How a Darker Bedroom Could Help Prevent Type 2 Diabetes

While lifestyle factors such as losing weight, exercising and eating healthful foods are typically touted as leading ways to help prevent type 2 diabetes, new research reveals there may be another even simpler way: avoiding light at night.

A recent study of nearly 85,000 men and women in the United Kingdom found that those who were exposed to the least amount of light between the hours of 12:30 a.m. and 6 a.m. were significantly less likely to develop type 2 diabetes than those who were exposed to the most light at night.

“We found that exposure to light at night predicted the likelihood of developing type 2 diabetes. The brighter the light at night, the higher the risk,” said Andrew Phillips, PhD, senior author of the report and an associate professor of medicine and public health at Flinders University in Bedford Park, Australia.

Why Nocturnal Light Increases Diabetes Risks

Why would light overnight have any relationship with type 2 diabetes? Nighttime light can interfere with the sleep-wake cycle or circadian rhythm, the body’s internal 24-hour clock, which is vital for a variety of physical functions including blood sugar regulation. The brain’s master clock largely takes its cues from the light around us, and unnatural light or darkness can throw off its finely tuned release of hormones, leading to metabolic dysfunction.

“Light exposure at night can disrupt our circadian rhythms, leading to changes in insulin secretion and glucose metabolism,” Dr. Phillips said, explaining that those changes may in turn affect the body’s ability to regulate blood sugar levels, which can promote the development of type 2 diabetes.

Participants in the study wore a wrist device equipped with a sensor that captured 24-hour light exposure over the course of one week. That gave researchers 13 million hours of light sensor data.

The researchers then tracked study participants over the next eight years to see how many went on to develop diabetes. The data revealed a dose-response relationship between exposure to light at night and type 2 diabetes, with the greatest nighttime light exposure corresponding to the highest risk. Researchers found that people in the top 10 percent of nighttime light exposure had up to a 67 percent greater chance of developing type 2 diabetes than those who had the least nighttime light exposure.

“What we found was that people who had very dim levels (less than 1 lux; below candlelight) were relatively protected from diabetes. Anyone with brighter levels than that had elevated risk of diabetes, with increasing risk the brighter the nighttime light levels were. Already, those who were exposed to an average of 1 to 6 lux at night had nearly a 30 percent increased risk of developing diabetes.” Lux is a measure of light intensity; 1 to 6 lux is about the darkness of twilight in the countryside on a clear evening.

The study controlled for a wide variety of potential confounding factors, including measures of health status and daylight exposure, urbanicity, and sleep duration. Overnight shift workers — who are known to have an elevated risk of diabetes progression and development — were excluded from the study.

The authors of the study concluded that “light at night was a robust predictor of type 2 diabetes for both males and females.”

The opposite was also true: Limiting light at night appeared to reduce type 2 diabetes risk, even in people who had an increased genetic risk of developing the disease.

“The findings are significant, as they suggest that reducing nighttime light exposure in our always-lit society could be an effective strategy to lower the prevalence of type 2 diabetes,” said Christian Benedict, PhD, associate professor of neuroscience at Uppsala University in Sweden, who was not involved in the research. “While this alone won’t counteract an unhealthy lifestyle marked by physical inactivity, poor sleep and junk food consumption, it may be a significant step toward better health, including reducing the risk of type 2 diabetes, a disease that’s increasing worldwide, so accessible preventive measures are crucial.”

Dr. Benedict coauthored a commentary on the new study calling for research into the efficacy of nocturnal light mitigation strategies such as sleep masks and blue light blocking glasses.

Dale P. Sandler, PhD, chief of the epidemiology branch at the National Institute of Environmental Health Sciences, says the latest study affirms her own previous research on the subject: “This study supports and extends previous research, including our own Sister Study of more than 50,000 U.S. women, that links exposure to light at night and poor sleep to poor metabolic health outcomes such as obesity and diabetes. The use of light sensors rather than self-reported data strengthens the findings.”

Dr. Sandler’s earlier study found that women exposed to any artificial light at night at all — whether from a nightlight in the room, light outside the room or light or television in the room — were more likely to gain weight and develop obesity than those who reported sleeping in a dark environment.

The growth of type 2 diabetes rates continues to represent a global public health crisis. As many as 1.3 billion people worldwide could be living with diabetes by 2050, up from 529 million in 2021, making the condition “a defining disease of the 21st century.” While it is unlikely to replace diet, exercise, or weight-loss medication as an important diabetes prevention tool, better overnight lighting habits “may hold some public health promise as an easily implementable strategy,” wrote Benedict.

Many other health issues have been linked to poor sleep quality or duration, including depression, heart disease, and cognitive decline. Greater nighttime light exposure is also associated with a number of psychiatric disorders.

Avoiding bright lights and bright screens, both before bedtime and overnight, is just one of several tips that experts routinely share for a better night’s sleep. Regular exercise and a consistent sleep schedule may also help reset circadian rhythms.

“Our findings suggest that reducing light exposure at night and maintaining a dark environment may be an easy and cheap way to prevent or delay the development of diabetes,” Phillips said.