The village of Kalachi, Kazakhstan, is located in the center of the country toward the north (here’s a map). It is home to about 600 people, although about half of them are looking to move — before they suddenly fall asleep, perhaps for days at a time.

In the spring of 2013, approximately ten villagers ended up in the hospital, each with a similar set of symptoms — dizziness, memory loss, some loss of motor control. But there was a stranger symptom common among these ten patients — many reported conking out during the course of everyday tasks. And this wasn’t just a cat nap nor were these sleeping spells driven by fatigue. The patients simply fell asleep, without warning — and many of them slept for days, unable to be awoken.

The cause was a mystery. Other than the fact that they all lived in the same area, there were few other similarities among the patients. The malady affected both men and women. The patients ranged in age from 14 to 70. Some were students, others worked long hours, and still others had a relatively less taxing schedule. If there were any other clues, they went unfound — given the size and location of Kalachi, the outside world didn’t know about and couldn’t send in the teams of researchers who would normally address such strange things. And then, in May of 2013, the sleeping sickness abated. Curiosity notwithstanding, any incentive to further explore the cause of these ten hospital visits disappeared.

Until the affliction came back in January of 2014.

And then again that May.

And again that August.

Increasingly, doctors and researchers have descended on Kalachi, trying to figure out what’s going on. One of the people struck by the August wave, a man named Viktor Kazachenko spoke to reporters about the ordeal (relayed by the Guardian). On August 28th, he was driving his motorcycle to a neighboring town, with his wife riding along with him. He woke up in the hospital five days later, without any recollection of the intervening events. While he suffered no injuries from driving a motorbike while asleep — more likely a telling clue about the disease than a miracle — he told the media that he felt disoriented for weeks after he awoke. (His wife was entirely fine.) And Kazachenko was probably more prepared for the after-effects of the sleeping sickness than others, because this wasn’t his first bout with the disease. He had previously fallen asleep for three days.

But despite repeat victims and, now, at least four different spells of the sleeping sickness, researchers have almost no idea what is causing this weird plague. As VICE reported, “over 7,000 tests on patients and the town’s air, food, water, and general environment, no signs of bacterial, viral, chemical, radiological, or any other contamination have been identified.” There’s a disused uranium plant which early reports believed could be the source of the trouble, but there is a village closer to the mine than Kalachi is and besides, residents of Kalachi show no symptoms typical with uranium poisoning. Even very rare causes such as psychosomatic ones (i.e., mass hysteria) have been ruled out for various reasons. The only promising lead? As VICE noted, “outbreaks tend to accord with shifts from cold to warm weather.” But no one knows what, if anything, the temperature change has to do with the problems.

In total, more than 150 people have been struck the Kalachi sleeping sickness, and as the cause hasn’t been identified, there’s no reason to believe that the recurrences will end. Many of Kalachi’s residents are leaving town, hoping to escape whatever is making them and their neighbors fall asleep without explanation. But others want to stay in the only home they know, despite the dangers of contracting mystery disease.

Bonus Fact : Another mystery about sleep is why we sleep in the first place. If anyone could explain why, it’d probably be William Dement, a sleep researcher — Wikipedia refers to him as “a leading authority on sleep, sleep deprivation and the diagnosis and treatment of sleep disorders such as sleep apnea and narcolepsy” who “is sometimes referred to as the father of sleep medicine.” But in 2010, when a National Geographic reporter asked him, according to his half-century of research, why people sleep, Dement replied, “As far as I know, the only reason we need to sleep that is really, really solid is because we get sleepy.”

Take the Quiz : Can you name these cities which have multiple Zs in their names? (Get it?)

From the Archives : Nodding Disease: Another sleeping sickness with an unknown cause. (The two are believed to not be related, as Nodding Disease only affects children.) And, Sleeping Beauty Syndrome, yet another one, which, again, isn’t likely what’s causing whatever is happening in Kalachi.