MANILA: It creeps into bedrooms in the dead of night and steals the souls of healthy young men in Asia.

"Lai tay," or sleep death, killed more than 5,000 Thais between 1981 and 1997, researchers say.

In the Philippines it is called "bangungot", literally nightmare.

Experts are divided on the cause, though they agree there is no fool-proof way to prevent it.

Some doctors have drawn comparisons with crib deaths among infants in the West. The Philippines does not keep records of cases, but the stealthy and mysterious killer otherwise known as "Sudden Unexplained Nocturnal Death Syndrome" claimed a famous victim during the Easter holidays.

Rico Yan, a popular 27-year-old movie and television actor, died in his sleep at a western Philippines resort, breaking the hearts of millions of fans.

Tens of thousands lined his funeral route Thursday.

"People who develop this syndrome are known to growl (in their sleep), and eventually they just die," Philippine Health Secretary Manuel Dayrit said.

It afflicts "healthy males, usually Filipinos of course, and Asians," he said.

But "nobody knows why it happens and how it happens." The Thai Public Health Ministry is conducting research to pinpoint the real cause, while Dayrit said the Philippines Health Department plans to set up a registry to receive case reports and "maybe to get a piece of epidemiologic information."

Somporn Triamchaisri, a researcher at the Faculty of Public Health of Thailand's Mahidol University said: "At the moment we suspect it comes from a genetic predisposition." The AIMNE gene, which controls sleeping patterns, is believed to be at the root of the disease, she said.

Somporn said the disease occurs widely among Thais, Filipinos, and Laotians, but particularly among hill tribe minorities who have resettled overseas.

Asked about the cause of the syndrome, World Health Organization regional director Shigeru Omi told AFP here: "We don't know."

After World War II, city officials in Manila formed a committee to study the syndrome and autopsied all suspected victims, Dayrit said: "They found that 50 percent of people with this syndrome had acute pancreatitis."

Anthropologist Michael Tan, who writes a medical column in the Philippine Inquirer newspaper, said that US Navy doctors had studied the phenomenon as early as the 1950s among sailors of Filipino origin.

They "suggested the deaths were due to pancreatitis resulting from the Filipino salty diets," he said.

Acute pancreatitis, the verdict in the Rico Yan police autopsy, involves blockage of the pancreatic duct, causing the digestive enzymes it produces to destroy the pancreas itself.

Dayrit stressed that experts have not established a conclusive link.

"Acute pancreatitis happens in all nationalities, all races. Bangungot' is a specific event where 50 percent have acute pancreatitis. It is not equal to acute pancreatitis," Dayrit said.

Tan said "investigations of similar deaths among Thai workers in Singapore in the 1990s also proposed dietary examinations, although in this case the doctors said it was a nutritional deficiency."

Some studies in Thailand have even postulated this could be caused by the fish sauce in the local diet, or eating rice, Dayrit added.

However, Tan said "there are other cultures - Thai, Cambodian, Japanese - with similar conditions, and it's striking that in all of them, just like the Philippines, most of the nightmare victims are male. This raises questions about relying on a dietary explanation alone."

Arrhythmia, a heart condition, or even an acute respiratory distress syndrome, have also been cited as possible causes, Dayrit said, though he acknowledged that victims usually had no history of heart problems.

As it is known that some victims had drank alcohol and had hearty meals shortly before going into their sleep of death, Dayrit said Asian men should "avoid having too heavy meals at night, and avoid going to sleep immediately after that." - AFP