Now that clocks have fallen back, many Americans face a dreary daily reality: finishing the workday after the sun has set. That could be problematic for health, since sunlight benefits us in many ways. It helps regulate our internal clocks, boosts mood, and supports the production of vitamin D, which is essential for bone and overall health. Emerging research offers yet another reason to soak up the sunshine while you can, linking more sunlight exposure to improved metabolism and lower blood sugar.
“Light is an underestimated lifestyle factor when it comes to metabolism,” says Patrick Schrauwen, a senior scientist at the German Diabetes Center in Düsseldorf who has been studying this relationship. “We focus on diet and exercise, but I think exposure to natural light also plays a role.”
Schrauwen was a co-author of a small study, published in the January issue of Cell Metabolism, in which 13 people with Type 2 diabetes spent several days during working hours in one of two environments. They were either exposed to natural daylight through windows or only artificial lighting from 8 a.m. to 5 p.m. Continuous glucose monitoring showed that people spent more time in the normal blood sugar range when they had access to daylight.
“It was one of the first experimental studies to show that natural light has this effect,” Schrauwen says.
An August study of more than 420,000 people in the U.K. linked greater sunlight exposure to lower blood sugar. The same study found that people who use tanning beds—which, like the sun, emit UV radiation—had significantly lower likelihood of developing Type 2 diabetes.
The researchers used blood-glucose test results from the UK Biobank, a large biomedical database, combined with satellite estimates of sunlight where participants lived, to examine whether sunlight exposure in the days before a blood test influenced glucose levels.
“The main overall finding is that blood glucose was lower if you'd just been through a period of sunnier weather, relative to the time of year, in the two days or seven days before the test,” says study co-author Chris Dibben, a health geographer at the University of Edinburgh in Scotland.
The finding was most pronounced in the winter months, says Dibben. “It makes sense that in the most sunlight-starved time of the year, the effect may be biggest,” he says.
In a separate analysis using the same medical records, Dibben and his colleagues found that people who used tanning beds were less likely to develop Type 2 diabetes compared to people who didn’t.
The researchers chose to study people who frequently use tanning beds as a sort of proxy for people who spend more time in the sun, Dibben says. In addition to getting sun-like light from the devices themselves, past research has shown that people who use tanning beds also get more sun exposure compared to non-users. “They enjoy sun-bathing too,” Dibben says. The researchers found that people who regularly used tanning beds had a roughly 20% reduced risk of developing Type 2 diabetes compared to non-users over the course of their lifetime, Dibben says: “This was quite a large effect.”
There are known downsides to sun and UV exposure. People who use tanning beds are more likely than non-users to develop skin cancer, for one. But Dibben says he was initially drawn to this area of research after seeing research indicating that even though people exposed to more sun had a higher risk of skin cancer, they also had an overall lower risk of dying and disease.
These results are largely drawn from correlational studies, not causative ones. But a growing number of studies have linked more sunlight exposure with improved cardiovascular health and a lower risk for obesity.
“The combination of these studies support the notion that light is important for metabolic and cardiovascular health,” says Frank A.J.L. Scheer, a neuroscientist at Harvard Medical School who studies how the body’s internal rhythms affect health.
Scientists are still working out how light affects metabolism, but the circadian system, which helps regulate blood sugar, body weight, and cardiovascular function, is thought to be a leading factor, Scheer says.
“If we zoom out a little bit, we know that light is the most important signal for our body clock, our circadian system,” says Scheer, who also heads the Medical Chronobiology Program at Mass General Brigham. “The evidence from these recent studies is supporting the idea that daytime light is a signal that can strengthen the circadian system.”
By contrast, too much light during the night can “disturb this carefully coordinated system,” Scheer says, pointing to research that shows how exposure to light during night hours can cause blood sugar levels to increase.
“In the modern world, we live in a weird lighting environment where we are typically indoors during the day,” a much dimmer environment compared to the outdoors, Scheer says. “During the night, we are introducing light where before there was none. We spend an enormous amount of time in front of screens—the TV, phones, computers—close to bedtime.”
Researchers think that light at night could activate the sympathetic nervous system—involved in the “fight or flight” response—which is “mismatched with what we should do at night” and could cause dysregulation of glucose metabolism, Scheer says.
Other biological pathways are also likely involved in how light affects metabolism, Dibben says. In laboratory experiments, for example, researchers have found that exposure to UVA, a type of ultraviolet light from the sun, triggers stores of nitrite in the skin to turn into nitric oxide. Nitric oxide widens blood vessels, which could potentially lower blood pressure and increase blood flow in ways that could improve glucose regulation, Dibben says.
As winter approaches and evenings darken earlier, Dibben says accumulating research supports the idea that people should get outside when they can.
Scheer agrees. “Go outdoors more during the day. If you cannot go outdoors, position yourself near a window, and cut down on artificial light in the few hours before and during sleep," he says.
Just remember to use sun protection when you venture outside.
