There has been a surge in the number of healthy young men in China dying their sleep over the last 10 years.
Researchers at the Zhongshan School of Medicine at Sun Yat-sen University, based in Guangzhou, found that hundreds of young men living in the manufacturing area of Dongguan have died from unexplained nocturnal death syndrome over recent years.
The paper, Epidemiological Study of Sudden Unexplained Nocturnal Death Syndrome, found that there were 893 cases of nocturnal sudden death syndrome in Dongguan between 2001 and 2013.
In comparison, between January 1990 and December 1999, there were just 231 cases.
Researchers analysed police records of deaths in the area over the last 20 years. Unexplained nocturnal sudden death occurs when a healthy individual dies in their sleep and the autopsy shows no fatal disease or injury. Normally, people experience difficulty breathing just before dying.
The researchers found that over 90% of the victims were factory workers – Dongguan is a major production base and is sometimes known as the "factory of the world", with the city producing construction goods, chemicals, electronics, home appliances, metal, furniture, garments, shoes and toys.
Findings showed that 93% of the victims were male, with most aged between 21 and 40.
Lead author Cheng Jianding told China's Xinhua news agency: "Sudden unexplained nocturnal death syndrome mainly happened to workers with high labour intensity and poor education."
Zhang Yiri, an associate professor at the Guangzhou City Polytechnic, said the increase in deaths could be due to the city's payment system that encourages people to work overtime.
"Many bosses pay very low base salaries to workers. But the payment for overtime work is higher. Therefore, many people apply for overtime to earn extra money," he said.
Zeng Feiyang, the director of the Guangdong Panyu Migrant Worker Centre in Guangzhou, told the South China Morning Post that worker's rights groups have been worried about the sudden deaths for years.
He said long working hours, poor sanitation and ventilation could increase their risk of sudden nocturnal death: "It is especially hard to help the victims get compensation since the concept of overwork has not been recognised by Chinese law," Zeng said.
"Factories owners could easily argue against the accusation, saying the victim's colleagues were working for the same number of hours a day healthily."