A new study finds that outdoor artificial light at night (LAN) is associated with impaired blood glucose control and an increased risk of diabetes. The research was published in Diabetologia by Dr. Yu Xu and colleagues at the Shanghai Institute of Endocrine and Metabolic Diseases, Shanghai, China.
Exposure to artificial LAN at night is a ubiquitous environmental risk factor in modern societies. The intensity of urban light pollution has increased to the point that it affects not only residents of big cities but also those in distant areas such as suburbs and forest parks that may be hundreds of km from the light source. The authors note, "Despite over 80% of the world's population being exposed to light pollution at night, this problem has gained limited attention from scientists until recent years.”
Earth's 24-hour day-night cycle has resulted in most organisms, including mammals, having an inbuilt circadian (roughly 24-hour) timing system which is adapted to the natural sequence of light and dark periods. Light pollution has been found to alter the circadian rhythm of insects, birds and other animals, resulting in premature death and loss of biodiversity.
Artificial LAN has also been implicated as a potential cause of metabolic dysregulation by altering the timing of food intake. Rats exposed to artificial LAN developed glucose intolerance, exhibiting elevated blood sugar and insulin. Another study found that mice exposed to the nocturnal dim white light of minimal brightness for 4 weeks had increased body mass and reduced glucose tolerance compared to animals whose environment was completely dark at night, despite having roughly equivalent energy consumption and expenditure.
Associations have also been found between artificial LAN and health problems in humans. A study of night-shift workers found that those exposed to brighter LAN were more likely to have disrupted circadian rhythms and a greater risk of coronary heart disease. Other research found that higher LAN exposure was associated with a 13% and 22% increase in the likelihood of being overweight and obese, respectively, while exposure to LAN in the bedroom was reported to be positively associated with the development of diabetes in elderly people.
The potential impact of outdoor artificial LAN was revealed by a study in South India which used satellite images to map light pollution and compared this with data on general health markers among adults across the region. With increasing LAN intensity, the exposed population had corresponding rises in average body mass index (BMI), systolic blood pressure and 'bad' (LDL) cholesterol levels.
Participants' body weight and height were measured to calculate BMI, and blood samples were taken to obtain levels of both fasting and postprandial (after meal) serum glucose, as well as glycated haemoglobin (HbA1c). This is a form of glucose bound to haemoglobin in red blood cells which acts as a moving average of blood sugar over the previous 8 to 12 weeks.
The study found that the highest quintile of LAN exposure was associated with a relative increase of 28% in the prevalence of diabetes than in the lowest quintile areas. Chronic exposure to residential outdoor LAN was positively associated with blood glucose levels, insulin resistance and diabetes prevalence and inversely associated with beta cell function, even after adjusting for many important diabetes risk factors. On average, for every 42 people living in regions in the highest quintile of LAN exposure, one more case of diabetes would not have occurred if those individuals had been living in areas in the lowest quintile. While the association between LAN exposure and diabetes might not be as strong as with better-known risk factors, outdoor artificial light's ubiquity means that the population exposure scale is vast.