What causes violent dreams?

The purposes of dreams are not fully understood,and have been debated throughout recorded history. The scientific study of dreams is known as oneirology. There is no universally agreed biological definition of dreaming. Dreams have been strongly associated with rapid eye movement (REM) sleep. Humans spend a total of about six years dreaming in their lifetime. (about two hours a night.) Most dreams last only 5 to 20 minutes. It is unknown where in the brain dreams originate,what potion or portions of the brain are involved, or what the purpose of dreaming is for the body or mind.