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, philosophical intrigue and religious interest throughout recorded history. The scientific study of dreams is called oneirology. Science has proven to an extent that all mammals dream. The scientific approach to dreams has been conducted through the discovery of REM sleep.

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. A person is more likely to remember the dream if he or she is awakened during the REM phase.

Dreams are a connection to the human subconscious. They can range from normal and ordinary to the overly surreal and bizarre. Dreams can at times make a creative thought occur to the person or give a sense of inspiration. Dream imagery is often absurd and unrealistic, and the events in dreams are generally outside the control of the dreamer, with the exception of lucid dreaming. Dreamers are usually not self-aware in their dreams; thus the dreams seem as reality. Dreams can have varying natures, such as frightening, exciting, magical, melancholic, adventurous, or sexual.