Early Morning Sun Exposure May Decrease Body Mass Index [Study]

People who get sun exposure before noon tend to have a lower body mass index (BMI) than those who get most of their sun exposure during the afternoon, according to a new study in PLoS One. I don’t know about you, but this announcement made me wonder as I powerwalked down the boardwalk just after dawn as I do most mornings. Is it something about the sun or lifestyle factors-or both-that is behind this curious finding? Or is it something else?

The good people at Northwestern University’s Feinberg School of Medicine, who conducted the study, noted that the lower BMI held true even after they controlled for physical exercise and diet. So those factors did not appear to be the key.

To support their findings, the researchers used a new way to measure the combined effect of the duration, timing, and intensity of sun exposure on a person’s BMI. They tested their method on 54 adults (28 males, 26 females) who averaged 30 years of age. All of them wore a special wrist monitor that recorded sleep duration and timing as well as exposure to sunlight. Records of each participant’s food intake, weight, and exercise also were noted.

According to Dr. Phyllis Zee, a professor of neurology and director of the Northwestern Medicine Sleep and Circadian Rhythms Research Program, for each hour later in the day in which a study participant received most of his or her exposure to light, the end result could be up to an additional 30 pounds of weight on an individual who is 5-foot-8 and who gets late-day sunlight.

Why? The researchers proposed that it has to do with circadian rhythms. That is, light exposure early in the day synchronizes our rhythms, which in turn have an impact on energy regulation and hormone levels (e.g., melatonin), all of which are intimately involved with weight and sleep.

In previous research, investigators studied the impact of early morning light on obese women. They discovered that at least 45 minutes of sunlight between 6 and 9 AM at 1300 lux for three weeks resulted in a reduction in body fat and appetite.

How much light do you need? The threshold noted in the new study was 500 lux, a lux being the power and amount of light. Normal inside room light can range from 100 to 500 lux, while direct sunlight can vary from about 32,000 to 100,000 lux.

Read about the benefits of sunlight on blood pressure

However, it’s not just how long one is exposed to sunlight but also the intensity and pattern of the light. The authors also pointed out that exposure to bright light later in the day has been linked to an increase in weight, so it’s best to get your light in the morning.

Much more research needs to be done in this area to determine the exact effects of sunlight on weight. But for now, it seems that the early bird doesn’t just get the worm; she gets a better shot at keeping off weight!

Image: butz2013