Nightmare experiences have been documented for hundreds, and in some cases, thousands of years. The advancement of sleep technologies has greatly contributed in clarifying the specific elements that define the nightmare phenomena. Contemporary understandings identify different types of nightmares and describe this phenomenon as the most common form of disturbed dreaming. In addition, epidemiological studies report that nightmares are more prevalent amongst the general population than once thought. The 'negative' aspect about having any type of nightmare, is that although the psychology (mental structures) of the dream are either fictitious or activations of past memories, the mind in the dream state is unable to distinguish reality from fiction. Therefore, the physiological and emotional reactions to those fictitious mental structures are very real to the dreamer.