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.