 54 adults (average age 30) from the Chicago area and had them wear wrist monitors that tracked their exposure to light and sleep patterns for a week. The participants recorded what they were eating in daily logs so the researchers could estimate caloric intake.

"We found that the earlier this light exposure occurred during the day, the lower individuals' body mass index," says study author Kathryn Reid, a research associate professor at the Northwestern University Feinberg School of Medicine.

That was true even when the researchers accounted for activity level, caloric intake, sleep timing, age or season. The people with the strongest correlation between light and BMI were exposed to more than 500 lux of light, equivalent to bright indoor lighting. Outdoor light can range from 1,000 lux up into the hundreds of thousands.

The study was published Wednesday in the journal PLoS One.

So there may be another reason my early morning jogs make me happy, beyond the release of endorphins.

"We weren't necessarily surprised by the findings," Reid told me, since she'd hypothesized that exposure to light plays a role in regulating metabolism. She points to animal studies in mice where disrupting the natural light-dark cycle with long periods of exposure to light leads to weight gain.

People should get more bright light between 8 a.m. and noon, the study authors say. They think about 20 to 30 minutes of morning light is enough to affect BMI (body mass index).

Now, to be clear, the relationship between morning light and a healthy body weight has not been nailed down as cause and effect.

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Good Day Sunshine: Could Morning Light Help Keep Us Lean?