How Sunlight May Help Keep Blood Sugar in Check

As the days grow shorter and many people find themselves leaving work after sunset, there is a growing health reason to seek out daylight when possible. Sunlight helps set the body’s internal clock, can lift mood, and supports vitamin D production, which is important for bone health and overall well-being. New research suggests it may also play a role in metabolism, with more exposure linked to better blood sugar control.

“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 the connection. “We focus on diet and exercise, but I think exposure to natural light also plays a role.”

Schrauwen helped write a small study published in the January issue of Cell Metabolism in which 13 people with Type 2 diabetes spent several workdays in one of two settings. Some were exposed to daylight through windows, while others had only artificial lighting between 8 a.m. and 5 p.m. Continuous glucose monitoring showed that participants had 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 involving more than 420,000 people in the U.K. found a similar pattern, linking greater sunlight exposure with lower blood sugar. It also reported that people who used tanning beds, which like sunlight emit UV radiation, were significantly less likely to develop Type 2 diabetes.

The researchers analyzed blood-glucose test results from the UK Biobank, a large biomedical database, along with satellite-based estimates of sunlight at participants’ homes, to see whether recent sunlight exposure 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 effect was strongest during the winter months, Dibben says. “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 than those who did not.

The team used tanning-bed users as a rough stand-in for people who spend more time in the sun, Dibben says. In addition to the sun-like light from the devices themselves, earlier research suggests tanning-bed users also tend to get more outdoor sun exposure than non-users. “They enjoy sun-bathing too,” Dibben says. The researchers found that regular tanning-bed users had about a 20% lower lifetime risk of developing Type 2 diabetes than non-users. “This was quite a large effect,” he says.

There are clear downsides to sun and UV exposure. For example, tanning-bed users are more likely than non-users to develop skin cancer. Still, Dibben says he was drawn to the topic after seeing research suggesting that people with more sun exposure, despite a higher skin-cancer risk, also had a lower overall risk of death and disease.

Most of the evidence so far comes from correlation rather than direct proof of cause and effect. Even so, a growing number of studies have connected more sunlight exposure with better cardiovascular health and a lower risk of 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 trying to understand exactly how light influences metabolism, but the circadian system is thought to be a leading factor, Scheer says. This body clock helps regulate blood sugar, body weight, and cardiovascular function.

“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 at night can upset that carefully coordinated system, Scheer says. Research has shown that exposure to light during nighttime hours can raise blood sugar levels.

“In the modern world, we live in a weird lighting environment where we are typically indoors during the day,” Scheer says, referring to the much dimmer indoor environment compared with outdoors. “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 night-time light may activate the sympathetic nervous system, which drives the body’s fight-or-flight response and is not aligned with what the body should be doing at night, potentially disrupting glucose metabolism, Scheer says.

Other biological pathways may also help explain the link between light and metabolism, Dibben says. In laboratory studies, for instance, researchers have found that UVA, a type of ultraviolet light from the sun, can cause nitrite stored in the skin to convert into nitric oxide. Nitric oxide widens blood vessels, which could lower blood pressure and improve blood flow in ways that may help regulate glucose, he says.

As colder months bring earlier sunsets, Dibben says the research increasingly supports a simple takeaway: 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 be sure to use sun protection when you do go out.