Dr. Yu Xu and colleagues from Shanghai Jiaotong University School of Medicine, Ruijin Hospital, and the Shanghai Institute of Endocrine and Metabolic Diseases in Shanghai, China, conducted the study.
An increase in the risk of developing diabetes and poor blood glucose control are linked to exposure to outdoor artificial light at night (LAN), according to a new study that was just published in Diabetologia, the journal of the European Association for the Study of Diabetes (EASD). LAN exposure is responsible for more than 9 million cases of diabetes in Chinese adults.
In contemporary communities, exposure to artificial LAN during night is a pervasive environmental risk factor. Urban light pollution has become so severe that it no longer just affects people who live in large cities but even those who live in far-off places like suburbs and forest parks, which may be hundreds of kilometres from the light source. Most species, including mammals, have an innate circadian (approximately 24-hour) timing system that is suited to the natural sequence of light and dark phases thanks to Earth’s 24-hour day-night cycle. It has been discovered that light pollution affects the circadian rhythm of insects, birds, and other species, causing early death and a decline in biodiversity.
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.”
Artificial LAN-exposed rats showed signs of glucose intolerance, including increased insulin and blood sugar levels. By changing the timing of food intake, artificial LAN has also been linked as a potential contributor to metabolic imbalance.
Despite having about equal energy intake and expenditure, a different study indicated that mice exposed to nocturnal dim white light of low brightness for 4 weeks had increased body mass and decreased glucose tolerance compared to animals whose environment was completely dark at night. Additionally, links between artificial LAN and human health issues have been discovered. According to a study of night shift employees, individuals who were exposed to brighter LAN had a higher chance of developing coronary heart disease as well as altered circadian rhythms.
While exposure to LAN in the bedroom was shown to be positively associated with the onset of diabetes in elderly persons, other research indicated that increased LAN exposure was connected with a 13% and 22% rise in the risk of becoming overweight and obese, respectively.
An earlier research done by Indian scientists in southern Indian region that used satellite photos to track light pollution and combined this with information on general health markers among individuals in the region highlighted the possible effects of outdoor artificial LAN. Average body mass index (BMI), systolic blood pressure, and “bad” (LDL) cholesterol levels all increased in the exposed population in direct proportion to rising LAN intensity.
In China, diabetes is a serious public health issue, and environmental and behavioural risk factors are primarily responsible for the disease’s genesis and progression. Urban illumination and the number of individuals exposed to it have dramatically increased as a result of the nation’s rapid urbanisation and economic expansion. For those who live in cities, the natural 24-hour cycle of day and night is more likely to be replaced by a 24-hour cycle of work and play, with frequent late-night outings and exposure to artificial LAN.
The study made use of information from the China Noncommunicable Disease Surveillance Study, which was conducted in 2010 at 162 sites across China and included a representative sample of the general population.
A total of 98,658 persons took part in the study and underwent interviews to provide data on their demographics, health, household income, way of life, education, and family history. Participants had an average age of 42.7 years, with almost 50% of them being female. Participants’ height and weight were evaluated to determine BMI, and blood samples were obtained to measure glycated haemoglobin, fasting and postprandial (after meal) serum glucose levels (HbA1c). A moving average of blood sugar over the past 8 to 12 weeks can be calculated using this type of glucose linked to haemoglobin in red blood cells.
Using nighttime low-light image data of the Earth’s surface from the US Defense Meteorological Satellite Program, participants at each study site were given an average simulated outdoor LAN exposure level for that area (DMSP). The median light intensity in the highest quintile was 69 times larger than in the lowest, and exposure levels were arranged from lowest to greatest and divided into five quintiles (groups of 20% from highest to lowest).
China’s outdoor LAN intensity varied greatly, with the majority of the country being exposed to low-intensity light and the eastern coastline cities receiving greater intensities. Participants were more likely to be older, had higher BMIs and household incomes, and reside in metropolitan locations if they lived in areas with higher outdoor LAN quintiles. The lower quintile groups, on the other hand, reported higher levels of physical activity but lower levels of education.
According to the study, areas with the highest quintile of LAN exposure had a relative 28% higher prevalence of diabetes than those with the lowest quintile. Even after controlling for a number of significant diabetes risk variables, chronic exposure to residential outdoor LAN was negatively linked with beta cell function and favourably associated with blood glucose levels, insulin resistance, and diabetes prevalence.
In locations with the highest quintile of LAN exposure, there is typically one additional case of diabetes for every 42 residents, which would not have happened if they had resided in areas with the lowest quintile.
Although the link between LAN exposure and diabetes may not be as strong as with other, better-known risk factors, the prevalence of outdoor artificial light implies that the population as a whole is exposed to a significant amount of LAN.
According to the researchers, exposure to outdoor LANs may be responsible for more than 9 million cases of diabetes in Chinese adults over the age of 18; this number is expected to rise in light of China’s accelerating urbanisation and the rising number of people moving from the country to its cities.
The fact that more than 99% of people in the US and Europe and an estimated 83% of the world’s population live beneath light-polluted skies highlights the scope and global significance of this issue. These findings show that LAN may be a potential unique risk factor for diabetes and add to the growing body of evidence that LAN is unhealthy.
To determine if LAN’s association with diabetes is causative, the authors state that “additional investigations incorporating the direct measurement of individual exposure to LAN are needed.”
Parvathy Sukumaran 
