The Tibetan tradition explains the process of falling asleep using a metaphor for the mind and prana. Often prana is compared to a blind horse and the mind to a person unable to walk. Separately they are helpless, but together they make a functional unit. When the horse and rider are together they begin to run, generally with little control over where they go. As we fall asleep, awareness of the sensory world is lost. The mind is carried here and there on the blind horse of karmic prana until it becomes focused in a particular chakra where it is influenced by a particular dimension of consciousness. Perhaps you had an argument with your partner and that situation (secondary condition) activates a karmic trace associated with the heart chakra, which pulls your mind into that location in the body. The subsequent activity of mind and prana manifests in the particular images and stories of the dream.
Tenzin Wangyal Rinpoche