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 and a subject of philosophical and religious interest throughout recorded history. The scientific study of dreams is called oneirology. Scientists believe that other mammals, birds and reptiles, also dream.
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 20 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, but some may have up to seven dreams in one night. The dreams tend to last longer as the night progresses. During a full eight-hour night sleep, two hours of it is spent dreaming.
In modern times, 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.
source: wikipedia. org