More than 100 residents in Kalachi, Kazakhstan, have been falling asleep for days without any warning.
Some of them actually fall asleep in the middle of walking.
When the townspeople wake up, they have a memory loss, notes TIME.
The incidents first began in 2010, but started happening very frequently in March 2013.
The small village is near a uranium mine, and officials suspected that radon gas from the mine might be the cause. However, tests have been run on residents and the gas was ruled out.
However, one possibility may be carbon monoxide in chimney smoke.
Sergei Lukashenko, who is the director of the country's National Nuclear Centre’s Radiation Safety and Ecology Institute, told the Siberian Times:
Carbon monoxide is definitely a factor, but I can’t tell you whether this is the main and vital factor. The question is why it does not go away. We have some suspicions as the village has a peculiar location and weather patterns frequently force chimney smoke to go down instead of up.
According to Lukashenko, there are 15 specialists working on the issue and five scientists taking samples in the town, but residents are leaving Kalachi in droves.
Officials plan to block off the town in May.