Could light be used as a tool to fight obesity? Researchers at Northwestern University in Evanston, IL, seem to think so. They publish their findings in PLOS One.
The benefits of sunlight on human health are well documented. As well as providing us with necessary vitamin D, exposure to sunlight has also been implicated in preventing infectious disease and even lowering blood pressure.
Recent studies have also suggested that manipulating sleep duration or light exposure can produce alterations in people's metabolic function, appetite and body fat.
One of these studies showed that 45 minutes of morning light between 6-9 am lowered body fat and appetite in obese women over 3 weeks. Another study in obese women found that exercise was more effective at reducing body fat when it was combined with a good quality of light exposure.
In the new study, 54 participants - 26 male and 28 female, with an average age of 30 - wore a wrist actigraphy monitor to measure their exposure to light and sleeping patterns over the course of 1 week. The participants also kept food logs so that the researchers could measure their intake of calories across the 7-day study period.
Timing, duration of exposure, or intensity of light?
Co-author Giovanni Santostasi, a research fellow in neurology at Northwestern University Feinberg School of Medicine, looked for an association between timing, duration or intensity of light and body mass index (BMI). However, no correlation was forthcoming, until he hit upon the idea of combining the three factors (timing, duration, intensity) together into a measure called "mean light timing" (MLiT).
In the mornings we are exposed to a higher amount of what is known as "blue light," which has been shown to have a strong influence on the circadian system.
"I saw that what seemed to be most associated with body mass index was not just how much light you receive but when you get it and for how long," Santostasi explains.
The team found a link between even moderate intensity light exposure (measured as 500 lux and above, or "MLiT500") and BMI. Participants who had most of their exposure to light earlier in the day were more likely to have a lower BMI. For every hour later of MLiT500 during the day, there was an associated 1.28 unit increase in BMI.
"The earlier this light exposure occurred during the day, the lower individuals' body mass index," clarifies co-lead author Kathryn Reid, research associate professor of neurology at Feinberg. "The later the hour of moderately bright light exposure, the higher a person's BMI."
The researchers also found that there was only a relationship between BMI and light exposure for light intensities between 170 lux and 850 lux. Putting this into perspective, in America, normal room light is usually between 200 and 300 lux. In terms of outdoor light, even on a cloudy day, the light is more than 1,000 lux of brightness. This implies that - according to the study - the most beneficial morning light is actually strong indoor light, rather than outdoor light.
Like sleep, light can be modified to help people lose weight
On average, the participants were exposed to light above 100 lux for around 4 hours a day. But exposure to light is something that can be changed to suit the person.
"Light is a modifiable factor with the potential to be used in weight management programs," Reid says. "Just like people are trying to get more sleep to help them lose weight, perhaps manipulating light is another way to lose weight."
But why does early morning light have this influence on BMI? The researchers think it is because light and dark provide signals that regulate our internal body clocks - what is known as "circadian health." And in the mornings we are exposed to a higher amount of "blue light" - sunlight of a shorter wavelength - that has been shown to have a strong influence on the circadian system.
Although this type of study cannot prove cause and effect, it does suggest that light plays a role in regulating metabolism. Further studies will be needed to test this link further.
Until then, it may be worth considering how much light we take in on a daily basis and how modifying this may benefit our overall health.
Senior author Dr. Phyllis C. Zee concludes:
"We focus on how too much light at night is bad; it's also bad not to get enough light at the appropriate time during the day. This is something we could institute early on in our schools to prevent obesity on a larger scale."