Study: Timing of Light Exposure Affects BMI

The timing of exposure to moderate levels of light could have an impact on body mass index (BMI) as well as body fat, according to a new study.

In a study of 23 healthy adults, Ivy N. Cheung, a doctoral candidate in the interdepartmental neuroscience program at Northwestern University, and colleagues found that individuals with more exposure to moderate or more intense light earlier in the day demonstrated a lower BMI and a lower percentage of body fat in comparison to those who had been exposed to moderate or higher intensity light exposure later on in the day.
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This finding provides “further support that changes to environmental light exposure in humans may impact body weight regulation,” says Cheung. “In terms of helping patients establish and maintain a healthy body weight, primary care practitioners may advise patients to take into account their environmental light exposure.”

In the study, the majority of participants were female, with a mean age of 26, and a mean body mass index of 29. Patients wore a wrist monitor for 7 days to determine light patterns. In addition, the investigators measured the total percentage of body fat for 17 participants using dual axis absorptiometry (DXA). Light data was binned into 2-minute epochs and smoothed using a 10-minute moving average, and then aggregated over 24 hours for each participant. Height and weight were objectively measured to determine BMI, according to the authors.

These results “emphasize the importance of getting the majority of exposure to moderate or higher intensity light during the morning, as well as limiting light exposure in the evening hours,” says Cheung. “Patients should make sure to get morning light, perhaps with a walk or eating breakfast outside, and should make sure to dim lights and limit use of light-emitting devices in the evening.”

The study findings were presented at the 29^th annual meeting of the Associated Professional Sleep Societies, held June 6-10 in Seattle, Wash.

—Mark McGraw