In May, Rhett Lamb, a 3-year-old boy from St. Petersburg, Fla., did something nobody thought he could: sleep.

Lamb, who suffers from an apparently unique strain of an already rare neurological condition called Chiari Malformation, had never slept for more than an hour or so in a single stretch since he was born. In many cases, the condition causes severe cycles of sleeplessness separated by periods of chronic fatigue. Unfortunately, Lamb only experienced the first half of the cycle. After braving the risks of experimental surgery in May, Lamb was finally able to get a decent night's rest, according local news reports.

Not everyone is so lucky. In 2005, sleep disorders caused 684 deaths in the U.S., according to the U.S. Center for Disease Control's Compressed Mortality Data. In many cases, how and why sleep disorders kill people is largely a medical mystery.

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The surreal deaths of more than 100 healthy adults in the U.S., primarily in Minnesota, are perhaps the most mysterious of all. Since 1977, more than a hundred Southeast Asian immigrants in the U.S., primarily ethnic Hmong from Laos, have died from a mysterious disorder known as Sudden Unexpected Nocturnal Death Syndrome, according to reports by the U.S. Center for Disease Control. The victims were mostly men in their 30s or older, who were apparently in good health when they died in their sleep for no apparent reason.

"The victim has no known antecedent illnesses, and there are no factors that might precipitate cardiac arrest," the Cambridge History of Disease notes. "At autopsy, no cause of death can be identified in the heart, lung or brain. Postmortem toxicologic screening tests reveal no poisons."

The syndrome remains a medical mystery. Shelley Adler, a professor of integrative medicine at the University of San Francisco, California School of Medicine, speculates that the cataclysmic psychological stress caused by war, migration and rapid acculturation created such wrenching nightmares among Hmong refugees that they died. In other words, nightmares killed them.

The Hmong refugees who survived long enough to migrate to the U.S. have more than ample fodder for nightmares. During the Vietnam War, the U.S. Central Intelligence Agency used the Hmong to wage a proxy war against communist forces in the hills of Laos. But like several other rare diseases, nobody really knows why southeast Asian refugees die from SUNDS.

Take Fatal Familial Insomnia, a rare genetic sleep disorder that affects fewer than 50 families around the world, according to the Merck Manual of Medical Information.

A healthy person in his mid-50s has trouble sleeping one otherwise normal night. The next night is worse, and the next is worse still. Days turn to weeks and sleep regimens shrink to less than an hour daily. The person's pupils shrink to a tiny size. Men become impotent. As weeks turn to months, the ability to sleep disappears entirely. A rare gene they inherited from their ancestors tricks their brain into thinking the body is always awake. They can close their eyes or lie down to rest, but they will literally never "sleep" again, according to a multiple case studies, including one by Ann Akroush in Case Studies in Virtual Genetics.

Hallucinations and paranoia begin to take hold and eventually deteriorate into a state of dementia, according to the Merck Manual. Soon, they slip into a coma-like fog and, mercifully, die. The whole process can take as little as seven months or as long as three years. There is no known cure.

Sleepwalking as a defence

For some, sleep itself can be dangerous. In May 1987, a 23-year-old man named Kenneth Parks stumbled into a police station in blood-spattered clothes near Toronto muttering "I think I have killed some people with my bare hands," according to a case study by Canadian sleep expert Roger Broughton. Parks was right. Hours earlier, he had bludgeoned his mother-in-law and father-in-law with a tire iron and then stabbed them with a butcher's knife after breaking into their house in suburban Toronto, according to Broughton.

When his murder trial began, Parks admitted to killing his mother-in-law and trying to kill his father-in-law (who miraculously survived the encounter) but said he wouldn't have done so if he had been awake at the time. Parks was the first defendant to claim he shouldn't be held responsible for what he does while sleepwalking because he couldn't willfully control those actions. A Canadian jury agreed with him.

In the years since Parks was acquitted of murder in 1988, dozens of defendants have argued with mixed success that they are innocent of murder on grounds of "homicidal sleepwalking," according to Mark Pressman's study, "Disorders of Arousal From Sleep and Violent Behavior: The Role of Physical Contact and Proximity," published by the Associated Professional Sleep Societies in 2006. Pressman is a sleep specialist at Lankenan Hospital in Wynnewood, Pa.

A similar sleepwalking defence has emerged in sexual assault cases. In 2003, Canadian researchers coined "Sexsomnia" in a paper called "Sexsomnia — A New Parasomnia?" published in the Canadian Journal of Psychiatry as the name of a rare form of automatism in which people carry out sexual acts in their sleep.

The researchers cited the case of an Australian woman who would walk out of her house while asleep and repeatedly have sex with strangers as an example of the condition. In the past three years, courts in Canada and England have acquitted defendants accused of rape who allegedly suffered from "sexsomnia," according to news reports.