Despite the rising number of deaths caused mainly by heart attacks, an increasing number of Nepalese seek work overseas.

The number of Nepali workers going abroad has more than doubled since the country began promoting foreign labour in recent years: from about 220,000 in 2008 to about 500,000 in 2015, mostly in Malaysia, Qatar or Saudi Arabia.

Yet the number of deaths among those workers has risen much faster in the same period.

One out of every 2,500 workers died in 2008; last year one out of every 500 died, according to an Associated Press analysis of data released by Nepal’s Ministry of Labour and Employment.

In total, more than 5,000 workers from this small country have died working abroad since 2008 – more than the number of US troops killed in the Iraq War.

The causes, in many cases, have been mysterious. Natural death, heart attack or cardiac arrest are listed for nearly half the deaths.

But now medical researchers say these deaths fit a familiar pattern: Every decade or so, dozens, or even hundreds, of seemingly healthy Asian men working abroad in poor conditions start dying in their sleep.

It happened in the US in the late 1970s, in Singapore about a decade later and more recently in China. The suspected killer even has a name: Sudden Unexplained Nocturnal Death Syndrome.

Authorities in Nepal say their citizens seem to die abroad more frequently than their equally vulnerable Sri Lankan, Bangladeshi and Indonesian co-workers, but the explanation for the increased mortality has been unclear.

“It’s usually sleeping disease,” said Kumud Khanal, vice president of the Nepal Association of Foreign Employment Agencies, which represents more than 400 registered agents.

“We get the report that he was talking with friends in the evening, had dinner and went to bed, and in the morning he was found dead,” he said

But medical experts say there have been waves of deaths before among young, rural southeast Asian men working abroad in physically stressful conditions.

“I see some familiar patterns here with Nepalese workers,” said Patrick Clarkin at the University of Massachusetts, Boston, who has written about the biology and epidemiology of Sudden Unexplained Nocturnal Death Syndrome.

No one has identified a single cause of SUNDS fatalities – medical journal discussions include genetics, infection and nutritional deficiencies.

And in Nepal, the syndrome is not being considered at this point. Instead, Nepali authorities say it could be stress, even homesickness, brought on by physically demanding jobs in extremely hot climates.

Top export: young men

Nepal exports iron and steel, carpets, some vegetables – but mainly it exports men. It even advertises them.

“Nepalese workers are well known for their hard work, dedication and loyalty,” boasts the Nepalese Embassy website in Doha, Qatar, where a pre-World Cup construction boom employs about 1.5 million migrants.

The Arab Gulf state recently made changes to its labour laws and will be subject to a review by the International Labour Organization in March 2017.

The Nepali workers “are comparatively cost-effective”, says the embassy, and they’re experienced at “working in the extreme climatic conditions”.

Nepali law bans recruiters and employers from charging fees.

Nepal worker returns home to loss and devastation

Some of the countries when the men go to work, including Qatar, also prohibit the fees. But in reality everyone has to pay for these jobs.

The men borrow at 36 percent interest rates from money lenders or sell off family land to get the $1,100 stake needed for recruiters, airline tickets and more.

According to a 2014 report by the Open Society Foundation, “migrant workers commonly encounter a range of abuses during their recruitment in Nepal which makes them more vulnerable to exploitation abroad”.

About 10 percent of Nepal’s 28 million residents are working abroad, sending home more than $6bn a year, amounting to about 30 percent of the country’s annual revenues. Only Tajikistan and Kyrgyzstan are more dependent on foreign earnings.

Some come back maimed or disabled, like Salit Mandal, who rolled off a third-level bunk in Malaysia and smashed in his skull. He is in debt, partially paralysed, and lives with his parents.

“I have no idea what I’m going to do, how I’m going to raise them, because I can’t move,” he says, gazing at his three children.

His family had pinned their hopes on him after he returned from an earlier stint in Qatar with enough money to build a five-bedroom house.

His little brother Jamun Mandal, 24, is next in line. He has abandoned his aspirations of going to college and has paid the recruiting broker. He holds up a passport.

“I know it sounds weird to be planning to go, because people die, disappear, they come back in comas,” he says, “But what to do?”