Dreams are successions of images , ideas , emotions, and sensations that occur usually 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. 

Dreams mainly occur in the rapid eye movement stages 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.

                                                        Why do we dream?