<P> A dream is a succession of images, ideas, emotions, and sensations that usually occur involuntarily in the mind during certain stages of sleep . The content and purpose of dreams are not fully understood, although they have been a topic of scientific, philosophical and religious interest throughout recorded history . Dream interpretation is the attempt at drawing meaning from dreams and searching for an underlying message . The scientific study of dreams is called oneirology . </P> <P> 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 . The length of a dream can vary; they may last for a few seconds, or approximately 20--30 minutes . People are more likely to remember the dream if they are awakened during the REM phase . The average person has three to five dreams per night, and some may have up to seven; however, most dreams are immediately or quickly forgotten . Dreams tend to last longer as the night progresses . During a full eight - hour night sleep, most dreams occur in the typical two hours of REM . </P> <P> Opinions about the meaning of dreams have varied and shifted through time and culture . Many endorse the Freudian theory of dreams--that dreams reveal insight into hidden desires and emotions . Other prominent theories include those suggesting that dreams assist in memory formation, problem solving, or simply are a product of random brain activation . </P>

How many dreams does a person have at night