Dream sequence

A dream sequence is a technique used in storytelling, particularly in television and film, to set apart a brief interlude from the main story. The interlude may consist of a flashback, a flashforward, a fantasy, a vision, a dream, or some other element.

Commonly, dream sequences appear in many films to shed light on the psychical process of the dreaming character or give the audience a glimpse into the character's past. For instance in Pee-wee's Big Adventure, the purpose of Pee-wee's dreams is to inform the audience of his anxieties and fears after losing his bike. Other times major action takes place in dreams, allowing the filmmaker to explore infinite possibilities, as Michel Gondry demonstrates in The Science of Sleep. Harvard psychologist Deirdre Barrett points out in the book The Committee of Sleep that, while the main content of dream sequences is determined by the film's overall plot, visual details often reflect the individual dream experience of the screenwriter or director. For Hitchcock's Spellbound, Salvador Dalí designed sharply angled sets inspired by his own dream space. Ingmar Bergman lit dream sequences in several films with a harsh glare of light which he says reflects his own nightmares (though most people's have dim light), and Orson Welles designed a scene of the trial to reflect the manner in which architecture constantly changed in his dreams.

Films normally present dreams as a visually accessible or objectively observed space, a discrete environment in which characters exist and interact as they do in the world rather than restricting themselves to the subjective point of view a dream is normally experienced from in real life. In this way films succeed in presenting a coherent dreamed world alongside the diegetic reality of the film. Via transition from one to the next, a film establishes not only the boundaries but resonances between the two worlds. These resonances can reveal a character's subjective observations or desires without breaking away from the objective viewpoint of the narrator, camera, or director with which some theorist, such as Christian Metz, believe the viewer identifies.

...
Wikipedia