Dreams are successions of images, ideas, emotions, and sensations that occur involuntarily in the mind during certain stages of sleep. The content and purpose of dreams are not definitively understood, though they have been a topic of scientific speculation, as well as a subject of philosophical and religious interest, throughout recorded history. The scientific study of dreams is called oneirology. Dreams mainly occur in the rapid-eye movement (REM) stage of sleep—when brain activity is high and resembles that of being awake. REM sleep is revealed by continuous movements of the eyes during sleep. At times, dreams may occur during other stages of sleep. However, these dreams tend to be much less vivid or memorable. Dreams can last for a few seconds, or as long as twenty minutes. People are more likely to remember the dream if they are awakened during the REM phase. Dreams have been seen as a connection to the unconscious. They range from normal and ordinary to overly surreal and bizarre. Dreams can have varying natures, such as frightening, exciting, magical, melancholic, adventurous, or sexual. The events in dreams are generally outside the control of the dreamer, with the exception of lucid dreaming, where the dreamer is self-aware. Dreams can at times make a creative thought occur to the person or give a sense of inspiration. Opinions about the meaning of dreams have varied and shifted through time and culture. Dream interpretations date back to 5000-4000 BC. The earliest recorded dreams were acquired from materials dating back approximately 5000 years, in Mesopotamia, where they were documented on clay tablets. In the Greek and Roman periods, the people believed that dreams were direct messages from the gods, or from the dead and that they predicted the future. Some cultures practiced dream incubation with the intention of cultivating dreams that are prophetic. The Austrian neurologist Sigmund Freud, who developed the discipline of psychoanalysis, wrote extensively about dream theories and interpretations. He explained dreams as manifestations of our deepest desires and anxieties, often relating to repressed childhood memories or obsessions. In The Interpretation of Dreams, Freud developed a psychological technique to interpret dreams and devised a series of guidelines to understand the symbols and motifs that appear in our dreams.