What Is Benign Neonatal Sleep Myoclonus? Benign neonatal sleep myoclonus is a sleep-related movement disorder that occurs in very young infants. The condition is characterized by fast, repetitive jerks of large muscles, such as the arms, legs, and torso, during sleep. In benign neonatal sleep myoclonus, myoclonic jerks occur multiple times while the infant is sleeping. The condition is considered harmless, though without measuring brain waves it may be difficult to tell whether an infant has ben