Correlation is not causation, but participants in a study who got exposure to bright light earlier in the day tended to have a lower body mass index than others.
The new study, published in PLoS ONE, looked for correlations among caloric intake, timing and duration of daily habitual ambient light exposure, sleep timing and duration, and body mass index (BMI). The researchers hypothesized that the intensity and timing of light exposure, especially early in the day, would be associated with a person's BMI, independent of sleep patterns. The data bore out this hypothesis.
"For every hour later in the day that you reach 500 lux that translates to an increase of 1.28 BMI," co-author Dr. Phyllis Zee told Healthday.com. "The earlier the light exposure, the lower the BMI."
Data from 56 study participants was used; slightly less than half the participants were men. The average age was 30 with a standard deviation of 11 years. Participants wore wrist monitors to track their activity and light exposure for an average of seven days. They kept logs of their food intake and sleep times. (Those times were combined with the activity monitors to calculate the start, midpoint, and end of sleep). BMIs were calculated from participants' self-reported height and weight. The researchers said that, for the subjects whose data were included, "there was fairly even distribution of data collection during all four seasons." Applicants who scored low on a test for depression were not included in the study.
The study defined a metric, MLiT500, that represented the average clock time of all aggregated data points for which the subject was in ambient light above 500 lux. The researchers actually calculated MLiT values for light levels from 100 to 1,000 lux, but they found that only MLiT500 was significantly correlated with BMI.
The image above shows a representative study participant's light exposure over seven days, with data taken every two minutes and smoothed to 10-minute intervals. The figure below shows a participant's seven days of light data on a single time-of-day graph (log scale), with MLiT500 marked by a ★.
Log linear light plot. Individual light data (lux) for up to seven days is plotted by time of day. The smoothed average is shown as a bold line; the 500-lux threshold is shown as a continuous horizontal line.
(Source: Reid et al., PLoS ONE)
What it means
This study does not claim that, if you get up earlier and get out into the sunlight, you will necessarily lose weight. What it does claim is that those habitually exposed earlier in the day to bright ambient light tend to have a lower BMI than others. We know that light exposure affects circadian levels of the sleep-related hormone melatonin. It seems that light also affects other hormones related to metabolism and eating activity, and that bluer (early) sunlight is particularly effective in this regard.
As we study human-centric lighting to enhance well-being via solid-state lighting, we might want to pay attention to the blue content and illuminance values of early-morning light.
Zee has these recommendations:
Whenever possible, be exposed to early light. 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.
— Keith Dawson , Editor-in-Chief, All LED Lighting