Streetlights and smartphones can raise the risk of diabetes by more than a quarter, new research has found.
Artificial light at night alters the human body clock, thereby impairing the body’s ability to control blood sugar levels.
Among the lights which could have a harmful effect are street, car and shopfront lamps as well as the light from smartphones and tablets.
The research was based on almost 100,000 men and women in China who were exposed to artificial light while it was dark outside.
Those exposed the most were 28 per cent more likely to develop the metabolic condition because of the interference the light had on the body’s production of melatonin - the hormone that helps regulate our circadian rhythms.
Evidence is growing that 24/7 lifestyles are wreaking havoc with our health by interfering with production of melatonin.
Chronic exposure to residential outdoor artificial light was linked to a rise in glucose levels, insulin resistance and diabetes prevalence.
It was also associated with poorer function of beta cells which control blood sugar by releasing insulin, a hormone stored in the pancreas.
Lead author Dr Yu Xu, of Ruijin Hospital, Shanghai, said: "Despite over 80 per cent of the world's population being exposed to light pollution at night, this problem has gained limited attention from scientists until recent years."
The study in Diabetologia estimated more than nine million cases of diabetes in Chinese adults can be attributed to it.
The researchers used data from the China Noncommunicable Disease Surveillance Study, a representative sample of the general population across 162 sites across the country.
A total of 98,658 participants underwent interviews to collect demographic, medical, household income, lifestyle, education and family history information.
Body weight and height were measured to calculate BMI, and blood samples were taken to obtain levels of both fasting and after meal glucose.
Participants were assigned an average artificial outdoor light exposure level for that location using satellite images. They were divided into five groups based on highest to lowest exposure.
On average, for every 42 people living in regions with the most light pollution, there was one more case of diabetes.
Dr Xu said: "Exposure to artificial light at night is a ubiquitous environmental risk factor in modern societies."
More than 99 per cent of people in the US and Europe live under light-polluted skies.
Earth's 24-hour day-night cycle has resulted in most organisms, including humans, having an inbuilt circadian timing system.
It is adapted to the natural sequence of light and dark periods, but light pollution has been found to alter the circadian rhythm of insects, birds and other animals, resulting in premature death and loss of biodiversity.
Dr Xu said the findings “contribute to the growing evidence that light at night is detrimental to health” and “points to outdoor LAN [light at night] as a potential novel risk factor for diabetes".
Associations have previously been found between light pollution and health problems in humans.
A study of night-shift workers found those exposed to brighter light were more likely to have disrupted circadian rhythms, as well as a greater risk of coronary heart disease.
Other research found that higher light exposure was associated with a 13 and 22 per cent increase in the likelihood of being overweight and obese, respectively.
He added: "The positive associations indicate an urgent need for countries and governments to develop effective prevention and intervention policies and to protect people from the adverse health effects of light pollution at night."
Type 2 diabetes, the form linked to unhealthy lifestyles that comes on in adulthood, affects around 4.5 million Britons.
Around five million people in the UK have diabetes - with 90 per cent of cases the type 2 form caused by unhealthy lifestyles. Obesity raises the risk sixfold.
