9 million people develop diabetes, which is strongly associated with this risk factor!

Exposure to outdoor artificial light at night is a common environmental risk factor in modern society, and urban light pollution affects not only residents of large cities, but also residents of suburban and remote areas such as forest parks hundreds of kilometers away from the light source. Although more than 80% of the world's population is exposed to light pollution at night, it is only in recent years that the issue has gained attention.

Under nature's 24-hour light-dark cycle, most organisms, including mammals, have developed an endogenous circadian timing system to adapt to the regular alternation of light-dark phases. Nocturnal exposure to artificial light may inhibit the nocturnal secretion of melatonin from the pineal gland, and animal studies have found that light pollution alters circadian rhythms in insects, birds and other animals, leading to premature death and biodiversity loss.

In addition, studies have found that bedroom exposure to artificial light is positively associated with the development of diabetes in the general elderly population. Diabetes is one of the serious public health problems in China, and the onset and progression of diabetes are mainly attributed to behavioral and environmental risk factors. With the rapid development of urbanization, China's urban artificial lighting has greatly increased, and people living in cities are more likely to change from nature's 24-hour circadian rhythm to all-weather work and contact with artificial light mode. Based on this, it is necessary to assess the extent of artificial light sources that cause or are associated with diabetes in order to implement effective prevention strategies.

To tangle the association between chronic exposure to outdoor artificial light and glucose homeostasis markers and diabetes prevalence, a team from the Department of Endocrinology at Ruijin Hospital affiliated to Shanghai Jiao Tong University School of Medicine found that the higher the exposure to artificial light at night, the higher the incidence of diabetes. The study was based on "Outdoor light at night in relation to glucose homoeostasis and diabetes in Chinese adults: a national and cross-sectional study of 98,658 participants from 162 study sites , published in Diabetologia.

The study included data from 98,658 participants in the NCD surveillance study in China, involving 162 study sites in different geographical regions of 31 provinces, autonomous regions and municipalities directly under the central government, with an average age of 42.7 years. Participants were assessed for BMI and blood samples were collected to measure glycated hemoglobin, fasting and postprandial blood glucose levels, and further assessed participants' insulin resistance index, β cell function, etc.

According to the average outdoor artificial light exposure level at each study site, the researchers divided the participants into five quintiles, which were the 1st percentile array, the 2nd percentile array, the 3rd percentile array, the 4th percentile array, and the 5th percentile array, and the median outdoor artificial light exposure levels at night were 1.0 nWcm-2sr-1, 3.9 nW cm-2sr-1, 7.0 nW cm-2sr-1, 17.0 nW cm-2sr-1, and 69.1 nW, respectively cm-2sr-1。

The researchers found that outdoor artificial light exposure levels at night were positively correlated with impaired glycemic regulation, elevated blood sugar, and increased diabetes risk, with a 7% increased risk of diabetes for every 1 percentile increase in outdoor artificial light exposure. Participants in the 5th percentile array had a 28 percent increased risk of diabetes compared to those in the 1st percentile array, with an estimated 9 million people with diabetes attributable to artificial light exposure.

Based on this nationwide survey of adult populations in China, long-term exposure to outdoor artificial light in residential homes was positively correlated with blood glucose levels, insulin resistance, and diabetes prevalence, and negatively correlated with β cell function. After adjusting for important diabetes risk factors, these associations remained robust. On average, there is one more diabetic person living in one out of every 42 people living in areas with the highest artificial light exposure rate than people living in areas with the lowest quintile of artificial light exposure. The findings are important for assessing the impact of light pollution on the health of Chinese citizens.

In addition, adverse effects of outdoor artificial light exposure on metabolic regulation and disease including obesity, psychiatric illness, and cancer have been found. For example, in an analysis based on a prospective cohort of older adults in Hong Kong, a 60 nW cm-2 sr-1 increase in outdoor artificial light was associated with a 10% increased risk of coronary heart disease.

Exposure to artificial light is so harmful because it can disrupt circadian rhythms, resulting in changes in physiological activity and metabolic variables, including exercise, body temperature, food intake, lipid profile, insulin sensitivity, glucose metabolism, and plasma melatonin, glucocorticoids, and fatty acids. In addition, disturbances in the central clock caused by artificial light exposure lead to disturbances in circadian rhythms in brain regions and disturbances in the interactions between different peripheral tissues.