In 2013, there were reports that the inhabitants of a small rural town called Kalachi in northeast Kazakhstan were succumbing to a mysterious illness. No one knew what it was – or what caused it – just that the victims would start to slur their words, sway their bodies, and see in double vision before falling into a deep sleep that could last days. Other than brief moments of blurry consciousness to eat, to smoke, to go to the bathroom, the ill were almost comatose. When they eventually awoke from their stupor, patients were completely unaware the entire episode had even happened.

The first person to fall ill was a middle-aged woman called Lyubov Belkova. She was working at a market in April 2010 when she was suddenly overcome with an intense drowsiness. Four days later, she woke up in a hospital where she was told she had had a stroke.

Soon afterward, several others were struck by this strange "sleeping sickness". One time, five women fell ill all at once. Misha Plyukhin, a child, saw hallucinations. There was even an incident involving an animal, Marquis the cat, who was affected by a "foolishness" before he fell asleep and started snoring "like a man".

But Kalachi (which has since been nicknamed “Sleepy Hollow”) was not the only place touched by this inexplicable sleeping sickness. Citizens of a neighboring town called Krasnogorsk similarly noticed an uptake in a mysterious condition that hit victims with an overwhelming urge to sleep. Krasnogorsk, now a ghost town, had once been home to the miners of nearby uranium mines, which had been left abandoned following the fall of the Soviet Union in the 1990s. 

Over the next few years, more than 140 incidents were reported between the two towns. In some cases, a patient would fall ill with sleeping sickness multiple times. Yet, the residents of Kalachi and Krasnogorsk could not work out what was causing their mystery illness. Was it alcohol or drugs? Some speculated it could be a government conspiracy.

As the cases started to rack up, authorities began to look into the matter, conducting environmental surveys and viral and bacterial tests on patients. At first, they suspected counterfeit vodka. Later, they thought it could be an example of mass hysteria, the same condition that caused the dancing plague of 1518, the laughing epidemic of 1962, and the Bin Ladin itch of 2002. Then, their attention turned to the deserted uranium mines.

At first, it appeared to be just another dead end. Radiation levels were measured in over 7,000 houses, only for them to find that concentrations of radioactive substances were too low to create any kind of problem. Besides, experts pointed out, the symptoms were all wrong. Radiation poisoning causes cancer and organ damage. It doesn't cause people to fall down unconscious. 

It was only after the results from the medical exams came back that they were able to identify the real culprit. By then, it was 2015 and the sleeping sickness had been tormenting the inhabitants of Kalachi and Krasnogorsk for five years. Many villagers had taken things into their own hands and left home of their own accord.

And it turned out it did have everything to do with the mines – just not the radiation levels, as originally thought. Instead, the sleeping sickness was the result of excessive levels of carbon monoxide and hydrocarbons in the air caused by the uranium mines. The gas carbon monoxide binds to the blood more effectively than oxygen – 200 times more effectively than oxygen, to be precise – which means that when levels get too high, it can starve the brain of oxygen (asphyxiation). This can trigger the brain to shut down, hence the uncontrollable feeling of fatigue.

“The uranium mines were closed at some point, and at times a concentration of carbon monoxide occurs there,” Kazakhstan’s deputy Prime Minister, Berdibek Saparbaev, announced in the summer of 2015, reports the Guardian. “The oxygen in the air is reduced accordingly, which is the real reason for the sleeping sickness in these villages.”

The remaining residents were evacuated but not everyone was persuaded by the carbon monoxide theory, not least because the mines hadn't been active since the early nineties and carbon monoxide is usually the product of combustion. Why was the sleeping sickness affecting residents two decades on? At the time, Claude Piantadosi, a pulmonologist at Duke University Medical Center, told Wired other gasses (including carbon dioxide) could be involved. 

According to a Mental Floss article in 2016, scientists at the National Nuclear Center of Kazakhstan did later confirm that excess carbon monoxide from the uranium mines was the cause of the sleeping illness, though there do seem to be several questions left unanswered.