Parasomnia

Fourteen-year-old Cassandra experiences sleep paralysis, a terrifying condition with a supernatural cause: a demonic incubus. After setting her classroom on fire during an apparent psychotic break at school, she is diagnosed as mentally ill. Unable to persuade her family that the demon's nocturnal assaults are real, she launches a desperate plan to save herself, with tragic consequences. Years later, Cassandra's daughter begins to experience sleep paralysis as well—or has the demon returned? When events eventually prove the incubus is not a product of mental illness, Cassandra must enlist the help of her skeptical sister, a sleep medicine physician, to find a way to destroy the demon. But they're running out of time: the creature will return on the next new moon to kill the young girl.