Obesity, family history, a history of smoking, lack of exercise: most of the major type 2 diabetes risk factors are not controversial. Nevertheless, the fundamental causes and mechanisms of diabetes are still somewhat mysterious, and every once in a while, we learn that there are surprising factors that help lead to diabetes progression and development.
Nocturnal light exposure is one of those sneaky factors. The science shows us that if there’s light in the room where you sleep, it’s not doing your metabolism any favors.
In the last year, three new scientific studies on the connections between metabolic health and light at night have been published:
Light at Night and Diabetes Risks: An Experiment
In July, a team from Northwestern University’s Center for Circadian and Sleep Medicine published the results of an experiment in which 20 healthy adults were exposed to nocturnal light for a single night’s sleep. This was a small but highly controlled laboratory experiment designed to dig into the details of light at night’s metabolic impact.
The results: Overnight light exposure did alter the metabolism of the volunteers, leading to increased insulin resistance, a root cause of type 2 diabetes. Their overnight heart rate was also elevated.
While these healthy individuals did not have higher glucose levels the next morning, their bodies secreted extra insulin during a glucose tolerance test. In short, the pancreas had to work harder just to maintain healthy blood sugar concentrations, a phenomenon that is known to predict the development of type 2 diabetes.
It’s not easy to run similar experiments over long periods of time (who wants to sleep in a laboratory every night for months?), but a rodent study showed that long-term exposure to nocturnal light eventually leads to failure of the pancreatic Beta cells, the second key element of type 2 diabetes progression.
Notably, the participants that were subjected to nocturnal light did not believe that their subjective sleep quality had been affected, suggesting that these metabolic changes can occur without any awareness of sleep disturbances.
Light at Night and Diabetes Risks: A National Survey
In November, Chinese researchers released the results of a very different study, a survey of nearly 100,000 people recruited from 162 different sites across the vast nation. Using satellite imagery, the researchers gauged the amount of artificial outdoor light at night each person was exposed to in their area.
Participants that lived near the highest levels of outdoor light pollution were significantly more likely to have diabetes than those exposed to the least amount of light. This was true even after the authors adjusted for everything they could think of, not just health factors like age and weight but also urban/rural status.
The difference wasn’t immense — the areas with the least light pollution had 41 people with diabetes for every 42 that lived in the areas with the most — but in a population the size of China’s, this has a dramatic effect. The authors estimate that “9 million cases of diabetes in Chinese adults … could be attributed to LAN exposure.”
The authors speculate that disturbing the circadian rhythms with light may lead to a huge array of physiological and metabolic changes, “including locomotor activity, body temperature, food intake, lipid profile, insulin sensitivity, glucose metabolism and levels of plasma melatonin, glucocorticoids and fatty acids.”
Light at Night and Diabetes Risks: A Real-World Study
The Northwestern University team also reported a real-world study on the topic this year. Researchers outfitted hundreds of older adults (63 to 84) with light sensors to find out exactly how much light they were exposed to overnight, and see if light exposure was connected to metabolic dysfunction. They learned that adults that were exposed to any amount of light during the darkest 5 hours of their night were about twice as likely to have diabetes, and nearly twice as likely to have obesity or high blood pressure, as adults that enjoyed complete darkness for those 5 hours.
The study could not determine if nocturnal light caused these metabolic dysfunctions — it is plausible that the causation goes in the opposite direction, and that people with diabetes, obesity, and hypertension are more likely to turn on a light in the middle of the night. But the connection may still be enough to convince experts that nocturnal light is a real problem.
An editorial written in response to this third study called for “more widespread recognition of the importance of light exposure at night for metabolic and cardiovascular health.”
Nocturnal light exposure is an issue that reaches far beyond the diabetes community. Other work has suggested that nocturnal light exposure is also related to obesity, depression and suicidal behavior, pancreatic cancer, and coronary heart disease.
Dr. Phyliss Zee, a co-author of both Northwestern sleep studies, stated, “It’s important for people to avoid or minimize the amount of light exposure during sleep.”
For many people, luckily, the solution will be easy. The authors of the editorial noted that,
Window shades and dark curtains can be used to reduce or eliminate light from outside the home, while turning off light sources inside the bedroom, and/or wearing a sleep mask are relatively easy and inexpensive ways to reduce [light at night.]
Nocturnal light exposure is a sneaky diabetes risk factor. Three very different studies in the past year have agreed that light at night is associated with several significant metabolic issues, including insulin resistance and type 2 diabetes.
While it is increasingly clear that sleep health is vital to diabetes health, this one specific aspect of good sleep hygiene may deserve special attention.
It may be worthwhile to take your own nocturnal light exposure seriously. Turn off your television, dim your nightlight, buy blackout curtains, or wear a sleep mask: Any of these could be simple ways to get your circadian rhythms and overall health back on track.