The surprising results suggested that the earlier light exposure occurred, the lower the body mass index
(BMI). The researchers point out that many people do not get enough natural light in the morning. Another concern is that people work in poorly lit environments, usually about 200 to 300 lux. In the study, 500 lux was "the magic number" for having a lower BMI. The report points out that even on a cloudy day, outdoor light is more than 1,000 lux of brightness. It is difficult to achieve this light level with usual indoor lighting. Lux
is a unit of illuminance.
The researchers drew their conclusions by correlating masses of data about people’s BMIs with their lifestyle’s. What they found was that body mass index was associated not just how much light you receive but when people got it and for how long.
To explore this further, a study was run with 54 participants (26 males, 28 females), with an average age of 30. The participants wore a wrist actigraphy monitor
(a non-invasive method of monitoring human rest/activity cycles) that measured their light exposure and sleep parameters for seven days in normal-living conditions. Their caloric intake was determined from seven days of food logs.
The study showed that the influence of morning light on weight was independent of physical activity, caloric intake, sleep timing, age or season. The researchers argue that the finding emphasizes the importance of "circadian health" in which exposure to light and dark is synchronized with your internal body clock.
The study was carried out by Northwestern medicine
and it has been reported to
the journal PLOS One
, in a paper titled “Timing and Intensity of Light Correlate with Body Weight in Adults”.