Specifically, having a majority of the mean light exposure above 500 lux (MLiT500) earlier in the day was associated with having a lower BMI. Note that lux is a unit for illuminance, which qualitatively means brightness. To place this number in perspective, the normal light in a room is roughly between 150 and 500 lux (2).
The results support the idea that the relationship between light and BMI is not particularly dependent on the number of minutes of light exposure received in a day, but rather more dependent on the timing pattern of light exposure above a certain threshold. The influence of the light on weight was independent of physical activity, caloric intake, sleep timing, age, and season. The influence of light likely occurs through its effect on the human body’s internal clock and sleep, both of which in turn has been shown to affect weight. In essence, not getting enough light at the right time of day can desynchronize the internal body clock and thus alter metabolism and lead to weight gain (1,2).
The participants were adults without any major unstable health conditions. They were recruited through advertisements for a study of circadian rhythms and sleep patterns. In total, there were 54 participants, 26 female and 28 male, who had an average age of 30. For seven days, they wore a wrist monitor that measured their light exposure and sleep timing and duration under normal living conditions. Caloric intake during those seven days was measured through food logs (1,2).
While duration and timing of sleep were not linked to the results found in the study, those who stay up later in the night and wake up later in the day would experience less exposure to bright morning light. Similarly, even people who wake up early yet stay inside would also suffer from lack of early light exposure (1,2).
Thus, the study suggests that getting up earlier to receive morning light exposure can promote a healthier body weight. On an institutional level, workplaces and schools might consider having more windows and improving indoor lighting. According to Zee, “This is something we could institute early on in our schools to prevent obesity on a larger scale.” Overall, this study offers society new information on how people can pursue a healthier lifestyle by making slight adjustments to their daily schedules (1).
(1) M. Paul, Morning rays keep off pounds (April 2 2014). Available at http://www.sciencedaily.com/releases/2014/04/140402212531.htm (4 April 2014).
(2) Reid KJ, Santostasi G, Baron KG, Wilson J, Kang J, et al. (2014) Timing and Intensity of Light Correlate with Body Weight in Adults. PLoS ONE 9(4): e92251. doi:10.1371/journal.pone.0092251. Available at http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0092251 (4 April 2014).