tl;dr: no known cause
Radiation. Government conspiracy. Mass hysteria. There are plenty of theories as to why the residents of a tiny Kazakh mining region keep falling asleep for days at a time, but no answers. Deputy Prime Minister Berdibek Saparbaev announced that the mystery of the sleeping sickness had been solved: It was the specific combination of reduced levels of oxygen and heightened levels of carbon monoxide and hydrocarbons. But as usual, rumors overshadowed reality. “The real moment is that we came to the conclusion that there are natural processes that lead to the combination of these kinds of factors and that specifically this combination of factors could give this effect. I can’t say for certain now. Our assumption is that all 3 components must coincide. When all 3 exist, that’s when people will start falling and sleeping again.” He had searched for similar cases in scientific literature, but so far had found nothing. He suggested the illness would return in September when people started to heat their homes.