Over the past year and a half, about a quarter of the population of a northern Kazakhstan village has sporadically stopped what they're doing and suddenly fallen into a deep sleep. And no one can explain why.

Dubbed a "sleeping sickness" by scientists, the episodes are marked by an instantaneous deep slumber that lasts days on end. Those afflicted wake up feeling confused and dizzy — most don't recall their time asleep, but some report frightening hallucinations, reports the tech-based news site Mashable.  

"To bring them into full consciousness is practically impossible on the first day," explained Professor Leonid Rikhvanov, who is investigating the disease, to Newsweek.

While college students during finals week may also experience similar symptoms, the sleeping sickness in Kalachi can't be explained away by late-night study sessions. In fact, it can't be explained away by anything.

Scientists and doctors have flown into Kalachi to determine the root of the illness, but after conducting several tests ranging from environmental toxicity to patient data, results have been unsubstantial. Bacterial and viral tests have been negative, knocking out the possibility that this is a parasitic disease such as African trypanosomiasis, which has similar effects. So beyond a few hunches, scientists don't have much explanation for what's going on.

Meanwhile, patients of the sleeping sickness have been reporting symptoms that endure after they wake up.

"After the slumber, my blood pressure started going up for no reason," a villager named Viktor Kazachenko explained to New York-based news website Eurasia Net. "For six weeks, I didn't know where to put myself. It strongly affects your mentality. I'm very on edge."

Kazachenko, like 30 other Kalachi residents, has been struck by the sleeping sickness twice already. Another villager named Tatyana Pavlenko told Eurasia Net that "we're all in fear of falling asleep."

The first incident was reported in March 2013 and since then 122 villager have fallen victim to the mysterious illness. The episodes tend to come in waves — lull periods are interrupted by a new outbreak of cases. According to the Russian Times, eight schoolchildren fell asleep within the same hour on the first day of school last September, and a few months later, 60 people fell victim on the same day.

A ninth wave hit at the beginning of the month, prompting local authorities to start evacuating the village.

So, what could be the force that's knocking out people in this tiny village? Here are the most popular theories:

Kalachi is located about 37 miles away from a former Soviet Union ore mine in a ghost town called Krasnogorsky. The mine hasn't been operational in the past two decades, but Rikhvanov told Newsweek that he believes radon gas could be emitting from underneath the ground.

In fact, elevated levels of radon have been found in the village, but doctors from Kazakhstan claim that it's not the cause of the sleeping sickness.

"I am an anesthesiologist myself and we use similar gases for anesthesia, but the patients wake up a maximum in one hour after surgery," Kabdrashit Almagambetov told The Siberian Times. "These people sleep for two to six days, what is the concentration of this gas then? And why one person falls asleep and somebody who lives with him does not?"

 Radon is a colorless and odorless radioactive gas formed by uranium breakdown in soil, water, and rock.  According to the EPA, radon has been linked to lung cancer, but symptoms don't typically match up with the sleeping sickness.

On the other hand, Sergei Lukashenko of Kazakhstan's National Nuclear Center Institute believes that carbon monoxide may play a role in the mystery. "Carbon monoxide is definitely a factor," he told The Siberian Times. "We have some suspicions as the village has a peculiar location and weather patterns frequently force chimney smoke to go down instead of up."

Carbon monoxide poisoning is usually accompanied by headache, vomiting, and dizziness, and while there may be an overlap in symptoms, this still doesn't explain the very specific behavior of Kalachi's sleeping sickness.

Sometimes, when doctors cannot find any medical cause for an ailment, they resort to the theory of mass hysteria. Often marked by inexplicable and bizarre occurrences in large groups of people, mass hysteria is a mysterious psychological concept of its own.

It's possible that the villagers of Kalachi, perhaps due to distress of some sort, have fallen into such a category.